Development of the model of an atom
Joseph Thompson (1856-1940)
Credited with the discovery of the atom. The 'Plum Pudding' model (pudding is positive and the things stuck in the pudding are negative)
Earnest Rutherford (1871-1937)
Famous 'Gold-Foil Experiment'. Discovered the atom has a small positive NUCLEAS with negative electrons moving around it - mostly empty space.
Gold Foil Experiment
Gold Foil Experiment
- Shot alpha particles are a thin gold foil. If Thompson's model was correct, they would have expected most of the particles to be deflected, and hardly any to pass though.
- However, they found most of the alpha particles passed through the foil. Some were deflected a bit and a few were deflected almost straight back towards the sources
- This meant that all the positive charge must be concentrated at the centre of the atom, with the electrons orbiting at the same distance; most of the atom is empty space.
Within the nucleus energy is stored. The energy is held in by the nuclear forces between the protons and neutrons, but the protons repel one another. Depending on the balance between the forces, the nucleus might be ether stable or unstable.
Unstable nuclei decay to try to become more stable. This may involve losing part of the nucleus, changing the particles in the nucleus and releasing energy.
This results in 3 types of nuclear radiation:
1. Alpha Particle (α Particle) - part of the nucleus is ejected.
This means that the nucleus becomes a different element.
The alpha particle is a Helium ion [He]2+
2. Beta Particle (β Particle) - one of the neutrons in the unstable nucleus decays into a proton and an electron, and the electron is ejected --> turns into a beta particle. This again means that the nucleus changes into a different element.
All beta particles are electrons but not all electrons are beta particles.
3. Gamma Radiation (ɣ Radiation) - this is not a particle; it is a photon --> a packet of electromagnetic energy. They are generated when an unstable nucleus loses extra energy.
e.g. during alpha and beta decay.
Penetration and Ionising Power
There are 3 types of nuclear radiation have differing penetration through materials. They can also ionise atoms of the materials they pass through, and this 'ionising power' is dependent on the mass of the particle.
This ionisation process can damage materials and cause changes in living tissue, leading to illness and some forms of cancer.
Alpha particles have the highest mass and the least energy so it doesn't take much to slow them down and stop them.
A radioactive element is one that emits nuclear radiation. Materials are radioactive because the nucleus is unstable. This may be because it has excess energy stored, or too many nucleons (protons and neutrons) for all forces in the nucleus to stay balanced.
Radioactivity occurs naturally as well as being made. Rocks and minerals give off nuclear radiation and even plants can be mildly radioactive because of the nutrients they use.
Alpha vs. Beta vs. Gamma
- High ionising power
- Positively charged (relative charge of +2) and relative mass of 4
- Slow, only travel a few cm in the air
- Stopped by a sheet of paper
- Moderate ionising power
- Negatively charged (relative charge -1) and relative mass of 1/2000
- Fast, travel up to 1m in air
- Stopped by a few mm of aluminium
- Low ionising power
- No Charge and no mass
- Travel at speed of light and for very long distances
- mostly stopped by thick concrete or several cm of lead
Properties of Nuclear Radiation
Alpha and Beta particles are charged and can therefore be deflected by magnetic and electric fields.
Beta Particles have a very small mass and so are deflected most.
Alpha particles are relatively heavy in comparison and so are deflected by a smaller amount.
Gamma Rays have no charge and are unaffected.
Dangers in the body
All forms of nuclear radiation are potentially damaging to the human body. The amount of danger they present depends of the type of radiation and whether the source of the radiation is inside the body.
- When the source is outside the body, the order of least to most dangerous is alpha --> beta --> gamma.
- This is because alpha particles will be stopped by clothing or a few cm of air.
- When the source is inside the body, the order of least to most dangerous is gamma --> beta --> alpha.
- This is because gamma rays will most probably pass out through the body, and they have lower ionising power, but alpha particles will by stopped by tissues in the body, and they have the highest ionising power.
Although nuclear decay of radioactive element (sometimes called radioisotopes) is a random process, we can make predictions using half-life of an isotope.
The half-life is the time taken for half of an amount of a radioisotope to escape.
The half-life o a radioisotope is the amount of time it takes for half the amount of the radioisotope nuclei to decay into another element. This process continues every half-life.
Uses of Nuclear Radiation
1. Medical Treatment:
- Gamma sources of radiation are used to destroy tumours. The gamma rays aimed at the tumour from a range of different angles. This is so the healthy tissue gets as small a dose as possible; but the tumour receives the maximum dose.
2. Paper Manufacture:
- Beta radiation source is used in paper manufacturing. The rollers that determine the thickness of the paper are controlled according to the beta count received at the detector.
3. Smoke Detectors
- Uses alpha source. Stopped by smoke particles, sets off alarm. Detector would stop detecting the radiation because the smoke blocks it so the alarm goes off.