How the model of the atom changed: Dalton to Bohr

0:00No one could see an atom. So how did scientists work out what one is like?

0:06They built models: simple pictures that fit the evidence.

0:10In the early 1800s, John Dalton pictured atoms as solid balls that cannot be split.

0:17That fitted his evidence: elements combine in fixed amounts.

0:22In 1897, J. J. Thomson found the electron, a tiny negative particle inside atoms.

0:30So atoms can be split. He pictured a ball of positive charge dotted with electrons, like plums in a pudding.

0:39Then Rutherford's team fired alpha particles, tiny positive particles, at thin gold foil.

0:46Most went straight through, a few turned aside, and a very few bounced back.

0:52In 1911, Rutherford said electrons surround a tiny, dense, positive centre, the nucleus, in mostly empty space.

1:02But heated hydrogen gives off only a few colours of light, not a full rainbow.

1:08In 1913, Niels Bohr put electrons in fixed energy levels, or shells, around the nucleus.

1:16When hydrogen's one electron drops a level, it gives off light of one colour.

1:21In the 1920s, scientists found electrons also act like waves, with no exact path.

1:28Today's model shows an electron cloud: the denser it is, the more likely an electron is there.

1:35Older models were not useless: each fitted the evidence of its time.

1:41New evidence changed the model: Dalton, Thomson, Rutherford, Bohr, then the cloud.