2 Democritus460 BC - Greek philosopher proposes the existence of the atomHis theory:all atoms are small hard particlesmade of a single material formed into different shapes and sizesalways moving, and that they form different materials by joining together
3 John DaltonBritish chemist; elements combined in specific proportions to form compounds.His theory:all substances are made of atoms that cannot be created, divided, or destroyedatoms join with other atoms to make new substancesatoms of the same element are exactly alike, and atoms of different elements are different in mass and size (elements)
4 Edward Frankland 1852 - English chemist developed the valence theory His theory:every atom has a fixed number of bonds (chemical links) that it can formfor the atom to be stable, all of these bonds must be used.
5 J.J. ThomsonEnglish chemist and physicist discovered 1st subatomic particlesHis theory:negatively charged particles called electrons and positively charged mattercreated a model to describe the atom as a sphere filled with positive matter with negative particles mixed inReferred to it as the plum pudding model
6 Ernest Rutherford 1912 - New Zealand physicist discovered the nucleus His theory:small, dense, positively charged particle present in nucleus called a protonelectrons travel around the nucleus, but their exact places cannot be described
7 Niels Bohr 1913 - Danish physicist discovered energy levels His theory:electrons travel around the nucleus in definite paths and fixed distanceselectrons can jump from one level to a path in another level
8 Erwin ShrodingerAustrian physicist developed the electron cloud modelHis theory:electrons exact path cannot be predictedregions, referred to as the electron cloud, are areas where electrons can likely be found.
9 James Chadwick 1932 - English physicist discovered neutrons His theory:neutrons have no electrical chargeneutrons have a mass nearly equal to the mass of a protonunit of measurement for subatomic particles is the atomic mass unit (amu)
10 Modern Theory of the Atom Atoms are composed of three main subatomic particles: the electron, the proton, and the neutron.Most of the mass of the atom is concentrated in the nucleus of the atom.The protons and neutrons are located within the nucleus while the electrons exist outside of the nucleus.In stable atoms, the number of protons is equal to the number of electrons.
11 Modern Theory of the Atom The type of atom is determined by the number of protons it has.The number of protons in an atom is equal to the atomic numberThe sum of the number of protons and neutrons in a particular atom is called the atomic massValence electrons are the outermost electrons and are where bonding takes place