Atomic structure

Atomic structure

In this chapter you will learn:

  1. Developing the model of the atom

  2. What are Atoms, isotopes and ions?

  3. Atoms and nuclear radiation

  4. Half life

  5. Irradiation and Contamination

  6. Uses and risk of radiation

  7. Nuclear fission and fusion

Developing the model of the atom

  • In 1804, John Dalton proposed his atomic theory that all matter was made up of tiny hard particles or spheres called atoms that could not be broken up.

  • In 1897, J J Thomson proposed that the atom looked like a plum pudding.

  • J J Thomson discovered that atoms contained even smaller negatively charged particle called electrons which could be removed from atoms.

  • He said that the atoms were spheres of positive charge with a lot of negative electrons stuck in it like fruit in a pudding.

  • Rutherford and his assistants had proven the existence of the nucleus. Rutherford performed alpha scattering experiment in which a beam of alpha particles was fired at a gold foil. They realised that most of the mass of the atom must be concentrated at the centre in a tiny nucleus because a few alpha particles were deflected back.

  • In 1913, Niels Bohr suggested that the electrons orbited the nucleus in different energy levels or certain distances from the nucleus.

Atoms, isotopes and ions

Atoms are made up of three particles: protons, neutrons and electrons.

The number of protons in an atom is called the atomic number.

The total number of protons and neutrons in an atom is called its mass number.

The table below gives information about the three types of particle, relative mass and charge.

Particle Relative mass Relative charge
Proton 1 +1
Electron very small -1
Neutron 1 0

Atoms can be represented as shown in this examples.


Isotopes are atoms that have same number of protons but different number of neutrons.

Most elements have one or two stable isotopes and other isotopes are unstable.


An ion is a charged atom. When an atom is attracted to another atom because it has an unequal number of electrons and protons, the atom is called an ION.

Atoms can lose or gain electrons due to collisions or other interactions. When they do this they form charged particles called ions.

  • Positive Ion - happens when an atom loses one or more electrons.

  • Negative Ion - happens when an atom gains one or more electrons.

Atoms and nuclear radiation

Alpha particle

Alpha particle are helium nuclei. They contain two protons and two neutrons.

Alpha decay (two protons and two neutrons) decreases the atomic mass number by 4 and atomic number by 2.

The nuclear equation for alpha decay would be:

  • Beta particles

    Beta particles (β) are simply high speed electrons.

    A beta particle has zero mass.

    Beta decay increases the nuclear charge by 1 and the atomic number by 1.

    The nuclear equation for beta decay would be:

    Gama rays

    Gamma-rays are a form of electromagnetic radiation with a small wavelength.

    Gamma rays have a high penetrating power. It takes a thick sheet of metal such as lead, or concrete to to absorb it all.

    Gamma rays have the lowest ionising power.

    Gamma rays has no mass and no charge.

    Nuclear equations represent radioactive decay by using element symbol.

    Radioactive decay is a random process. so it is impossible to tell when a particular nucleus will decay next and which one will decay next.

    Half life

    It is possible to find out the time it takes for the amount of radiation emitted by a source to halve, this is known the half life.

    A short half life means the nuclei are very unstable and decay rapidly, therefore, the activity falls quickly.

    A long half life means the most nuclei do not decay for a long time, therefore, the source releases small amounts of radiation for a long time.

    Background radiation is all around us all the time. It comes from natural sources and from artificial sources.

    Irradiation and Contamination

    When an object is exposed to radiation from a radioactive source it is called irradiation.

    Exposure to radiation is called irradiation. It can damage living cells.

    Contamination occurs when material that contains radioactive atoms are deposited on materials or surfaces or where their presence is undesirable.

    Contamination is very dangerous because radioactive particles could get inside your body.

    Outside the body, Alpha radiation is less harmful because they would not penetrate the skin cells and is unlikely to reach living cells below the skin.

    Outside the body, Gamma and Beta sources are dangerous because they can penetrate the body and will reach to the vital organs in the body.

    Inside the body, Alpha radiation is the most dangerous. Gamma radiation is the least dangerous, as they have lowest ionising power.

    Uses and risk of radiation

    • Radiation can be used to sterilize medical and scientific equipment.

    • Gamma emitters are used in medical tracers. They are ideal because of their low ionising power.

    • Gamma emitters are also used in industrial tracers.

    • Radiotherapy treats cancer.

    • Some invasive surgical procedures can be replaced by imaging processes utilising medical contamination.

    • Radiation can cause cancer.

    • Radiation can damage or kill living cells.

    Nuclear fission

    Nuclear fission is the splitting of a larger unstable nucleus into smaller nuclei.

    When an atom splits two or three neutrons are released and they can collide with other uranium nuclei to cause further fission reactions. This is known as a chain reaction.

    The heat energy from nuclear fission processes in the nuclear reactor is used to heat water to drive the turbines and generators.

    If fission is uncontrolled the result would be a quick release of lots of energy like in the case of the explosion of an atomic bomb.

    Nuclear fusion

    In nuclear fusion, two small, light nuclei join together to create one large and heavy nucleus. In this process some of the mass may be converted into the energy of radiation.

    Fusion releases a lot more energy than fission for a given mass of fuel.

    Fusion needs high temperature and pressure. Fusion reactors are really hard and expensive to build.

    The hydrogen atomic bomb uses nuclear fusion, not fission.

    The hydrogen fusion bomb is more powerful than a fission bomb.