Chemistry 2a AQA

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Atomic Number and Mass Number

These two numbers tell you how many of each kind of particle an atom has.

The Mass Number - Total number of protons and neutrons.

The Atomic Number - Number of protons.

1) The atomic number tells you how many protons there are.

2) Atoms of the same element all have the same number of protons - so atoms of different elements will have different numbers of protons.

3) To get the number of neutrons,just subtract the atomic number from the mass number. Electrons aren't counted in the mass number because their relative mass is very small.

Particle / Mass

Proton = 1, Neutron = 1, Electron = Very Small

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Compounds are formed when atoms of two or more elements are chemically combined together.

For example, carbon dioxide is a compound formed from a chemical reaction between carbon and oxygen.

It is difficult to seperate the two original elements out again.

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Isotopes are: different atomic forms of the same element, which have the same number of protons but a different number of neutrons.

Isotopes must have the same atomic number but different mass number.

If they had different atomic numbers, they'd be different elements altogether.

Carbon-12 and Carbon-14 are a very popular pair of isotopes.

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Ionic Bonding - Transferring Electrons

In ionic bonding, atoms lose or gain electrons to form charged particles (called ions) which are then strongly attracted to one another (because of the attraction of opposite charges, + and -).

Atoms such as sodium, potassium, calcium etc. have just one or two electrons in their outer shell (highest energy level). They're pretty keen to get shot of them, because then they'll only have full shells left, which is how they like it. (They try to have the same electronic structure as a noble gas.) This leaves the atom as an ion instead. Ions tend to stick with the first passing ion with an opposite charge.

Elements in Group 6 and Group 7, such as oxygen and chlorine, have outer shells which are nearly full. They are keen to gain that extra one of two electrons to fill the shell up. When they do they become ions and latch onto the atom (ion) that gave up the electron a moment earlier.

The reaction of sodium and chlorine is a classic case:

The sodium atom gives up its outer electron and becomes an Na+ ion. The chlorine atom has picked up the spare electron and becomes a Cl- ion.

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Ionic Compounds

  • Ionic compounds always have giant ionic latices.
  • The ions form a closely packed regular latice arrangement.
  • There are very strong electrostatic forces of attraction between opposiely charged ions, in all directions.
  • A single crystal of sodium chloride (salt) is one giant ionic latice, which is why salt crystals tend to be cuboid in shape. The Na+ and Cl- ions are held together in a regular lattice.
  • They all have high melting points and high boiling points due to the strong attraction between the ions. It takes a large amount of energy to overcome this attraction. When ionic compounds melt, the ions are free to move and they'll carry electric current.
  • They do dissolve easily in water though. The ions seperate and are all free to move in the solution, so they'll carry electric current.
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  • Atoms that have lost or gained an electron (or electrons) are ions.
  • Ions have the electronic structure of a noble gas.
  • The elements that most readily form ions are those in Groups 1, 2, 6 & 7.
  • Group 1 & 2 elements are metals and they lose elctrons to form positive ions.
  • Group 6 & 7 elements are non-metals.They gain electrons to form negative ions.
  • The charge on the positive ions is the same as the group number of the element.
  • Any of the positive ions can combine with any of the negative ions to form an ionic compound.
  • Only elements at the opposite sides of the periodic table will form ionic compounds, e.g. Na and Cl, where one of them becomes a positive ion and one of them becomes a negative ion.


The + and - charges we talk about, e.g. Na+ for sodium, just tells us what type of ion the atom will form in a chemical rection. In sodium metal there are only neutral sodium atoms, Na. The Na+ ions will only appear if the sodium metal reacts with something like water or chlorine.

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Ionic Compound Formulas

  • Ionic compounds are made up of a positively charged part and a negatively charged part.
  • The overall charge of any coumpound is zero.
  • So all the negative charges in the compound must balance all the positive charges.
  • You can use the charges on the individual ions present to work out the formulas for the ionic compounds.

Sodium chloride contains Na+ (+1) and Cl- (-1) ions.

(+1) + (-1) = 0

The charges are balance with one of each ion, so the formula for sodium chloride = NaCl.

Magnesium chloride contains Mg2+ (+2) and Cl- (-1) ions.

Because a chloride ion only has a 1- charge we will need two of them to balance out the 2+ charge of a magnesium ion. This gives us the formula MgCl2.

