Atoms and isotopes · Grade 7

Atoms have a tiny, dense, positive nucleus of protons and neutrons, surrounded by electrons in energy levels. The atomic number (protons) decides the element; the mass number counts protons plus neutrons. Atoms of the same element with different numbers of neutrons are isotopes. Our model of the atom was built up from experiments – most famously Rutherford’s gold foil experiment.

Key facts

  • Relative charges: proton +1, neutron 0, electron −1. Protons and neutrons have mass 1; electrons almost 0.
  • Atomic number Z = protons. Mass number A = protons + neutrons. Neutrons = A − Z.
  • A neutral atom has as many electrons as protons. Ions have gained (−) or lost (+) electrons.
  • Isotopes: same number of protons, different numbers of neutrons.
  • The nucleus is about 100,000 times smaller than the atom but holds almost all its mass.
  • Models: plum pudding (Thomson) → nuclear (Rutherford) → shells (Bohr) → neutron discovered (Chadwick).

Worked examples

Example 1

Chlorine is 75% chlorine-35 and 25% chlorine-37. Find its relative atomic mass.

Answer: 35.5

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