Atoms and isotopes

Atoms have a tiny, dense nucleus of protons and neutrons, surrounded by electrons. Our model of the atom changed over time as new experiments showed the old ideas were wrong.

Key facts

  • Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge −1, mass almost 0.
  • Atomic number = protons. Mass number = protons + neutrons.
  • Isotopes: same number of protons, different numbers of neutrons.
  • Models: Dalton’s solid spheres → Thomson’s plum pudding → Rutherford’s nucleus → Bohr’s electron shells.

Worked examples

Example 1

Carbon-14 has atomic number 6. How many protons, neutrons and electrons does a neutral atom have?

  1. Protons = atomic number = 6.
  2. Neutrons = 14 − 6 = 8.
  3. Neutral, so electrons = protons = 6.

Answer: 6, 8, 6

Example 2

What did Rutherford’s gold foil experiment show?

  1. Most alpha particles passed straight through: the atom is mostly empty space.
  2. A few bounced back: the positive charge and mass are concentrated in a tiny nucleus.

Answer: A tiny positive nucleus