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400 BCE
DEMOCRITUS (460 BCE - 370 BCE)
Democritus was an Ancient Greek pre-Socratic philosopher. He assumed the existence of the atom as an indivisible part of matter, and think that there were different types of atoms that when combined in ways and with different arrangements formed the different existing substances. -
JOHN DALTON (1766 - 1844)
John Dalton FRS was an English chemist, physicist, and meteorologist. He determined that the elements are made up of
discrete, tiny and indivisible particles, called atoms. The atoms of the same element are equal to each other in mass, size, and in the rest of the physical or chemical properties. Compounds are formed by the union of atoms. -
WILLIAM CROOKES (1832 - 1919)
Sir William Crookes OM PRS was a British chemist and physicist. He discovered the chemical element thallium.
He was examining the emission spectrum of a piece of raw selenium, observed a bright, new line, which led him to isolate a new chemical element, thallium, and to examine their chemical properties. -
JOSEPH JOHN THOMSON (1856 - 1940)
Sir Joseph John Thomson OM PRS was a British physicist and Nobel Laureate in Physics. Thomson proposed a model of the atom, which consists of positive and negative charges, which exist in the same amount, He proposed that the atom is a positive sphere and it have negative charges embedded in it. -
ROBERT MILLIKAN (1868 - 1953)
Robert Andrews Millikan was an American experimental physicist honored with the Nobel Prize for Physics. His earliest major success was the accurate determination of the charge carried by an electron, using the elegant “falling-drop method”; he also proved that this quantity was a constant for all electrons, thus demonstrating the atomic structure of electricity. -
ERNEST RUTHERFORD (1871 - 1937)
Ernest Rutherford was a New Zealand physicist who came to be known as the father of nuclear physics. He discovered that there are two types of radiation, alpha and beta particles, coming from uranium. He found that the atom consists mostly of empty space, with its mass concentrated in a central positively charged nucleus. -
HENRY GWYN JEFFREYS MOSELEY (1887 - 1915)
He was an English physicist. Moseley set out to learn if each element had a unique X-ray spectrum. To find out, he placed a
sample of an element inside an X-ray tube. When a beam of electrons struck the sample, the element gave off X-rays. Moseley
could then determine the element’s X-ray spectrum. -
NIELS BOHR (1885 - 1962)
Niels Henrik David Bohr was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory. Niels Bohr proposed a theory for the hydrogen atom based on the quantum theory that energy is transferred only in certain well-defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. -
ERWIN SCHRÖDINGER (1887 - 1961)
Erwin Rudolf Josef Alexander Schrödinger was a Nobel Prize-winning Austrian-Irish physicist. A powerful model of the atom was developed by Erwin Schrödinger. He combined the equations for the behavior of waves with the de Broglie equation to generate a mathematical model for the distribution of electrons in an atom. -
JAMES CHADWICK (1891 - 1974)
James Chadwick was a British physicist who was awarded the 1935 Nobel Prize. He discovered the final piece of the atom: the
neutron. The neutron has almost the same mass as the proton, and they both occupy the nucleus. But the neutron is electrically neutral
