Unit 1 - Atomic Structure and the Periodic Table

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Atoms

The nucleus is in the middle of the atom and contains protons and neutrons. The nucleus has a radius of  1 x 10  and a positive charge because of the protons.

Electrons orbit around the nucleus in electron shells and they have a negative charge. They are tiny meaning they virtually have no mass but they cover a lot of space. The volume of their orbits determines the size of an atom.

Atoms are neutral meaning they have no charge because the number of protons is equal to the number of electrons. The charge on the electrons is the same size as the charge on the protons but opposite so the charges cancel out.

In an ion, the number of protons doesn't equal the number of electrons. This causes the ion to have an overall charge. For example, an ion with a charge of 2-, has two more electons than protons.

The nuclear symbol of an atom tells you its atomic number and mass number. The atomic number tells you how many protons there are. The mass number tells you the total number of protons and neutrons in the atom. To get the total number of neutrons, just subtract the atomic number from the mass number.

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Elements

Elements consist of atoms with the same atomic number. Atoms can have different numbers of protons, neutrons and electrons. It's the number of protons in the nucleus that decides what type of atom it is. If a substance only contains atoms with the same number of protons it's called an element.

Atoms of each element can be represented by a one or two letter symbol e.g carbon is C.

Isotopes are different forms of the same element, which have the same number of protons but a different number of neutrons. So Isotopes have the same atomic number but different mass numbers. For example, carbon 12 and carbon 13.

Relative Atomic Mass = sum of (isotope abundance x isotope mass number)                                                                                                 sum of abundances of all the isotopes

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Compounds

A compuond is when two or more atoms are chemically bonded together. Making bonds involves giving away, taking or sharing electrons. It's usually difficult to seperate the original elements of a compuond as a chemical reation needs to have taken place. 

Compouns are often represented by formulas. The formulas are made up of elemental symbolsin the same porportions that the elements can be found in the compound. For example Carbon Dioxide, CO2,  is a compound formed from a chemical reaction between Carbon and Oxygen. It contains 1 Carbon atom and 2 Oxygen atoms.

  • Ammonia - NH3 
  • Water - H2O
  • Sodium Chloride - NaCl                                                     
  • Carbon Monoxide - CO                   
  • Hydrochloric Acid - HCl                                                                                                         
  • Calcium Chloride - CaCl2                 
  • Sodium Carbonate - Na2CO3       
  • Sulfuric Acid - H2SO4
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Mixtures

Mixtures are two or more elements / compounds that aren't chemcillay bonded which mean they can be easily be seperated. The properties of a mixture are just a mixture of the properties of the seperate parts, the chemical properties of a substance aren't affected by it being part of a mixture.

They can be physically sperated with methods such as filtration, crystillisation,  simple distillation, fractional distillation and chromatography. 

Air is a mixture of gases, mainly nitrogen, oxygen, carbon dioxide and argon which can all be seperated out easily. Crude oil is a mixture of different length hydrocarbon molecules.

For example, a mixture of  ion powder and sulfur powder will show the properties of both iron and sulfur. It will contain grey magnetic bits and brght yellow bits of sulfur.

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Chromatography

Chromatography is one method of seperating substances in a mixture. This technique can be used to seperate different dyes in an ink.

  • Using a pencil  (pencil marks are insoluble), draw a line near the bottom of a sheet of paper   
  • Add a spot of ink to the line and place the sheet of filter paper in the beaker of solvent e.g water.
  • The solvent used depends on what's being tested. Some compounds dissolve well in water, but sometimes other solvents, like ethanol, are needed. 
  • Make sure the ink isn't touching the solvent as you don't want it to dissolve into it.
  • Place a lid on top of the container to stop the solvent from evapourating. 
  • The solvent seeps up the paper, carrying the ink with it.
  • Each different dye in the ink will move up the paper at a different rate so the dyes will seperate out.
  • Each dye will form a spot in a different place - 1 spot per dye in the ink. 
  • If any of the dyes in the ink are insoluble in the solvent, they will stay on the baseline.
  • When the solvent has nearly reached the top of the paper, take the paper out of the beaker and leave it to dry.                                                                                                     
  • The end result is a pattern of spots called a chromatogram.
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Filtration and Evapouration

Filtration                                                                                                   

It can be used to sperate insoluble solids from liquids. It can also be used in purification. For example, solid impurities in the reaction mixture can be seperated out using filtration.

Evaporation

 It is a really quick way of seperating a soluble salt from a solution, but you can only use it if the salt doesn't decompose when it's heated.

