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Atoms and atomic structure

Science

Atoms and atomic structure

Atoms consist of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] surrounding a nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] that contains protons [protons: Sub-atomic particles with a positive charge and a relative mass of 1. ] and neutrons [neutrons: Uncharged sub-atomic particles, with a mass of 1 relative to a proton. ].

Protons and neutrons have a relative mass [relative mass: The relative mass is the number of times heavier a particle is, compared to another. ] of 1 and electrons have a negligible [negligible: So small as to be not worth considering. ] mass. Neutrons are neutral, but protons and electrons are electrically charged. Protons have a relative charge of +1 and electrons have a relative charge of -1.

Early ideas about atoms

The word atom [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ] comes from atomos, an ancient Greek word meaning indivisible. The Greek philosopher Demokritos (460-370 BCE) maintained that all matter could be divided and sub-divided into smaller and smaller units, and eventually there would be a tiny particle that could not be divided any further - an atom. This was remarkable because there was no way ancient Greeks could support this theory by observation or experiment.

John Dalton

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_83.jpg)

John Dalton (1766-1844)

Understanding of atoms didn’t progress much beyond Demokritos’ theory until the English chemist John Dalton (1766 - 1844) started to look at it in the 1800s. Dalton did experiments, worked out some atomic weights, and invented symbols for atoms and molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ]. His most important conclusions are summarised below.

  • all matter is made of atoms, and atoms are indestructible and cannot be broken down into pieces
  • all the atoms of a particular element are identical to each other and different from the atoms of other elements
  • atoms are rearranged in a chemical reaction
  • compounds are formed when two or more different kinds of atoms join together

the atoms are represented as spheres and joined by a double bond (http://www.bbc.co.uk/schools/gcsebitesize/science/images/oxygen_multi.gif)

Molecules of oxygen - the two atoms are joined by a double bond

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/carbondioxide.jpg)

A molecule of carbon dioxide. Atoms are represented as spheres and are colour-coded - carbon (green) and oxygen (red). The atoms are joined by double bonds

Dalton's theories about atoms took a long time to be accepted by scientists. Some of his ideas about gases were incorrect, and it was difficult for many years to do the experiments needed to support his theories, because atoms are too small to see.

Atoms and elements

atoms have a small central nucleus surrounded by electrons (http://www.bbc.co.uk/schools/gcsebitesize/science/images/4_atoms_elements.gif)

The structure of the atom

Although the word 'atom' comes from the Greek for indivisible, we now know that atoms are not the smallest particles of matter. Instead, they have a small central nucleus surrounded by even smaller particles called electrons.

All substances are made from atoms. And, as Dalton suggested, any given element is made of atoms of just one particular sort. The atoms of any element are different from the atoms of any other element. So iron contains a different sort of atoms from those of sulfur, and the atoms in carbon are different from those of oxygen.

Chemical symbols

The atoms of each element are represented by chemical symbols. These usually consist of one or two different letters, but sometimes three letters are used for newly-discovered elements. The first letter in a chemical symbol is always an UPPERCASE letter, and the other letters are always lowercase. So, the symbol for magnesium is Mg and not mg, MG or mG.

Every element has its own chemical symbol. For example, iron is Fe, sulfur is S, sodium is Na and oxygen is O.

The periodic table

There are more than 100 different elements. The periodic table is a chart showing all the elements arranged in a particular way. The vertical columns in the periodic table are called groups. Each group contains elements that have similar properties.

Group 1 - alkali metals, group 7 - halogens, group 0 - noble gases. Transition metals are between group 2 and 3.  (http://www.bbc.co.uk/schools/gcsebitesize/science/images/38_modern_periodic_table.jpg)

The modern periodic table

The periodic table has eight main groups. For example, group 1 contains very reactive metals such as sodium - Na - while group 7 contains very reactive non-metals such as chlorine - Cl.

Note that you will never find a compound in the periodic table, because these consist of two or more different elements joined together by chemical bonds.

Reactions and compounds

New substances are formed by chemical reactions. When elements react together to form compounds their atoms join to other atoms using chemical bonds. For example, iron and sulfur - often spelt 'sulphur' - react together to form a compound called iron sulfide - often spelt 'sulphide' - and sodium and oxygen react together to form sodium oxide.

 mixture of powders (http://www.bbc.co.uk/schools/gcsebitesize/science/images/react_comp_2.jpg)

Mixture of iron (grey) and sulphur (yellow) powders.

 mixture heated in a test tube (http://www.bbc.co.uk/schools/gcsebitesize/science/images/react_comp_1.jpg)

The mixture is heated in a test tube.

chemical reaction occurs (http://www.bbc.co.uk/schools/gcsebitesize/science/images/react_comp_3.jpg)

A chemical reaction occurs and iron sulphide is formed.

