Atoms and atomic structure
Atoms consist of electrons surrounding a nucleus that contains protons and neutrons. Protons and neutrons have a relative mass of 1 and electrons have a negligible 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.
The word atom 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.
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. 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
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.
Molecules and atoms
Molecules of oxygen - the two Atoms are represented as
molecule of carbon dioxide. spheres and are colour-coded
carbon(green) and oxygen red).
The atoms are joined by double bonds
Atoms and elements
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.
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 structure of the atom
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. 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. You will never find a compound in the periodic table, because these consist of two or more different elements joined together by chemical bonds.
The modern periodic table
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.
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
- electrons are shared between two atoms.
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. 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 & H
Carbon dioxide units contain one carbon atom and t
The diagram shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide - its chemical formula should be written as CO2.
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. 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. This is the word equation for the reaction: 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.
Two atoms of copper react with two atoms of oxygen
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.
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. 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.
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.
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 or large hydrocarbons with many carbon atoms have high boiling points and are solids.
Structure of alkanes
Distillation is a process that can be used to separate a pure liquid from a mixture of liquids when the liquids have different boiling points. Distillation is commonly used to separate ethanol from water.
Step 2 - the ethanol evaporates first, cools, then
Step 3 - the water left evaporates, cools, then co
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 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.
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.
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
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
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.
Most hydrocarbon fuels naturally contain some sulfur compounds. When the fuel burns, the sulfur it contains is oxidised to sulfur dioxide.
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 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.
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.
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.
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.
The process of removing sulfur dioxide
Carbon dioxide from burning fuels causes global warming, a process capable of changing the world’s climate significantly.
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.
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.
- Sun’s rays enter the Earth’s atmosphere
- 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
- The Earth becomes hotter as a result
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.
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.
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.
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 calcium oxide + carbon dioxide
CaCO3 CaO + CO2
Other metal carbonates decompose in the same way. Here are the equations for the thermal decomposition of copper carbonate:
copper carbonate copper oxide + carbon dioxide
CuCO3 CuO + CO2
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.
The process of thermal decomposition
Quicklime and slaked lime
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
calcium oxide + carbon dioxide
CaCO3 CaO + CO2
This is a thermal decomposition reaction. 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 cause the water to boil. Using common names instead of chemical names, this is what happens:
limestone 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).
- 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 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.
Advantages and disadvantages of various building m
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.
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.
The Earth's crust contains metals and metal compounds 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, while a less-reactive metal such as iron may be extracted by reduction with carbon or carbon monoxide.
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
Steel and alloys
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. 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.
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.
It is more difficult for layers of atoms to slide
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. 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.
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.
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.
Aluminium and titanium
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. 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 and 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. 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.