  • Pour the solution into an evapourating basin
  • Slowly heat the solution. The solvent will evapourate and the solution will get more concentrated. Eventually, crystals will start to form.
  • Keep heating the evapourating dish until all you have left are dry crystals

 

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Crystillation

Crystillation is used for when salts that are soluble have dissolved in a solution and decompose when you heat them.

  • Pour the solution into an evapourating dish and gently heat the solution. Some of the solvent will evapourate and the solution will get more concentrated.
  • Once some of the solvent has evapourated or when you see crystals start to form (the point of crystillisation), remove the dish from the heat and leave the solution to cool.
  • The salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution.
  • Filter the crystals out the solution, and leave them in a warm place to dry. You could also use a drying oven or a desiccator.

 

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Rock Salt

Filtration and Crystillation can be used to seperate rock salt. Rock salt is simply a mixture of salt and sand. Salt and Sand are both compounds - but salt dissolves in water and sand doesn't. The vital difference in their physical properties gives a great way to seperate them.

  • Grind the mixture to make sure the salt crystals are small, so will dissolve easily.
  • Put the mixture in water and stir. The salt will dissolve, but the sand won't.
  • Filter the mixture. The grains of san won't fit through the tiny holes in the filter paper, so they collect on the paper instead. The salt passes through the filter paper as it's part of the solution.
  • Evapourate the water from the salt so that it forms dry crystals.
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Simple Distillation

Simple distillation is used to seperate out a liquid from a solution.

  • The solution is heated. The part of the solution which has the lowestboiling point evapourates first.
  • The vapour is then cooled, condenses and is collected.
  • The rest of the solution is left behind in the flask.

You can use simple distillation to get pure wter from sea water. The water evapourates and is condensed and collected. Eventually you'll end up with salt left in the botom of the flask.

The problem with simple distialltion is you can only use it to seperate out liquids with very different boiling points. If the temperature goes higher than the boiling point of the sunbstance with the higher boiling point, they will mix again.

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Fractional Distillation

Fractional Distillation is used to seperate a mixture of liquids with similar boiling points.

  • You put your mixture in a flask and place a fractionating column on top. Then you heat it up.
  • The different liquids will all have different boiling points so they will evaporate at different temperatures.
  • The liquid with the lowest boiling point evaporates first. When the temperature on the thermometer matches the boiling point of this liquid, it will reach the top of the column.
  • Liquids with the higher boiling points might also start to evaporate. But the column is cooler towards the top, so they will only get part of the way up before condensing and running back down towards the flash.
  • When the first liquid has been collected, you raise the temperature until the next one reaches the top.

 

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The History Of The Atom

At the start on the 19th century, John Dalton described atoms as solid spheres, and said that different spheres made up different elements.

In 1897, JJ Thomson concluded from his experiments that atoms weren't solid spheres. His measurements of charge and mass showed that an atom must contain even smaller, negatively charged  particles called electrons. The new theory was known as the 'plum pudding model'. The plum pudding model showed an atom as a ball of positive charge with electrons stuck in it.

In 1909, Ernest Rutherford and his student Ernest Rutherford conducted the famous alpha particle scattering experiments where they shot alpha particles through a thin sheet of gold. They expected the particles to pass straight through however more particles were deflected then expected which proved the plum pudding model wrong. He then came up with the nuclear model of the atom where there's a tiny, positively charged nucleus at the centre (where most of the mass is concentrated).

Neil Bohr's nuclear model of the atom suggested that all the electrons were contained in shells. Bohr proposed that electrons orbit the nucleus in fixed shells (same distance apart) and aren't anywhere inbetween. This theory was supported by many experiments and it helped to explain lots of other scientists' observations at the time.

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Electron Shells

Electron shell rules:

  • electron always occupy shells sometimes called energy levels
  • the lowest energy levels are always filled first - these are the ones closest to the nucleus
  • only a certain number of electrons are allowed in each shell - 1st shell is allowed 2, 2nd shell is allowed 8 and the 3rd shell is allowed another 8
  • atoms are much happier when they have a full electron shells like the noble gases in Group O
  • in most atoms, the outer shell is not full and this makes the atom want to react to fill it 
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Development Of The Periodic Table

In the early 1800s elements were arranged by atomic mass. Until quite recently, there were two obvious ways to categorise elements: their physical and chemical properties and their realtive atomic mass. 

In 1869, Dimitri Mendeleev overcame some of the problems of the early periodic tables by taking 50 known elements and arranging them into his Table of Elements - with various gaps for elements that haven't been discovered yet. Mendeleev put the elements mainly in order of atomic mass but did switch the order if the properties meant its should be changed.