Chemical bonds involve electrons from the reacting atoms. Bonds can form when:

  • electrons are transferred from one atom to another, so that one atom gives electrons and the other takes electrons, or
  • electrons are shared between two atoms.

You don’t need to know any more details about these bonds for GCSE Science.

Chemical formulae

The chemical formula of a compound shows how many of each type of atom join together to make the units that make the compound up. For example, in iron sulfide every iron atom is joined to one sulfur atom, so we show its formula as FeS. In sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as Na2O. Notice that the 2 is written as a subscript, so Na2O would be wrong.

The diagram below shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide - its chemical formula should therefore be written as CO2.

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/aqa_science_03.gif)

Carbon dioxide units contain one carbon atom and two oxygen atoms

Sometimes you see more complex formulae such as Na2SO4 and Fe(OH)3:

  • a unit of Na2SO4 contains two sodium atoms, one sulfur atom and four oxygen atoms joined together
  • a unit of Fe(OH)3 contains one iron atom, three oxygen atoms and three hydrogen atoms - the brackets show that the 3 applies to O and H

Equations

When elements are joined to cause a chemical reaction, no atoms are made or lost during the process - but at the end of it they are joined differently from the way they were at the start. This means that the mass of the substances at the start - the reactants - is the same as the mass of the substances at the end - the products.

Copper and oxygen reaction - getting a balanced equation

We use balanced equations to show what happens to the different atoms in reactions. For example, copper and oxygen react together to make copper oxide.

Take a look at the word equation for the reaction, here:

copper + oxygen    →    copper oxide

You can see that copper and oxygen are the reactants, and copper oxide is the product.

If we just replace the words shown above by the correct chemical formulae, we will get an unbalanced equation, as shown here:

Cu + O2    →    CuO

Notice that we have unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, we need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides of the arrow.

Here is the balanced symbol equation:

2Cu + O2    →    2CuO

You can see that now we have two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/4_equations.gif)

Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide

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Fuels from crude oil

Fuels from crude oil

Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil. It can be separated into different fractions using fractional distillation, and some of these can be used as fuels. Unfortunately, there are environmental consequences when fossil fuels such as crude oil and its products are used.

Hydrocarbons and alkanes

Hydrocarbons

Most of the compounds in crude oil are hydrocarbons. This means that they only contain hydrogen and carbon atoms, joined together by chemical bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes.

Alkanes

The alkanes are a family of hydrocarbons that share the same general formula. This is:

CnH2n+2

The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6. Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.

Structure of alkanes

alkaneformulachemical structureball-and-stick model methane CH4 H - C - H, with an H above and below the C. (http://www.bbc.co.uk/schools/gcsebitesize/science/images/methane_chem_struc_2.gif) one carbon atom and four hydrogen atoms (http://www.bbc.co.uk/schools/gcsebitesize/science/images/methane_model_2.gif) ethane C2H6 two C's and six H's (http://www.bbc.co.uk/schools/gcsebitesize/science/images/ethane_chem_struc.gif) two carbon atoms and six hydrogen atoms (http://www.bbc.co.uk/schools/gcsebitesize/science/images/ethane_model.gif) propane C3H8 three C's and eight H's (http://www.bbc.co.uk/schools/gcsebitesize/science/images/propane_chem_struc.gif) three carbon atoms and eight hydrogen atoms (http://www.bbc.co.uk/schools/gcsebitesize/science/images/propane_model.gif) butane C4H10 four C's and ten H's atoms (http://www.bbc.co.uk/schools/gcsebitesize/science/images/butane_chem_struc.gif) four carbon atoms and ten hydrogen atoms (http://www.bbc.co.uk/schools/gcsebitesize/science/images/butane_model.gif)

Notice that the molecular models on the right show that the bonds are not really at 90º.

Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air, which we call burning or combustion.

Boiling point and state at room temperature

Hydrocarbons have different boiling points, and can be either solid, liquid or gas at room temperature:

  • Small hydrocarbons with only a few carbon atoms have low boiling points and are gases.
  • Hydrocarbons with between five and 12 carbon atoms are usually liquids.
  • Large hydrocarbons with many carbon atoms have high boiling points and are solids.

