The AtomThe Atom

Particle Proton Neutron Electron

Symbol 1 p1

1n0

0 e-1

Location Nucleus Nucleus Electron cloud

Charge 1+ 0 1-

The Atomic Number = number of protons( = number of electrons in neutral atom)The Mass Number = Number of protons +

number of neutrons.Isotope = atoms with same atomic number but a

different mass – different number of neutrons.

RadioactivityRadioactivity

The stability of atom depends on how many protons and neutrons it has.

If the numbers of protons are approximately equal to the number of neutrons then the atom will be stable. Example – the lighter elements.

However – if the is a large difference between the number of protons and neutrons the atom is unstable.

As the atoms get heavier, the neutron:proton ratio increases. The neutrons help keep the nucleus from flying apart since the + protons repulse each other.

What is radioactivity?What is radioactivity?

The unstable nuclei start to release energy spontaneously – we call this decay.

The nuclei will keep doing this until the neutron:proton ratio lie in the band of stability.

Radioisotopes are isotopes that are radioactive.Radioactive decay is unlike normal chemical

reaction because it is the nucleus that changes and not the electrons.

The nature of RadiationThe nature of Radiation

When a nucleus undergoes decay it emits small radioactive particles.

There are 3 types: alpha (α), beta (β)and gamma (γ).

Radiation Alpha Beta GammaSymbol α β γMass 4 V light 0Charge 2+ 1- 0

Alpha and beta are particles.Gamma is electromagnetic radiation.Alpha 4 He 2+

2

Beta are high energy electrons o e –-1

Each type of radiation has unique properties.Alpha is the heaviest and least penetrative – it

travels for a very short distance and can be stopped by paper.

Beta – very light will travel almost 0.6 cm and be stopped by Aluminium foil.

Gamma is the most penetrative – it is stopped by lead or thick concrete.

Nuclear ReactionsNuclear Reactions

When a radioactive atom starts to decay the original nuclei will change.

We can balance the change by making sure the total mass on the LHS and RHS are the same and the total atomic number on the LHS and RHS are the same.

Nuclear EquationsNuclear Equations

Alpha decayWhen the original atom releases an alpha particle we

can figure out the name of the new atom by taking away the mass and atomic number of an alpha particle from the original atom.

ExamplePolonium with a mass of 210 emits an alpha particles.210 P —> 4 α 2+ + 206 Pb

84 2 82

Beta Decay90 Sr —> 0 e + 90 Y 38 -1 39

Gamma DecayGamma particles have no mass and charge –

there emission has no effect on the mass and atomic number of the radioisotope.

Making RadioisotopesMaking Radioisotopes

This can be done by bombarding the target nucleus with small particles: alpha, neutrons or protons.

ExampleNeutron bombardment238 U + 1 n —> 239 U 92 0 92

Alpha bombardment27 Al + 4 He 2+ —> 30 P + 1 n 13 2 15 0

Proton Capture14 N + 1P —> 15 O7 1 8

Half Life ( t ½)Half Life ( t ½)

This is the time taken for a radioisotope to to half it’s initial activity. The time taken for half the radioactive atoms to disintegrate.

The rate of decay depends on the mass of the isotope.However – the half life of a particular isotope will always

be the same regards of the mass of the sample size – 1 g of Uranium will have the same half life as 100g of Uranium.

Temperature and pressure and particle size etc have no effect on radioactive decay.

n = number of half lives ( t ½)After n half lives the fraction of the original sample left is ( 1/2)

n

The length of half life various from one radioisotope to the next – some are very short, others very long.

The half life of an atom will always be the same if it is in element form or in a compound.

This is because radioactive decay is nuclear – unlike chemical reactions. The nucleus of an atom in an element and in a compound will be the same. Only their electron arrangement will be different.

Calculations using half life ( t ½)

Calculations using half life ( t ½)

Phosphorous has a half life of 14 days.The sample has a mass of 80g.Calculate the mass of the sample of the

radioisotope after 56 days.Step 1 – Work out how many t ½ have passed.