The discovery of isotopes in the early 20th century confirmed that Mendeleev was correct to not place elements in a strict order of atomic mass but to also take account of their properties. Isotopes of the same elemnet have different atomic masses but have the same chemical properties so occupy the same position on the periodic table.

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The Modern Periodic Table

In the periodic table elements are laid out in order of increasing atomic number. Arranging the elemnets like this means there are repeating patterns in the properties of the elements.

Metals are placed on the left side of the periodic table and metals are found on the right side of the periodic table.

  • elements with similar properties form columns called groups
  • the group number tells you how many electrons there are on the outer shell for example, Group 7 have 7 electrons on the outer most shells
  • if you know the properties of one element, you can predict properties of other elements in that group 
  • you can also make predictions about trends in reactivity
  • the rows are called periods, each new period represents another full shell of electrons
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Metals

Most elements are metals which can form positive ions when they react and are found towards the bottom and to the left of the periodic table.

Metals to the left of the periodic table don't have many electrons to remove and metals towards the bottom of the table have outer electrons which are from the nuclues so feel a weaker attraction. Both these effect means that not much energy is needed to remove the electrons so it's feasible for the elements to react to form positive ions with a full outer shell.

All metals have metallic bondng which cause them to have similar basic properties. They are:

  • strong (hard to break) 
  • malleable
  • good at conducting heat and eletricity
  • have high melting and boiling points
  • dense

Transition metals can be found between Group 2 and 3. They are typical metals so will have similar properties but can ave one more ion and are often coloured so compounds that contained them are colouful.

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Non-Metals

Non-Metals are at the far right and top of the periodic table which don't generally form positive ions when they react.

Non-metals, forming positive ions is much more difficult because they have lots of electrons on their outer shell or the lectrons are closer to the nucleus so feel a stronger attraction. This means it is easier to gain electrons than lose so they often form negative ions.

As non-metals don't have metallic bonding, they don't tend to exhit the same properties as metals. They tend to be:

  • brittle
  • dull looking
  • aren't always solids are room temperature
  • don't generallly conduct heat or electricity
  • have a lower density

Non-metals form a variety of different structures so have a wide range of chemical properties.

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Group 1 Elements

The Group 1 Elements are reactive, soft metals. The alkali metals are lithium, sodium, potassium, rubidium, caesium and francium.

They all have one electron on the outer shell hence being group 1. The alkali metals are all soft and have low density. The trends as you go down Group 1 include:

  • increasing reactivity beacuse the outer electron is further away from the nucleus so the atrraction between them decreases
  • lower melting and boiling points
  • higher realtive atomic mass

Group 1 elements don't need much enegry to lose their one outer electron to form an outer shell, so they readily form 1+ ions. They only evere react to form ionic compounds which are generally white solida that dissolve in water to form colourless solutions.

When reacted with water Group 1 are vigorus and produce hydrogen gas and metal hydroxides. When reacted with Chlorine Group 1 are vigorus again but form whtie meatl chloride salts. When reacted with Oxygen Group 1 forms a metal oxide.

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Group 7 Elements

The Halogens are all non-metals with coloured vapours. They all exist as molecules which are pairs of atoms.

  • Fluorine is very reactive, poisonous yellow gas.
  • Chlorine is fairly reactive, poisonous dense green gas. 
  • Bromine is a dense, poisonous, red-brown volatile liquid.
  • Iodine is a dark grey crystalline solis or pupple vapour.

As you go down Group 7:

  • the elements become less reactive because the electrons are further away from the nucleus
  • have higher melting and boiling points
  • have higher relative atomic masses

Halogens can form molecular compounds and can form ionic bonds with metals. More reactive halogens will displace a less recative halogen, this is called a displacement reaction. For example, Chlorine can displace Bromine and Iodine from an aqueous solution of its salt.

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Group 0 Elements

The noble gases are all inert, colourless gases. The elements are helium, neon, argon, krypton, xenon and radon. They have eight electrions on the outer energy level which means they are energetically stable so don't need to lose or gain electrons. They don't recat with much.

They exixst as monatomic gases (single atoms not bonded to each other). All elements in Group O are colourless gases at room temperature. Also they are non-flammable meaning they won't set on fire.

There are patterns in the properties of noble gases:

  • the boiling points increases as you go down the group due to an increase in the electron number in each atom leading to greater intermolecular forces between them which need to be overcome
  • the realtive atomic masses increase as you go down the group
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