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Electronic Structure of Ions

A useful way of representing ions is by drawing out their electronic structure.

Use a big square bracket and a + or a - to show the charge.


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Covalent Bonds

strong covalent bonds: atoms share electrons with each other so they've got full outer shells.

  • Sometimes atoms prefer to make covalent bonds by sharing electrons with other atoms.
  • They only share elctrons in their outer shells (highest energy levels).
  • This way both atoms feels that they have a full outer shell, that makes them happy. Having a full outer shell gives them the electronic structure of a noble gas.
  • Each covalent bond provides one extra shared electron for each atom.
  • So, a covalent bond is a shared pair of electrons.

There are 7 important examples to learn.

In a dot and cross diagram you only have to draw the outer shell of electrons.

Covalent bonds can be shown by a line.

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Hydrogen & Chlorine

Hydrogen, H2

Hydrogen atoms have just one electron. They only need one more to complete the first shell...

Chlorine, Cl2

...chlorine atoms also need only one more electron... they often form single covalent bonds to achieve this.

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Methane, CH4

Carbon has four outer electrons which is half a full shell. So it forms four covalent bonds to make up its outer shell.

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Hydrogen Chloride

Hydrogen Chloride, HCL

This is very similar to H2 and Cl2. Again, both atoms only need one more electron to complete their outer shells.

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Ammonia, NH3

Nitrogen has five outer shell electronsso it needs to form three covalent bonds to make up the extra three electrons needed.

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Water, H2O

Oxygen atoms have six outer electrons. They sometimes form ionic bonds by taking two electrons to complete their outer shell. However they'll also cheerfully form covalent bonds and share two electrons instead. 

In water molecules, the oxygen shares electrons witht the two H atoms.

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Oxygen, O2

In oxygen gas, oxygen shares two electrons with another oxygen atom to get a full outer shell. A double covalent bond is formed.

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Simple Molecular Substances

Substances with covalent bonds (electron sharing) can either be simple molecules or giant structures.

  • The atoms form very strong covalent bonds to form small molecules of several atoms.
  • By contrast, these forcees of attraction between these molecules are very weak.
  • The result of these feeble intermolecular forces is that the melting and boiling points are very low, because the molecules are easily parted from each other. Its the intermolecular forces that get broken when simple molecular substances melt or boil - not the much stronger covalent bonds.
  • Most molecular substances are gases or liquids at room temperature, but they can be solids.
  • Molecular substances don't conduct electricity - there are no ions so there's no electrical charge.

The very weak intermolecular forces can be represented as a dotted line on a diagram between the molecules.

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Giant Covalent

Substances with covalent bonds (electron sharing) can either be simple molecules or giant structures. Giant covalent structures are macromolecules.

  • These are similar to giant ionic structures (lattices) except that there are no charged ions.
  • All the atoms are bonded together by strong covalent bonds.
  • This means that they have very high melting and boiling points.
  • They dont conduct electricity - not even when molten (except graphite).
  • The main examples are diamond and graphite, which are both made only from carbon atoms and silicon dioxide (silica).
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Each carbon atom forms four covalent bonds in a very rigid giant covalent structure. This structure makes diamond the hardest natural substance, so its used for drill tips. And it's pretty and sparkly too.

Silicon Dioxide (Silica):

Sometimes called silica, this is what sand is made of. Each grain of sand is one giant structure of silicon and oxygen.


Each carbon atom only forms three covalent bonds. This creates layers which a free to slide over each other, like a pack of cards - so graphite is soft and slippery.The layers are held together so loosely that they can be rubbed off onto paper - thats how pencil works. This is because there are weak intermolecular forces between the layers. Graphite is the only non-metal which is a good conductor of heat and electricity. Each carbon atom has one delocalised (free) electron and it's these free electrons that conduct heat and electricity.

