atomic physics

Aristotle

described all matter as being composed of the "four basic elements" fire, earth, air and water.

Democritus

JJ Thompson

discovered electrons with cathode ray tubes. since atoms have an overall charge of zero, the existence of a positive charge was inferred.

plum pudding model

An atom consists of a uniformly distributed sphere of positive charge 

Negative vibrating electrons are embedded evenly inside, in fixed positions, like raisins in a pudding.

Ernest Rutherford

proved plum pudding model incorrect

shot alpha particles at a very thin piece of gold foil. Thompson’s model predicted that, most alpha particles passed straight through (with deflections less than 10 deg) because alpha particles were heavier and denser than electrons, and therefore would pass straight through.

in this experiment some alpha particles were deflected by a large angle and even rebounded from the foil.

This suggested that the alpha particles were being deflected by a heavy, positive mass concentrated at the centre of the atom. Not the uniform distribution of positive masses throughout the atom like Thompson suggested.  

Rutherford suggested that electrons orbit the nucleus like planets orbit the sun. He proposed that the majority of an atom is empty space. The nucleus (positive mass) was located in the center and the electrons surrounded.

ionisation

Ionisation is the process where radiation (radioactive particles or rays) interact with atoms and knock electrons out of them, creating positively charged ions.
This can happen either by direct collision (common with alpha and beta particles) or by transferring energy (common with gamma rays).

Half-life

time taken for half of the atoms in a sample of radioactive material to decay OR time taken for the detected radioactivity rate (count/min.) to reduce to half.