Explain how ideas about the atom have changed over time.
22 points for a good answer !

John Dalton was the first to record his ideas about the atom; he thought that matter was made up of these tiny particles called atoms which he thought of as tiny spheres that couldn’t be divided.
Then J J Thompson carried out experiments that led him to discover the electron. Putting his discoveries together he settles on the now famous ‘plum pudding model’ that describes the atom as a ball of positive charge with negative electrons embedded throughout - much like that of currants in a Christmas pudding.
Ernest Rutherford then put together an experiment to test the ideas of the plum pudding model. He fired positively charged alpha particles at thin gold foil; most alpha particles went straight through however some were scattered in differing directions. From this he realised that the mass of the atom is concentrated in the centre (now know as the nucleus) and that the nucleus is positive.
The most up to date model as we know it is the idea of the positive nucleus containing protons and neutrons with orbitals containing negative electrons surrounding the nucleus in shells.

The correct answer is "atomic weight".