Distillation

Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works when the liquids have different boiling points. Distillation is commonly used to separate ethanol (the alcohol in alcoholic drinks) from water.

Distillation process to separate ethanol from waterwater and ethanol solution are heated in a flask over a bunsen burner, pure vapour is produced in the air above the solution within the flask.  (http://www.bbc.co.uk/schools/gcsebitesize/science/images/12_1_distillation.gif)

Step 1 - water and ethanol solution are heated

Distillation process to separate ethanol from watertemperature reaches 78 degrees celcius, vapour condenses in a condenser, ethanol drips out into a beaker  (http://www.bbc.co.uk/schools/gcsebitesize/science/images/12_2_distillation.gif)

Step 2 - the ethanol evaporates first, cools, then condenses

Distillation process to separate ethanol from waterwater and ethanol solution has reached 100 degrees celcius. pure water now drips into the beaker, from the test tube. (http://www.bbc.co.uk/schools/gcsebitesize/science/images/12_3_distillation.gif)

Step 3 - the water left evaporates, cools, then condenses

The mixture is heated in a flask. Ethanol has a lower boiling point than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid. The thermometer shows the boiling point of the pure ethanol liquid. When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates.

This is the sequence of events in distillation:

heating → evaporating → cooling → condensing

Fractional distillation

Fractional distillation differs from distillation only in that it separates a mixture into a number of different parts, called fractions. A tall column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with low boiling points condense at the top. Like distillation, fractional distillation works because the different substances in the mixture have different boiling points.

Fractional distillation of crude oil

Because they have different boiling points, the substances in crude oil can be separated using fractional distillation. The crude oil is evaporated and its vapours allowed to condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.

Oil fractions

The diagram below summarises the main fractions from crude oil and their uses, and the trends in properties. Note that the gases condense at the top of the column, the liquids in the middle and the solids stay at the bottom.

The top of the column is cool (25 degrees celsius). Fractions taken from here have small molecules, low boiling points, are very volatile, flow easily and ignite easily. Crude oil enters at the bottom of the column and is heated to 350 degrees celsius. Fractions taken here have large molecules, high boiling points, are not very volatile, and don't flow or ignite easily. From top to bottom the fractions are: Refinery gases (bottled gas), gasoline (petrol), naptha (used for making chemicals), kerosene (aircraft fuel), diesel oil (fuel for cars, and lorries, etc), fuel oil (fuel for ships, power stations), residue (bitumen for roads and roofs). (http://www.bbc.co.uk/schools/gcsebitesize/science/images/5_fractional_distillation.gif)

The fractionating column

The main fractions include refinery gases, gasoline (petrol), naphtha, kerosene, diesel oil, fuel oil, and a residue that contains bitumen. These fractions are mainly used as fuels, although they do have other uses too.

Hydrocarbons with small molecules make better fuels than hydrocarbons with large molecules because they are volatile, flow easily and are easily ignited.

Combustion of fuels

Complete combustion

Fuels burn when they react with oxygen in the air. The hydrogen in hydrocarbons is oxidised to water (remember that water, H2O, is an oxide of hydrogen). If there is plenty of air, we get complete combustion and the carbon in hydrocarbons is oxidised to carbon dioxide:

hydrocarbon + oxygen    →    water + carbon dioxide

Incomplete combustion

If there is insufficient air for complete combustion, we get incomplete combustion instead. The hydrogen is still oxidised to water, but instead of carbon dioxide we get carbon monoxide. Particles of carbon, seen as soot or smoke, are also released.

Sulfur

Most hydrocarbon fuels naturally contain some sulfur compounds. When the fuel burns, the sulfur it contains is oxidised to sulfur dioxide.

Summary

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/smoke_chimneys.jpg)

Clouds of smoke and other combustion products are emitted from chimneys

The combustion of a fuel may release several gases into the atmosphere, including:

  • water vapour
  • carbon dioxide
  • carbon monoxide
  • particles
  • sulfur dioxide

These products may be harmful to the environment.

Sulfur dioxide

Sulfur dioxide is produced when fuels that contain sulfur compounds burn. It is a gas with a sharp, choking smell. When sulfur dioxide dissolves in water droplets in clouds, it makes the rain more acidic than normal. This is called acid rain.

Effects of acid rain

Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result. Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a result. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.