Time = 56 daysT ½ = 14 daysTherefore 4 t1/2 have passed.

Step 2Time ( days) Mass (g) 0 80 14 ( 1 t ½) 40

28 ( 2 t ½) 20 42 ( 3 t1/2) 10

56 ( 4 t1/2) 5The remaining mass is 5 g.

If a radioisotope has a t ½ of 5.6 years. How long will it take 50g of the sample to decay leaving 12.5g of the original sample?

Step 1 How many t ½ has the sample gone through to become 12.5

gTime Mass0 501 t1/2 252 t1/2 12.5g

The sample has gone through 2 half lives to leave 12.5g.Therefore 2 x 5.6 years = 11.2 years.What is the half life of a sample if it’s radioactive count

changes from 200 counts per minute to 50 counts per minute in 36 days?

Time Activity ( counts per min)0 2001 t ½ 1002 t1/2 50Therefore 2 half lives have passed in 36 days – t ½ = 18 days.

Uses of radioisotopesUses of radioisotopes

MedicalCobalt 60 is a gamma emitter and is used to treat deep rooted

tumours.32 P is a beta emitter and is used to treat skin cancer.Radioactive Iodine is used to monitor the Thyroid gland. It is taken

orally or injected and the gland is scanned over a certain period – plotting the concentrations of the isotope in the body. The isotope can also be used to kill cancerous cells.

Gold 198 – a wire of the element is placed directly into the tumour – the isotope decays over a few days – dosing the tumour.

Industrial Uses of Radioisotopes

Industrial Uses of Radioisotopes

Co 60 is a gamma emitter and is used to examine castings and welds for imperfections.

Radioisotopes can be used to measure thickness of sheet material. Beta and gamma is used for this. 2 identical sources used – i as a reference and the other passes through the material. The 2 are compared and the distance can be measured.

Smoke alarms contain an alpha emitter – not very penetrative . 241 americium is used.

Scientific ResearchScientific Research

Carbon dating14 C is produced in the atmosphere by the bombardment of

N by neutrons.14 N + 1 n —> 14 C + 1 p 7 0 6 1The 14 C is always being taken up by plants the same as 12 C

so there is a known ratio for 14 C : 12CWe measure the ratio in the substance to be dated and then

using the half life of 14C we can age the object.

ExampleExample

Piece of wood contains 3.75 counts per minute.New wood has an activity of 15 cpm, 14C has a t ½ of

5600 years.t1/2 Counts 0 15 1 t ½ 7.5 2 t ½ 3.75 2 t ½ have passed therefore the wood is 2 x 5600 =

11,200 years old.

3 H ( tritium) is used to date the age of vintage wines. ( t ½ = 12.4 years)

32P is used to trace how plants use phosphate fertilisers.

Background RadiationThere will always be a safe level of radiation in the air

called background radiation.This comes from cosmic rays, igneous rocks, gamma

rays.

Nuclear FusionNuclear Fusion

This is when to light nuclei fuse to form a heavier nuclei.

1H + 2 H —> 3 He 1 1 2Nuclear fusion happens in the sun releasing vast

amounts of energy.2H + 2 H —> 3 He + 1 n1 1 2 0

Because the T and P in the centre stars are so high further fusion reaction take place producing even heavier nuclei. All natural elements where produced in nuclear fusion in the stars!

The Hydrogen Bomb uses nuclear fusion. Very high T allow the small nuclei to join and not repulse one another.

Nuclear FissionNuclear Fission

This is when heavy nuclei split into lighter nuclei.The energy can be harnessed – this is the type

of reaction used in nuclear power stations, generate electricity, submarines etc.

235 U + 1n —> 90 Sr + 144 Xe + 2 1n 92 0 38 54 0The 2 neutrons produced will bombard more 235

Uranium starting a chain reaction.

The reaction can be controlled by adding a non fissionable material to the reactor that will absorb the neutrons – preventing further bombardment of 235 U.

The Atomic Bomb was a sample of 235U above critical size and so leading to a nuclear explosion.

239 Plutonium is used in fast breeder reactors.