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History of the Atom C1.1

• John Dalton thought that all matter was made of tiny particles called atoms, which he imagined as tiny solid balls. Dalton’s model included these ideas:
• Atoms cannot be broken down into anything simpler
• The atoms of a given element are Identical to each other
• The atoms of different elements are different from one another
• During chemical reactions atoms rearrange to make different substances
• J.J. Thomson discovered the electron. Atoms are neutral overall, so in Thomson’s ‘plum pudding model’:
• Atoms are spheres of positive charge.
• Electrons are dotted around inside.
• Geiger-Marsden:
• Hans Geiger and Ernest Marsden tested the plum pudding model. They aimed beams of positively-charged particles at very thin gold foil. These particles should have passed straight through, according to the plum pudding model. However, many of them changed direction.
• Atoms have a central, positively charged nucleus with most of the mass.
• Electrons orbit the nucleus.