Reducing acid rain

Sulfur dioxide can be removed from waste gases after combustion of the fuel. This happens in power stations. The sulfur dioxide is treated with powdered limestone to form calcium sulfate. This can be used to make plasterboard for lining interior walls, so turning a harmful product into a useful one.

Waste gases from the power station is treated with limestone slurry to form calcium sulfate. The clean gases then go to the chimney. (http://www.bbc.co.uk/schools/gcsebitesize/science/images/6_sulfur_dioxide.gif)

The process of removing sulfur dioxide

Sulfur can be removed from fuels at the oil refinery. This makes the fuel more expensive to produce, but it prevents sulfur dioxide being produced. You may have noticed 'low sulfur' petrol and diesel on sale at filling stations.

Global warming

Carbon dioxide from burning fuels causes global warming, a process capable of changing the world’s climate significantly.

The percentage of carbon dioxide in the atmosphere has risen from 0.028 in 1700 to 0.035 in 1990. (http://www.bbc.co.uk/schools/gcsebitesize/science/images/global_warming_graph.gif)

Carbon dioxide in the atmosphere has risen at a higher rate since the 19th century

The earth's global average temperature has risen from 13.5º C in 1860 to 14.4º C in 1995 (temperatures over a 5 year average).  (http://www.bbc.co.uk/schools/gcsebitesize/science/images/global_temp_graph.gif)

The temperature of the earth has risen over the years

As you can see from the graphs, the amount of carbon dioxide in the atmosphere has increased steadily over the past 150 years, and so has the average global temperature.

Carbon dioxide is a greenhouse gas. It absorbs heat energy and prevents it escaping from the Earth’s surface into space. The greater the amount of carbon dioxide in the atmosphere, the more heat energy is absorbed and the hotter the Earth becomes.

Greenhouse effect

Earth absorbing and reflecting some solar radiation (http://www.bbc.co.uk/schools/gcsebitesize/science/images/global_warm_1.jpg)

  1. Sun’s rays enter the Earth’s atmosphere
  2. Heat is reflected back from the Earth’s surface
  3. Heat is absorbed by carbon dioxide (greenhouse gas) and as a result becomes trapped in the Earth’s atmosphere
  4. The Earth becomes hotter as a result

Results of global warming

A rise of just a few degrees in world temperatures will have a dramatic impact on the climate:

  • Global weather patterns will change, causing drought in some places and flooding in others.
  • Melting of polar ice caps will raise sea levels, causing increased coastal erosion and flooding of low-lying land – including land where major cities lie.

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/global_warm_2.jpg)

The Triftgletscher glacier, Switzerland, 2002

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/global_warm_3.jpg)

The Triftgletscher glacier, Switzerland, 2003. As the glacier melts further, the lake's water level rises.

Global dimming

Tiny particles that are released when fuels are burned cause global dimming. Like global warming, this process may change rainfall patterns around the world.

The amount of sunlight reaching the Earth’s surface has decreased by about 2 per cent every ten years, because more sunlight is being reflected back into space. The particles from burning fuels reflect sunlight, and they also cause more water droplets to form in the clouds. This makes the clouds better at reflecting sunlight back into space.

Watch

You may wish to view this BBC News item from 2006 about air pollution controls.

It is likely that global dimming has hidden some of the effects of global warming, by stopping some of the Sun’s energy reaching the Earth’s surface in the first place. Governments around the world are introducing controls on pollution. There is the possibility that as the air becomes less polluted by smoke and soot, global dimming will decrease, causing the effects of global warming to become more obvious.

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Limestone

Limestone

Limestone is mainly calcium carbonate, CaCO3. When it is heated, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide reacts with water to produce calcium hydroxide.

Limestone and its products have many uses, including being used to make mortar, cement, concrete and glass.

Thermal decomposition

Metal carbonates such as calcium carbonate break down when heated strongly. This is called thermal decomposition. Here are the equations for the thermal decomposition of calcium carbonate:

calcium carbonate right facing arrow with heat (http://www.bbc.co.uk/schools/gcsebitesize/science/images/arrow_heat.gif) calcium oxide + carbon dioxide

CaCO3right facing arrow with heat (http://www.bbc.co.uk/schools/gcsebitesize/science/images/arrow_heat.gif) CaO + CO2

Other metal carbonates decompose in the same way. Here are the equations for the thermal decomposition of copper carbonate:

copper carbonate right facing arrow with heat (http://www.bbc.co.uk/schools/gcsebitesize/science/images/arrow_heat.gif) copper oxide + carbon dioxide

CuCO3right facing arrow with heat (http://www.bbc.co.uk/schools/gcsebitesize/science/images/arrow_heat.gif) CuO + CO2

Notice that in both examples the products are a metal oxide and carbon dioxide. The carbon dioxide gas can be detected using limewater. Limewater turns cloudy white when carbon dioxide is bubbled through it.

