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He purposed the concept of nucleus in an atom by using Gold foil experiment and interpreted that an atom was made up of mostly empty space with a small dense portion at center which deflected the positively charged alpha particles.The unstable radioactive elements show nuclear decay which is a spontaneous cleavage of an atomic nucleus with the formation of some other nucleus and release a certain amount of energy.Beta particles ionise atoms that they pass, but not as strongly as alpha particles do. We don't find pure gamma sources - gamma rays are emitted alongside alpha or beta particles.Strictly speaking, gamma emission isn't 'radioactive decay' because it doesn't change the state of the nucleus, it just carries away some energy. You can think of different isotopes of an atom being different "versions" of that atom. It has 6 protons and 6 neutrons - we call it "carbon-12" because it has an atomic mass of 12 (6 plus 6).There is some evidence of slight seasonal variations in the rates of radioactive decay of some isotopes, which may be due to temperature differences. Or may be due to something else we don't know about yet.For now, for GCSE exams, let's go along with the idea that the rate of radioactive decay does not depend on external factors.

They proposed the term "radioactivity" which was the spontaneous emissions of radiation from certain elements. They studied some radioactive components like uranium and discovered two new radioactive elements which they named polonium and radium.Alpha particles are made of 2 protons and 2 neutrons.This means that they have a charge of 2, and a mass of 4 (the mass is measured in "atomic mass units", where each proton & neutron=1) We can write them as , or, because they're the same as a helium nucleus, . They have a low penetrating power - you can stop them with just a sheet of paper.The next important step on the road of discovery of radioactivity was taken by Ernest Rutherford in 1904.He established the concept that the rays emitted by the radioactive substance are of three types designated as alpha, beta and gamma rays.

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