Properties of Radiation

What an atom is

• An atom is the basic building block of all matter. Atoms are tiny (about 1 × 10⁻¹⁰ m in radius), so around 100 million atoms could sit side by side across a thumbnail
• Every atom has two parts:
  • a tiny dense nucleus at the centre
  • a cloud of electrons orbiting the nucleus
• The nucleus is about 10 000 times smaller than the whole atom but contains almost all of the mass. If the atom were scaled up so the nucleus was a marble in the middle of a football stadium, the electrons would be orbiting the seats in the back row

The three sub-atomic particles

• Together protons and neutrons make up the nucleus and are called nucleons
• In a neutral atom, the number of electrons equals the number of protons, so the positive nuclear charge is exactly cancelled by the negative electron charge
• If an atom loses or gains electrons, the charges no longer balance and the atom becomes an ion: positive if electrons have been lost, negative if extra electrons have been gained

Atomic and mass numbers

• Atomic number (Z) = the number of protons in the nucleus. This single number defines which element an atom is:
  • Hydrogen: Z = 1
  • Carbon: Z = 6
  • Sodium: Z = 11
  • Uranium: Z = 92
• Mass number (A) = the total number of nucleons (protons + neutrons) in the nucleus
• The number of neutrons in an atom can therefore be found by subtraction:

number of neutrons = mass number − atomic number = A − Z

Nuclear notation

• A nucleus is described by writing its element symbol together with its mass number and atomic number:

ᴬ_Z X

• where:
  • A = mass number (top, total nucleons)
  • Z = atomic number (bottom, protons)
  • X = the element's chemical symbol
• Some examples:
  • ¹₁H is hydrogen (1 proton, 0 neutrons, 1 electron)
  • ²³₁₁Na is sodium (11 protons, 12 neutrons, 11 electrons)
  • ²³⁸₉₂U is uranium-238 (92 protons, 146 neutrons, 92 electrons)

Example — an atom of gold has the nuclear notation ¹⁹⁷₇₉Au. How many protons, neutrons and electrons does it have?

• Number of protons = atomic number = 79
• Number of neutrons = mass number − atomic number = 197 − 79 = 118
• Number of electrons = number of protons (atom is neutral) = 79