Metals high up in the reactivity series - such as calcium - have carbonates that need a lot of energy to decompose them. Metals low down in the reactivity series - such as copper - have carbonates that are easily decomposed. This is why copper carbonate is often used at school to show these reactions. It is easily decomposed, and its colour change, from green copper carbonate to black copper oxide, is easy to see

Copper carbonate + heat -> Copper oxide + Carbon dioxide (http://www.bbc.co.uk/schools/gcsebitesize/science/images/7_thermal_decomposition_v2.gif)

Quicklime and slaked lime

For your exam, you need to know how quicklime and slaked lime are obtained from limestone.

Making quicklime

If limestone is heated strongly, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide is also called quicklime. It is yellow when hot, but white when cold.

Here are the equations for this reaction:

calcium carbonate right facing arrow with heat (http://www.bbc.co.uk/schools/gcsebitesize/science/images/arrow_heat.gif) calcium oxide + carbon dioxide

CaCO3right facing arrow with heat (http://www.bbc.co.uk/schools/gcsebitesize/science/images/arrow_heat.gif) CaO + CO2

This is a thermal decomposition reaction.

Making slaked lime

Calcium oxide reacts with water to form calcium hydroxide, also called slaked lime.

Here are the equations for this reaction:

calcium oxide + water → calcium hydroxide

CaO + H2O → Ca(OH)2

A lot of heat is produced in the reaction, which may even cause the water to boil.

Summary

Using common names instead of chemical names, this is what happens:

limestone right facing arrow with heat (http://www.bbc.co.uk/schools/gcsebitesize/science/images/arrow_heat.gif) quicklime + carbon dioxide

quicklime + water → slaked lime

Uses of limestone

Limestone, quicklime and slaked lime are all used to neutralise excess acidity - which may be caused by acid rain - in lakes and in soils.

Limestone is used as a building material, and to purify iron in blast furnaces. It's also used in the manufacture of glass, and of cement (one of the components of concrete).

The main uses of limestone and its products

  • Limestone (CaCO3) can be used as a building material and in the manufacturing of iron.
  • Glass - heated with sand and soda (sodium carbonate).
  • Cement - heated with clay in a kiln.
    • Concrete - mixed with sand, water and crushed rock
    • Mortar - mixed with sand and water
  • Quicklime - heated.
    • Slaked lime (Calcium Hydroxide Ca(OH)2) - mixed with water
      • Lime motar - mixed with water

Glass

Glass is made by melting sand and then cooling it. Flat sheets of glass for windows are made by floating molten glass on a layer of molten tin.

Glass manufacturers add sodium carbonate to sand during the manufacturing process, to reduce the melting temperature of the sand and so save energy. The sodium carbonate decomposes in the heat to form sodium oxide and carbon dioxide, but this makes the glass soluble in water. Calcium carbonate (limestone) is therefore also added, to stop the glass dissolving in water. The calcium carbonate decomposes in the heat to form calcium oxide and carbon dioxide. About 90 per cent of glass is soda-lime glass, or bottle glass.

Environmental, social and economic considerations

The limestone industry

You need to be able to evaluate some of the effects of the limestone industry.

The main advantages and disadvantages of the limestone industry

AdvantagesDisadvantages Limestone is a valuable natural resource, used to make things such as glass and concrete. Limestone quarries are visible from long distances and may permanently disfigure the local environment. Limestone quarrying provides employment opportunities that support the local economy in towns around the quarry. Quarrying is a heavy industry that creates noise and heavy traffic, which damages people's quality of life.

Links

You may wish to read this BBC News item from 2006 about a public inquiry into a limestone quarry in the Peak District National Park. Consider the arguments for and against quarrying limestone in a national park.

Advantages and disadvantages of various building materials

Limestone, cement and mortar slowly react with carbon dioxide dissolved in rainwater, and wear away. This damages walls made from limestone, and it leaves gaps between bricks in buildings. These gaps must be filled in or "pointed". Pollution from burning fossil fuels makes the rain more acidic than it should be, and this acid rain makes these problems worse.

