Democritus 442 BCSaid atoms were completely solid, homogeneous, indestructible objects.Also that all matter is composed of indivisible atoms.
John DaltonAtomic theoryEnglish chemist who proposed the Atomic Theory. Which
says that all matter was composed of small indivisible
particles called atoms. 1.Matter is made up of indivisible atoms.
2.All atoms of an element are identical.
3.Atoms are neither created nor destroyed.
4.Atoms of different elements have different weights and chemical properties.
5.Atoms of different elements combine in simple whole numbers to form compounds.
J.J. ThompsonFirst to measure the electron. Found that cathode rays could be deflected by an electric field. Showed that cathode "rays" were actually particles. Developed the
plum-pudding model of the atom. Describes the atom as a
slightly positive sphere with small electrons inside.
Ernest RutherfordIn 1909 he and his friends did the gold foil experiment, from which he concluded that the greater part of the mass of the atom was concentrated in a minute nucleus. In the model the positively charged nucleus was surrounded by alot of empty space through which the electrons moved.
Robert MillikanIn 1909 started his oil drop experiment which allowed him to measure the charge on an electron. Combining his results with those of Thomson, Millikan found the mass of the electron to be 9.11x10-28 g
Niels BohrIn 1913 bohr improved the Rutherford atomic model. So the planetary model is called the Rutherford-Bohr model. Bohr added the idea of fixed orbits, or energy levels for the electron traveling around the nucleus
RutherfordRutherford proposed the name proton for the positively charged particles in the nucleus of an atom. At the same time, he also proposed that the nucleus also contained electrically neutral particles which accounted for the remaining mass of the atom. He called this yet unknown particle the neutron.
James ChadwickProved that neutrons, neutral particles in the nucleus that made up approximately half the mass of an atom, did exist. Summary of Subatomic Particles :
Atomic number: protons (and electrons if neutral)
Mass number: protons + neutrons (neutrons = mass number - atomic number)