AS OCR Salters Chemistry - Elements of Life

• Made up of protons, neutrons and electrons.
• Protons have relative mass of 1 and +1 charge.
• Neutrons have relative mass of 1 and no charge.
• Electrons have a negligible mass (ignore it) and a -1 charge.
• Ions have different numbers of protons and electrons.
• Isotopes - same element but a different number of neutrons.

In the 1800's John Dalton described atoms as solid spheres, and that different spheres made up different elements.

In 1897 JJ Thomson concluded that atoms weren't solid and indivisible. He discovered that atoms must contain smaller, negatively charged particles. He called these corpsucles (but we now call them electrons). His model was called the plum pudding model which was a positively charged sphere with negative electrons embedded in it.

In 1909 Rutherford conducted the gold foil experiment, it involved firing alpha particles at a very thin sheet of gold. According the plum pudding model most of the alpha particles should have been deflected but Rutherford found that most of them went straight through. This led to the nuclear model, a positive nucleus with electrons around it.

The Bohr model was another improvement as it showed that the electrons are in fixed shells with fixed energies, and the radiation has a fixed frequency.

RAM - average mass of an atom of an element.

RIM - mass of an atom of an isotope of an element.

RFM/Mr - average mass of a molecule.

Relative masses measured using a mass spectrometer;

• Sample turned into a gas.
• Positive ions formed by bombardment with high energy electrons.
• Ions accelerated by electric field.
• Detection time measured - lighter ions are quicker, heavier ions are slower.
• Mass spectrum produced.

Radioactive Decay - breaking down of atoms and the emission of radiation.

Alpha Decay - Helium nuclei - stopped by paper - strong ioniser - slight deflection in electric fields. STRONGLY +VE, KNOCK OUT e- QUICKLY.

Beta Decay - electrons - stopped by aluminium sheets - moderate ioniser - large deflection in electric fields. LOWER CHARGES, HIGHER SPEEDS.

Gamma Rays - short EM waves - thick lead sheets stop them - weak ioniser - not deflected in electric field.

Half Life - time taken for half the atoms is a sample to decay, has a constant value for any particular isotope.

Tracers - easy to detect radiation, very long half life is dangerous - exposed to radiation for a long time, very short half life is inconvenient - it needs time to travel around body.

Finding out how old things are;

Radioactive Decay - breaking down of atoms and the emission of radiation.

Alpha Decay - Helium nuclei - stopped by paper - strong ioniser - slight deflection in electric fields. STRONGLY +VE, KNOCK OUT e- QUICKLY.

Beta Decay - electrons - stopped by aluminium sheets - moderate ioniser - large deflection in electric fields. LOWER CHARGES, HIGHER SPEEDS.

Gamma Rays - short EM waves - thick lead sheets stop them - weak ioniser - not deflected in electric field.

Half Life - time taken for half the atoms is a sample to decay, has a constant value for any particular isotope.

Tracers - easy to detect radiation, very long half life is dangerous - exposed to radiation for a long time, very short half life is inconvenient - it needs time to travel around body.

Finding out how old things are using radioactive isotopes;

All living things contain the same % of carbon-14, when organism dies the carbon-14 starts to decay, so the less carbon-14 present in something, the older it must be.

You can then work out an approximate age of the animal or plant.

Nuclear Fusion - fusing nuclei to create bigger nuclei.

Combination of small nuclei under high temperature and pressure (in stars) creates larger nuclei, eventually bigger and bigger nuclei are created.

Number of moles = number of particles you have/number of particles in a mole.

Number of particles in a mole = Avogadro's Constant = 6.02 x 10^23

Number of moles = mass of substance/molar mass

Electrons are in fixed energy levels, the first shell holds 2 electrons, the second shell holds 8 electrons, the third 18 electrons and the fourth 32 electrons.

However the 3rd and 4th shell overlap slightly so before the 3rd shell has been filled the 4th shell gains two electrons.

Atoms in their ground state have electrons at their lowest possible energy levels.

Electrons can gain energy from surroundings and jump to higher energy levels because they are excited.

They emit energy when they fall back down.

Absorption spectra - electrons only absorb certain frequencies of radiation. The diagrams show a coloured background with black lines (imagine the colour being absorbed).

Emission spectra - emit this energy again when they fall back down, the diagram is black with coloured lines, (imagine the colour being emitted again).

ΔE = hν

ΔE is the change in energy between two shells the units are Joules (J).

h is Planck's constant (6.63 x10^-34) the units are Joule seconds (Js).

ν is the Greek letter 'nu', it's the symbol for frequency, the units are Hertz (Hz).

On spectra the lines converge as the energy increases. If the electron arrow points down, the electron is emitting energy, if the electron arrow is pointing up the electron absorbs energy.

Ions stuck together by electrostatic attraction.

Ions formed when electrons are transferred from one atom to another.

Electrostatic attraction - holds together positive and negative ions, and its very strong - IONIC BONDING.

You will need to know dot-and-cross diagrams not forgetting the [square brackets] around the products and also the charge.

Negative charge if atom has taken an electron.

Positive charge if atom has donated an electron.

Ionic Compounds :-

• Conduct electricity when molten or dissolved, NOT SOLID. In liquid form the ions are free to move around and take their charges with them, in solid form they cannot.

• Have high melting points, it takes a lot of energy to overcome strong electrostatic forces.

Often dissolve in water, water is polar so charged parts of water pull away the oppositely charged parts on the compound.

When atoms share electrons with each other, so the all get full outer shells.

Linear molecules - 180^o bond angle O=C=O 

3 electron pairs on central atom - 120^o bond angles  

4 electron pairs on central atom - no lone pairs - 109.5^o  

one lone pair - 107^o

2 lone pairs/bent - 104.5^o

5 electron pairs on central atom - 120^o / 90^o  bond angles

6 electron pairs on central atom - 90^o bond angles