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460
Democritus (Greece 460 BC-370BC)
Democritus’ idea was that all matter is composed of tiny indestructible units, called atoms. He was one of the first people to develop a theory for matter. -
Isaac Newton (England)
Isaac Newton proposed a mechanical universe with small solid masses in motion. -
John Dalton (England)
John Dalton's theory of atoms rested on four basic ideas: chemical elements were composed of atoms; the atoms of an element were identical in weight; the atoms of different elements had different weights; and atoms combined only in small whole-number ratio to form compounds. -
G.J. Stoney (England)
Stoney proposed that electricity was made of negative particles called “electrons”. -
J.J. Thomson (England)
This English physicist discovered the electron and proposed a model for the structure of the atom. He knew that electrons had a negative charge and he thought that matter must have a positive charge. -
Max Planck (Germany)
Max Planck showed that when you vibrate atoms strong enough, you can measure the energy only in discrete units. He called these energy packets, quanta. -
Hantaro Nagaoka (Japan)
Hantaro Nagaoka proposed a model in which a positively charged centre is surrounded by a number of revolving electrons, in the manner of Saturn and its rings. -
Robert A. Millikan (America)
Robert A. Millikan discovered that the charge of an electron is negative with his oil drop experiment. He concluded that all the other charges were simple multiples of this basic charge. -
Ernest Rutherford (New Zealand)
Ernest Rutherford's experiments with alpha rays led him to describe the atom as a small, heavy nucleus with electrons in orbit around it. This nuclear model of the atom became the basis for the one that is still accepted today. He discovered the proton. He thought that since the atom consists of negative charged electrons there must be positive charged protons to balance the atom. -
H.G.J. Mosely (England)
Moseley discovered that each element contains a unique positive charge in its nucleus called the proton. Therefore the number of protons will determine an element’s identity. He suggested that the periodic table should be arranged in ascending atomic number, instead of ascending atomic weight. -
Bohr (Denmark)
Niels Bohr came up with a theory that said the electrons do not spiral into the nucleus and came up with some rules for what does happen. The rules are: electrons can orbit only at certain allowed distances from the nucleus; and atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit while an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit. -
James Chadwick (England)
James Chadwick discovered the neutron and found it slightly heavier than the proton with a mass of 1840 electrons and with no charge. The proton-neutron together received the name, “nucleon”. This dicovery completed the basic structure of an atom.