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Metal Properties

  • Metals also consit of a giant structure.
  • Metallic bonds involve the all-important 'free electrons' which produce all the properties of metals. These delocalised (free) electronscome from the outer shell of every metal atom in the structure.
  • These electrons are free to move through the whole structure and so metals are good conductors of heat and electricity.
  • These electrons also hold the atoms together in a regular structure. There are strong forces of electrostatic attraction between the positive metal ions and the negative electrons.
  • They also allow the layers of atoms to slide over each other, allowing metals to be bent and shaped.
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  • Alloys are harder than pure metals.
  • Pure metals often aren't quite right for certain jobs. S o scientists mix two or more metals together - creating an alloy with the properties they want.
  • Different elements have different sized atoms. So when another metal is mixed with a pure metal, the new metal atoms will distort the layers of metal atoms, making it more difficult for them to slide over each other.
  • So alloys are harder.
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Identifying the Structure of a Substance

You should be able to identify most substances by the way they behave as either:

giant ionic, simple molecular, giant covalent or giant metallic

They may test you in an exam by describing the pysical properties of a substance and asking you to decide which type of structure it has.

You must also be able to explain why.

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Smart Materials

  • Smart materials behave differently depending on the conditions e.g. temperature.
  • A good example is nitinol - a "shape memory alloy". It's an alloy (about half nickel, half titanium) but when its cool you can bend it and twist it like rubber. Ben it too far, though, it stays bent. If you heat it abover a certain temperature, it goes back to its "remembered shape".
  • Its really handy for glasses frames. If you accidentally bend them, you can just pop them in a bowl of hot water and they'll jump back into shape.
  • Nitinol is also used for dental braces. In the mouth it warms and tries to return to a 'remembered' shape, and so it gently pulls the teeth with it.
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  • Really tiny particles, 1-100 nanometres across, are called 'nanoparticles' (1nm = 0.000000001m).
  • Nanoparticles contain roughly a few hundred atoms.
  • Nanoparticles include fullerenes. These are molecules of carbon, shaped like hollow balls or closed tubes. These carbon are arranged into hexagonal rings. Different fullerenes contain different numbers of carbon atoms.
  • A nanoparticle has very different properties from the 'bulk' chemical that its made from - e.g. fullerenes have different properties from big lumps of carbon.

1) Fullerenes can be joined together to form nanotubes - tenny tiny hollow carbon tubes, a few nanometres across.

2) All those covalent bonds make carbon nanotubes very strong. They can be used to reinforce graphite in tennis rackets.

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Uses of Nanoparticles

Using nanoparticles is known as nanoscience. Many new uses of nanoparticles are being developed:

  • They have a huge surface area to volume ratio, so they could help make new industrial catalysts.
  • You can use nanoparticles to make sensors to detect one type of molecule and nothing else. These highly specific sensors are already being used to test water purity.
  • Nanotubes can be used to make stronger, lighter building materials.
  • New cosmetics e.g. sun tan cream and deoderant, have been made using nanoparticles. The small particles do their job but dont leave white marks on the skin.
  • Nonmedicine is a hot topic. This is the idea that tiny fullerenes are absorbed more easily by the body than most particles. This means they could deliver drugs right into the cells where they're needed.
  • New lubricant coatings are being developed using fullerenes. These coatings reduce friction a bit like ball bearings and could be used in all sorts of places from artificial joints to gears.
  • Nanotubes conduct electricity, so they can be used in tiny electric circuits for computer chips.
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Properties of Plastics

Srong covalent bonds hold the atoms together in long chains. But it's the bonds between the different molecule chains that determine the properties of plastics.

Weak Forces: Individual tangled chains of polymers, held together by weak intermolecular forces, are free to slide over each other.

Thermosoftening polymers don't have cross-linking between chains. The forces between the chains are really easy to overcome so it's dead easy to melt the plastic. When it cools, the polymer hardens into a new shape. You can melt these plastics and remould them as many times as you like.

Strong Forces: Some plastics have stronger intermolecular forces between the polymer chains, called cross-links, that hold the chains firmly together.

Thermosetting Polymers have cross-links. These hold the chains together in a solid structure. The polymer doesn't soften when its heated. Thermosetting polymers are the tough guys of the plastic world. They are strong, hard and rigid.

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Affecting the Properties of Polymers

1) The starting materials and reaction conditions will both affect the properties of a polymer.

2) Two types of polythene can be made using different conditions:

  • Low Density (LD) polythene is made by heating ethene to about 200 degrees C under high pressure. It's flexible and is used for bags and bottles.
  • High Densitiy (HD) polythene is made at a lower temperature and pressure (with a catalyst). It's more rigid and is used for water tanks and drainpipes.
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The Use of Plastic

The use of a plastic depends on its properties.

