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460
Idea of atoms
Around 460BCE a Greek philosopher, Democritus, developed the idea of atoms. He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms. -
Period: 460 to
Atomic theory
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John Dalton's discovery
In the 1800's an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms). Although he did not know about their structure, he knew that the evidence pointed to something fundamental. -
Thomson's proposed model
In 1897, the English physicist J.J. Thomson discovered the electron and proposed a model for the structure of the atom. Thomson knew that electrons had a negative charge and thought that matter must have a positive charge. His model looked like raisins stuck on the surface of a lump of pudding. -
Albert Einstein's theory
Atoms not only emit photons, but they can also absorb them. In 1905, Albert Einstein wrote a ground-breaking paper that explained that light absorption can release electrons from atoms, a phenomenon called the "photoelectric effect." -
Ernest Rutherford's Atom
In 1911 Ernest Rutherford knew that atoms consist of a compact positively charged nucleus, which negative electrons circulate aropund it. The nucleus is less than one thousand million millionth of the atom, but contains most of the atom's mass. For example, If an atom was the size of Earth, the nucleus would a football stadium.
But there appeared something wrong with Rutherford's atom. The theory of electricity and magnetism predicted that opposite charges attract eachother and spiral inwards. -
Niels Bohr's Theory
In 1912 Niels Bohr, a Danish physicist came up with a theory that electrons do not spiral inwards towards the nucleus and he came up with two rules for what does happen to the electrons.
RULES
1 - Electrons can orbit only at certain allowed distances from the nucleus.
2 - Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit. -
Paul Dirac's Theory
(The image represetns anti-matter)
In 1928, Paul Dirac produced equations which predicted an unthinkable thing at the time - a positive charged electron. In 1932 in experiments with cosmic rays, Carl Anderson discovered the anti-electron, which proved Dirac's equations. Physicists call it the positron.
For each variety of matter there should exist a corresponding 'opposite'. Physicists now know that antimatter exists. -
James Chadwick's Discovery
Not until 1932 did the English physicist James Chadwick finally discover the neutron. He found it to be slightly heavier than the proton with a mass of 1840 electrons and to be neutral.
