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400
Democritus
Democritus
Theory- All things are composed of invisible tiny particles of matter, which can’t be destroyed. -
Period: 400 to
Atomic theories
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John Dalton
John Dalton
Theory-
· 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. Thomson
J.J. Thomson
Theory-
· Atoms contain electrons, which have a negative charge.
· Atoms are solid with electrons and protons randomly placed through them. -
Ernest Rutherford
<ahref='http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1908/rutherford-bio.html' >Ernest Rutherford</a>
Theories-
· 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. -
Neils Bohr
Niels Bohr
Theories-
· 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
Sir James Chadwick
Theories-
The nucleus also contains neutrons, which are unchanged particles with a similar mass to protons.