A disposable cup for hot drinks:

Low cost (disposable) and a high melting point (for hot drinks).


Flexible (essential for clothing) and able to be made into fibres (clothing is usually woven).

A measuring cylinder:

Transparent and resistant to chemicals (you need to be able to see the liquid inside and the liquid and meassuring cylinder musn't react with each other).

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Relative Atomic Mass

Relative Atomic Mass, Ar

This is just a way of saying how heavy different atoms are compared with the mass of an atom of carbon-12. So carbon-12 has an Ar of exactly 12. It turnes out that the relative atomic mass Ar is usually just the same as the mass number of the element. In the periodictable, the elements all have two numbers. The small one is the atomic number (how many protons it has). But the bigger one is the mass number or the relative atomic mass.

Helium has Ar = 4, Carbon has Ar = 12, Chlorine has Ar = 35.5

When an element has more than one stable isotope, the relative atomic mass is an average value of all the different isoptopes (taking into account how much there is of each isotope).

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Relative Formula Mass

Relative Formula Mass, Mr

If you have a compound like MgCl2, then it hasa relative formula mass, Mr, which is just all the relative atomic masses added together.

For MgCl2 it would be:

24 + (35.5 x 2) = 95

(The relative atomic mass of chlorine is multiplied by 2 because there are two chlorine atoms.)

You can easily get Ar for any element from the periodic table, but in a lot of questions they give you them anyway.

Relative Formula Mass is simply "add up all the relative atomic masses".

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"One Mole"

"One Mole" of a substance is equal to it Mr in grams.

The relative formula mass (Ar or Mr) of a substance in grams is known as one mole of that substance.


Iron has an Ar of 56. So one mole of iron weighs exactly 56g.

Nitrogen gas, N2 gas an Mr of 28 (2x14). So one mole of N2 weighs exactly 28g.

NUMBER OF MOLES = MASS IN G (of element or compound) / Mr (of element or compound)

Example: How many moles are there in 42g of carbon?

Answer: No. of moles = Mass (g) / Mr = 42/12 = 3.5 moles

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% Mass

Calculating % mass of an element in a compound


[Ar x No. of atoms (of that element) / Mr (of whole compound)] x 100


Find the percentage mass of sodium in sodium carbonate, Na2CO3.


Ar of sodium = 23, Ar of carbon = 12, Ar of oxygen = 16

Mr of Na2CO3 = (2 x 23) + 12 + (3 x 16) = 106

Now use the formula:(23 x 2) / 106 ] x 100 = 43.4%

Sodium makes up 43.4% of the mass of sodium carbonate.

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Empirical Formula

Finding the empircal formula from masses or percentages.

1) List all the elements in the compound (there's usually only two or three!)

2) Underneath them, write their experimental masses or percentages

3) Divide each mass or percentage by the Ar for that particular element

4) Turn the numbers you get into a nice simple ratio by multiplying and/or dividing them by well chosen numbers

5) Get the ratio in its simplest form, and that tells you the empirical formula of the compound

You need to realise (for the exam) that this empirical method (i.e. based on experiment) is the only way of finding out the formula of a coumpound. Rust is iron oxide, sure, but is it FeO, or Fe2O3? Only an experiment to determine the empirical formula will tell you for certain.

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Empirical Formula Example


Find the empirical formula of the iron oxide produced when 44.8 g of iron react with 19.2 g of oxygen. (Ar of iron = 56, Ar for oxygen = 16)


1) List the two elements:                                   Fe                              O

2) Write in their experimental masses:              44.8                           19.2

3) Divide by the Ar for each element:         44.8 / 56 = 0.8             19.2 /16 = 1.2

4) Multiple up... then simplify:                          8 = 2                         12 = 3

5) So the simplest formula is 2 atoms of Fe to 3 atoms of O, i.e. Fe2O3

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Calculating Masses in Reactions

Three IMPORTANT Steps:

1) Write out the balanced equation

2) Work out the Mr - just for the two bits you want

3) Apply the rule: Divide to get one, then multiply to get all.

(but you have to apply this first to the substance they give you information about, and then the other one!)

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Calculating Masses in Reactions Example

What mass of magnesium oxide is produced when 60 g of magnesium is burned in air??

1) Write out the balanced equation: 2Mg + O2 --> 2MgO

2) Work out the relative formula masses: (Don't do the oxygen - we don't need it!)

