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442 BCE
Democritus
Democritus proposed that matter was made of indivisible particles and empty space . He believed it was impossible to change or destroy an atom. His atomic model had atoms that were homogeneous and came in different positions, sizes, shapes and arrangements. -
Antoine-Laurent de Lavoisier
Lavoisier identified 23 elements and compiled them into the first extensive list of elements. He also discovered that mass is conserved in chemical reactions. Another discovery of Lavoisier was the composition of water. -
John Dalton
From 1803 to 1807, Dalton proposed an atomic theory based on four postulates. His theory stated that elements were composed of atoms, all atoms of an element were identical, atoms cannot be created or destroyed in chemical reactions, and compounds are formed when the atoms of more than one element combine. -
Henri Becquerel
Becquerel discover that a compound of uranium gives off high-energy radiation. This property is called radioactivity. -
J.J. Thomson
Thomson discovered the electron , a negatively charged part of an atom, by using cathode rays in 1897. Later he would propose the plum pudding model of an atom. The model proposed that atoms were spheres of positively charged matter in which electrons are embedded. -
The Curies
Marie and Pierre Curie conducted experiments to identify and isolate possible sources of radioactivity in the uranium compound that Becquerel worked with. They discovered that uranium atoms were the source of the radioactivity. All three scientists involved were awarded a Nobel Prize in physics in 1903. Marie Curie also discovered the elements polonium and radium. -
Ernest Rutherford
Rutherford discover three types of radiation: alpha, beta, and gamma. His experiments showed that there were positively charged particles, negatively charged particles, and particles with no charge. He proposed the nuclear model which had a dense, positively charged region called the nucleus. His theory also stated that most of the atom was empty space where the electrons moved around the nucleus. Rutherford also discovered positive particles called protons in 1919. -
Niels Bohr
Niels Bohr suggested a theory that electrons travel in specific orbits around the nucleus. As well, that the amount of electrons held in each orbit varies, with the outer orbits holding more electrons than the inner orbits. His theory explained that the electrons orbit influenced the chemical properties of an atom. -
Henry Moseley
Moseley discovered that an elements atomic number was the same as the number of protons it had. This discovery explained why the position predicted on the table by an element's atomic weight did not always match with the chemical properties of the element. -
Louis de Broglie
Louis de Broglie proposed that matter behaved in a wave-like manner and he discovered the wave nature of electrons. His work gave an explanation of questions brought up by calculations of the movement of electrons. -
Hans Geiger
Geiger introduced the first successful detector of individual alpha particles and other ionizing radiations. He used his particle counter and other radiation detectors in experiments that led to the discovery that every atom contains a nucleus, where its positive charge and most of its mass are concentrated. This led him to propose a planetary model. -
Erwin Schrödinger
Schrödinger developed the electron cloud model. His model proposed a dense nucleus surrounded by a cloud of electrons. This model was based on mathematical equations for the behavior of waves based on earlier work by Louis de Broglie. -
James Chadwick
He discovered the neutron, a subatomic particle with no charge with a mass nearly equal to the proton. He discovered the neutron by using beryllium radiation and measuring the energy of recoiling particles.