Concrete is easily formed into different shapes before it sets hard. It is strong when squashed, but weak when bent or stretched. However, concrete can be made much stronger by reinforcing it with steel. Some people think that concrete buildings and bridges are unattractive.

Glass is usually brittle and easily shattered, but toughened glass can be used for windows. While glass is transparent and so lets light into a building, buildings with lots of glass can be too hot in the summer.

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Metals

Metals

Metals are very useful. Ores are naturally occurring rocks that contain metal or metal compounds in sufficient amounts to make it worthwhile extracting them. For example, iron ore is used to make iron and steel. Copper is easily extracted, but ores rich in copper are becoming more difficult to find. Aluminium and titanium are metals with useful properties, but they are expensive to extract. Most everyday metals are mixtures called alloys.

Methods of extracting metals

The Earth's crustcrust: The outer layer of the Earth, on top of the mantle. It is between 6 and 48 kilometres thick and includes the continents and the ocean floor. contains metals and metal compounds [compounds: Substances formed by the chemical union (involving bond formation) of two or more elements. ] such as gold, iron oxide and aluminium oxide, but when found in the Earth these are often mixed with other substances. To be useful, the metals have to be extracted from whatever they are mixed with. A metal ore is a rock containing a metal, or a metal compound, in a high enough concentration to make it economic to extract the metal.

The method used to extract metals from the ore in which they are found depends on their reactivity. For example, reactive metals such as aluminium are extracted by electrolysis [electrolysis: Electrolysis is the decomposition (separation or break-down) of a compound using an electric current. ], while a less-reactive metal such as iron may be extracted by reduction [reduction: Reduction is a reaction in which oxygen is removed from a substance. Reduction also means a gain in electrons. ] with carbon or carbon monoxide.

Thus the method of extraction of a metal from its ore depends on the metal's position in the reactivity series:

Reactivity and extraction method

Metals - in decreasing order of reactivityReactivity

  • potassium
  • sodium
  • calcium
  • magnesium
  • aluminium

extract by electrolysis carbon

  • zinc
  • iron
  • tin
  • lead

extract by reaction with carbon or carbon monoxide hydrogen

  • copper
  • silver
  • gold
  • platinum

extracted by various chemical reactions

Note that gold, because it is so unreactive, is found as the native metal and not as a compound, so it does not need to be chemically separated. However, chemical reactions may be needed to remove other elements that might contaminate the metal.

Making iron

In the blast furnace

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/blast_furnace.jpg)

Blast furnace in a modern steel works

Iron is extracted from iron ore in a huge container called a blast furnace. Iron ores such as haematite contain iron oxide. The oxygen must be removed from the iron oxide to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions.

Carbon is more reactive than iron, so it can push out or displace the iron from iron oxide. Here are the equations for the reaction:

iron oxide + carbon    →    iron + carbon dioxide

2Fe2O3 + 3C    →    4Fe + 3CO2

In this reaction, the iron oxide is reduced to iron, and the carbon is oxidised to carbon dioxide.

In the blast furnace, it is so hot that carbon monoxide can be used to reduce the iron oxide in place of carbon:

iron oxide + carbon monoxide    →    iron + carbon dioxide

Fe2O3 + 3CO    →    2Fe + 3CO2

Raw materials for the reaction

The raw materials for extracting iron and their function in the process

Raw materialContainsFunction iron ore (haematite) iron oxide a compound that contains iron coke carbon burns in air to produce heat, and reacts to form carbon monoxide (needed to reduce the iron oxide) limestone calcium carbonate helps to remove acidic impurities from the iron by reacting with them to form molten **** air oxygen allows the coke to burn, and so produces heat and carbon monoxide

Steel

Iron

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/steel.gif)

Layers of atoms slide over each other when metals are bent or stretched

Pure iron is soft and easily shaped. This is because its atoms are arranged in a regular way that lets layers of atoms slide over each other. Pure iron is too soft for many uses.

Iron from the blast furnace is an alloy of about 96 per cent iron with carbon and some other impurities. It is hard, but too brittle for most uses. So, most iron from the blast furnace is converted into steel by removing some of the carbon.

Steel

Carbon is removed by blowing oxygen into the molten metal. It reacts with the carbon producing carbon monoxide and carbon dioxide. These escape from the molten metal. Enough oxygen is used to achieve steel with the desired carbon content. Other metals are often added, such as vanadium and chromium.

