GCSE CHEMISTRY UNIT 2AAQA

Atoms and Isotopes

ISOTOPESIsotopes are different atomic forms of the same element, which have the SAME number of PROTONS AND ELECTRONS, but a DIFFERENT number of NEUTRONS.

Number of Neutrons = Atomic Mass – Atomic Number

Can you see that they are all still carbon? This is because it is the electrons which determine whether or not something is a different element.

Ionic BondingIONSElectrically charged particles which are formed when an atom loses or gains electrons. Metals form positively charged (+) ions. Non-metals form negatively charged (-) ions. Hydrogen forms a positive ion (H+). Atoms will want to either lose 1 or 2 electrons to gain a full outer shell OR gain

1 or 2 electrons.

In the exam, you may need to be able to show the electronic structure of some ions.

For example, this lithium atom has one electron in its outer shell. Therefore, it will lose this electron to get a full outer shell.

When this electron is lost, the positive lithium ion is formed.

You put charge here (+ or -)

Draw these brackets around the ion, to distinguish it from the atom.

YOU MAY WANT TO WATCH THIS VIDEO FIRST!I will come to covalent bonding later!

Ionic BondingIONIC COMPOUNDSWhen metals react with non-metals, electrons are transferred from the metal atoms (which become +) to the non-metal atoms (which become -), forming ions. The result is called an ionic compound.There is a strong electrostatic force of attraction between the oppositely charged ions, forming an ionic bond. Remember the rule; OPPOSITES ATTRACT!

Sodium Chloride (NaCl) is formed when sodium and chlorine react together. It contains oppositely charged ions held together by strong electrostatic forces of attraction – the ionic bonds. These ionic bonds then form into what is called a giant ionic lattice (pictured right).

Ionic compounds all:• Have high melting and boiling points due to strong attraction between

ions. It takes lots of energy to break this attraction.• Have free electrons when melted, so can therefore carry electric

current when melted.• Dissolve easily in water so the ions separate out, meaning it can then

also carry electric current when dissolved in water.

Ionic BondingFORMULAEThe formula of ionic compounds can be worked out if the ions within it are known. First, check the charge on the ions see whether the charges

cancel each other out. If they do, for example with potassium (K+) and chloride (Cl-), you only need to write one lot of each ion, KCl. It is the same if ions have 2+ and 2- charges.

However, if the ions have different numbers next to the charges, you will need to change the number of ions within the compound so that the charges cancel each other out.

For example, the compound formed from calcium (Ca2+) and iodine (I-) will contain two I- ions to every one Ca2+ ion, so that it is neutral.

The finished formula is CaI2.

Covalent Bonding Covalent bonding is when two non-metal atoms share electrons. They only share electrons in their outer shell. This completes both atom’s outer shells. Each covalent bond provides one extra shared electron for

each atom. So, covalent bond is one shared pair of electrons. Covalent bonds are strong, so need a lot of energy to break

them. Check page 43 and 44 in CGP Additional Science Revision for

more info.

Covalent BondingSIMPLE MOLECULAR SUBSTANCES Made up of molecules where the atoms are joined with strong

covalent bonds but the molecules are only held together by weak intermolecular forces.

They do not conduct electricity. They have low melting and boiling points because the forces

are broken easily. It is the forces between the molecules that get broken when a substance melts or boils NOT the bonds between atoms.

Giant Covalent Structures All atoms bonded by strong covalent bonds. High melting and boiling points. Most don’t conduct electricity (graphite is the exception.)

Metallic StructuresMETALS Metals are malleable. This is because they are made up

of layers of atoms which can slide over each other. Metals make up a giant metallic structure. This is

because the positive metal ions and negative electrons form strong metallic bonds between them, because OPPOSITES ATTRACT!

Metals have free electrons which can carry electric current and heat throughout the structure.

Alloys Alloys contain more than one metal. Alloys are harder than pure metals as the alloy contains atoms

of different sizes from the various different metals. These different sizes of atoms distort the regular arrangement of

atoms in metals. This makes it more difficult for the layers to slide over each other

as they do in pure metals.

• Shape memory alloys can return to their original shape after being bent or twisted.

• Nitinol is a shape memory alloy made from nickel and titanium. It is used in dental braces.

Nanoparticles Nanoparticles are MUCH smaller than ordinary-sized particles. They have a large surface area compared to their volume so they

can react quickly. This means that they can be very effective as catalysts. They often have different properties to what the ordinary-sized

particles would have in the same substance. Could be used to make stronger and lighter building materials. Nanoparticles could be used in medicine, as they are more easily

absorbed by the body. Nanoparticles include fullerenes. These are molecules of carbon,

which are shaped like hollow balls (buckyballs) or closed tubes.

Polymers

Relative Atomic/Formula Mass The symbol for relative atomic mass is Ar. You will ALWAYS be

given this value in the exam. The symbol for relative formula mass is Mr. WORKING OUT Mr

To work out the relative formula mass of a compound, all you need to do is add the relative atomic masses of the elements within the substance, together.

For example:

Moles

CalculationsPERCENTAGE MASS

CalculationsEMPIRICAL FORMULA

CalculationsCALCULATING MASSES IN REACTIONSWATCH VIDEO!

CalculationsPERCENTAGE YIELDThe percentage yield compares what you calculate you should get (predicted yield), with what you actually got (actual yield). Percentage yield = (actual yield ÷ predicted yield) x 100.Yields are always less than 100% either because: The reaction is reversible. This means that the reactions will

never be complete because the reaction is always going both ways.

You lose liquid or solid through filtration, as some is nearly always left behind in apparatus.

There could be a weighing error. When weighing out the products to be used in the reaction, there has been an error. This means the actual yield will differ from the predicted yield.

Paper Chromatography Used to analyse coloured substances, such as artificial

food colouring. Paper chromatography works because some of the

colours are better at dissolving in the liquid than they are at bonding with the paper, so they will travel further up the paper.

Instrumental methods of analysis Fast Accurate (reliable). Sensitive (can detect a small amount of a substance in a small

amount of sample).

GAS CHROMATOGRAPHY