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Elements in the same group have the same number of outer-shell electrons or "valence" electrons, so they tend to react similarly, as they can form similar bonds.
JJ Thomson discovered electrons using the cathode ray. Electrons are negatively charged particles that orbit around the nucleus of an atom. Finding out that there was a negative charge to balance out the positive charge. This later helped develop Hund's Rule and the Pauli Exclusion Principle.
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