Atoms and isotopes · Grade 9
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).