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400
Democritus
• All things are composed of invisible tiny particles of matter, which can’t be destroyed. -
Period: 400 to
Time
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John Dalton
• Described tiny indivisible particles called atoms.
• Atoms can’t be divided or destroyed.
• Atoms can join together to form molecules.
• Atoms of one element are all the same. -
J.J. Thompson
• Atoms contain electrons, which have a negative charge.
• Atoms are solid with electrons and protons randomly placed through them. -
Ernest Thompson
• Atoms have a dense nucleus which contains protons, and most of the rest is empty space.
• Electrons orbit around the nucleus like planets around the sun. -
Niels Bohr
• Electrons are arranged in layers or shells around the nucleus.
• The shells have different energy levels, and electrons fill up the lowest levels first. -
James Chadwick
•The nucleus also contains neutrons, which are unchanged particles with a similar mass to protons.