There are many different types of steel, depending on the other elements mixed with the iron. The table summarises the properties of some different steels.

A summary of the properties of some different steels

type of steeliron alloyed withpropertiestypical use low carbon steel about 0.25 per cent carbon easily shaped car body panels high carbon steel up to 2.5 per cent carbon hard cutting tools stainless steel chromium and nickel resistant to corrosion cutlery and sinks

Alloys

The properties of a metal are changed by including other elements, such as carbon. A mixture of two or more elements, where at least one element is a metal, is called an alloy. Alloys contain atoms of different sizes, which distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.

atoms of differing sizes create an irregular arrangement (http://www.bbc.co.uk/schools/gcsebitesize/science/images/130_alloy.gif)

It is more difficult for layers of atoms to slide over each other in alloys

Copper, gold and aluminium are too soft for many uses. They are mixed with other metals to make them harder for everyday use. For example:

  • Brass, used in electrical fittings, is 70 per cent copper and 30 per cent zinc.
  • 18 carat gold, used in jewellery, is 75 per cent gold and 25 per cent copper and other metals.
  • Duralumin, used in aircraft manufacture, is 96 per cent aluminium and 4 per cent copper and other metals.

Smart alloys can return to their original shape after being bent. They are useful for spectacle frames and dental braces.

The transition metals

You need to know where to find the transition metals in the periodic table. The transition metals are found in the large block between Groups 2 and 3 in the periodic table. Most metals are placed here, including iron, titanium, copper and nickel.

periodic table showing the transition metals, including manganese (Mn), iron (Fe), nickel (Ni), copper (Cu) zinc (Zn), silver (Ag), platinum (Pt), gold (Au) and mercury (Hg) (http://www.bbc.co.uk/schools/gcsebitesize/science/images/6_the_transition_metals.gif)

The transition metals

Common properties

The transition metals have these properties in common:

  • They are metals.
  • They form coloured compounds.
  • They are good conductors of heat and electricity.
  • They can be hammered or bent into shape easily.
  • They are less reactive than alkali metals such as sodium, they have higher melting points - but mercury is a liquid at room temperature -and they are hard and tough.
  • They have high densities [densities: Density is the ratio of mass to volume. It is usually measured in grams per cubic centimetre or grams per cubic decimetre. ].

Copper

Copper is a transition metal. It is soft, easily bent and it is a good conductor of electricity. This makes copper useful for electrical wiring. Copper does not react with water, which makes it useful for plumbing.

Copper is purified by electrolysis. Electricity is passed through solutions containing copper compounds, such as copper sulfate - sometimes spelt sulphate. Pure copper forms on the negative electrode. The animation shows how this works, but note that you do not need to know the details of the extraction process for your examination.

Problems

We are running out of ores rich in copper. Research is being carried out to find new ways to extract copper from the remaining ores, without harming the environment too much. This research is very important, as traditional mining produces huge open-cast mines, and the remaining ores are low-grade, which means that they contain relatively little copper and produce a lot of waste rock.

Watch

You may wish to view this BBC News item from 2005 about a huge copper mine in Chile, South America.

Aluminium and titanium

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/aluminium_1.jpg)

Block of aluminium metal - image does not show the transparent oxide layer

Aluminium and titanium are two metals with a low density. This means that they are lightweight for their size. They also have a very thin layer of their oxides on the surface, which stops air and water getting to the metal, so aluminium and titanium resist corrosion. These properties make the two metals very useful.

Aluminium is used for aircraft, trains, overhead power cables, saucepans and cooking foil. Titanium is used for fighter aircraft, artificial hip joints and pipes in nuclear power stations.

Extraction

Unlike iron, aluminium and titanium cannot be extracted from their oxides by reduction with carbon:

  • Aluminium is more reactive than carbon, so the reaction does not work.
  • Titanium forms titanium carbide with carbon, which makes the metal brittle.

Aluminium extraction is expensive because the process needs a lot of electrical energy. Titanium extraction is expensive because the process involves several stages and a lot of energy. This especially limits the uses of titanium.

Recycling

Aluminium is extensively recycled because less energy is needed to produce recycled aluminium than to extract aluminium from its ore. Recycling preserves limited resources and requires less energy, so it causes less damage to the environment.

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Rahul

make more simple and easy to read

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