2 x 24 --> 2 x (24 + 16)              48 --> 80

3) Apply the rule: Divide to get one, then multiply to get all.

The two numbers, 48 and 80, tell us 48 g of Mg react to give 80 g of MgO.

First you need to divide by 48 to get 1 g of magnesium and then you need to times by 60 to get 60 g of magnesium.

Then you must work out the numbers on the other side by doing the same two steps.

80 g / 48 = 1.67 g        then       1.67 g x 60 = 100 g of MgO

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Percentage Yield

Percentage Yield tells you about the overall success of an experiment. It compares what you should get (predicted yield) with what you get in practice (actual yield).

The amount of product you get is known as the yield. The more reactants you start with, the higher the actual yield will be. But the percentage yield doesn't depend on the amount of reactants you started with - it's a percentage.

1) The predicted yield of a reaction can be calculated from the balanced reaction equation.

2) Percentage yield is given by the formula: [actual yield (g) / predicted yield (g)x 100

3) Percentage yield is always somewhere between 0 and 100%.

4) A 100% percentage yield means that you got all the producted you expected to get.

5) A 0% percentage yield means that no reactants converted into product i.e. no product at all was made.

The predicted yield is sometimes called the theoretical yield.

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Yields Are ALWAYS Less Than 100%

Even though no atoms are gained or lost in reactions, in real life, you never get a 100% percentage yield. Some product or reactant always gets lost along the way - and that goes for big industrial proccesses as well as school lab experiments. There are several reasons for this:

1) The reaction is reversible: the products of the reaction can themeselves react to produce the original products. A + B = C + D

for example: ammonium chloride = ammonia + hydrogen chloride

This means that the reactants will never be completely converted to products because the reaction goes both way. Some on the products are always reacting together to change back to the original reactants. This will mean a lower yield.

2) When you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or bit of solid. So, some of the product may be lost when it's seperated from the reaction mixture.

3) Things dont always go exactly to plan. Sometimes there can be other unexpected reactions happenng which use up the reactants. This means there's not as much reactant to make the product you want.

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Sustainable Development

Thinking about product yield is important for sustainable development.

Sustainable development is about making sure that we don't use resources faster than they get replaced - there needs to be enough for future generations too.

So, for example, using as little energy as possible to create the highest product yield possible means that resources are saved.

A low yield means wasted chemicals - not very sustainable.

A high percentage yield means there's not much waste - which is good for preserving resources, and keeping production costs down. 

If a reaction's going to be worth doing commercially, it generally has to have a high percentage yield or recyclable reactants.

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Paper Chromatography

Artificial colours can be seperated using paper chromatography. A food colouring may contain one dye or it might be a mixture of dyes. Here's how you can tell:

1) Extract the colour from a food sample by placing it in a small cup with a few drops of solvent (can be water, ethanol, salt water etc).

2) Put spots of the coloured solution on a pencil base line on filter paper. (Don't use pen because it might dissolve in the solvent and confuse everything.)

3) Roll up the sheet and put it in a beaker with some solvent - but keep the baseline above the level of the solvent.

4) The solvent seeps up the paper, taking the dyes with it. Different dyes form spots in different places.

5) Watch out though - a chromatogram with four spots means at least four dyes, not exactly four dyes. There could be five dyes, with two of them making a spot in the same place. It can't be three dyes though, because one dye can't split into two spots.

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Machines can also analyse unknown substances.

You can identify elements and compounds using instrumental methods - this just means machines.

Advantages of using machines:

  • Very sensitive - can detect even the tiniest amounts of substances.
  • Very fast and tests can be automated.
  • Very accurate
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Gas Chromatography

Gas chromatography can seperate out a mixture of compounds and help you identify the substances present.

1) A gas is used to carry substances through a column packed with solid material.

2) The substances travel through the tube at different speeds, so they're seperated.

3) The time they take to reach the detector is called the retention time. It can be used to identify the substances.

4) The recorder draws a gas chromatograph. The number of peaks shows the number of different compounds in the sample.

5) The position of the peaks shows the retention time of each substance.

6) The gas chromatography column can also be linked to a mass spectrometer. This process is known as GC-MS and can identify the substances leaving the column very accurately.

7) You can work out the relative molecular mass of each of the substances from the graph it draws. You just read of the molecular ion peak.

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