Democritus’ Theory (400 BCE) Smallest particle is the atom. Atoms are Of different size In constant motion Separated by empty spaces
Aristotle’s Theory All matter is made up of earth, water air or fire. They had four specific qualities: dry, wet, cold and hot.
Dalton’s Theory (1807 AD) All matter is made up of tiny indivisible particles called atoms. All atoms of an element are identical. Atoms of different elements are different. Atoms are rearranged to form new substances in chemical reactions but not created or destroyed.
Thomson’s Theory (1897 AD) Atoms contained negatively charged electrons. Since atoms are neutral, the rest of the atom is a positively charged sphere. Negatively charged electrons are evenly distributed throughout the atom.
Nagaoka’s Theory (1904) Each atom contains a very massive nucleus. Electrons revolve around the nucleus. Like Saturn and its rings.
Rutherford’s Theory (1909) The centre of the atom has a positive charge and is called the nucleus. The nucleus contains most of the atom’s mass but occupies a very small space. The nucleus is surrounded by a cloud of negatively charged electrons. Most of the atom is empty space.
Bohr’s Theory (1913) Electrons orbit the nucleus of the atom and they have a definite amount of energy. The farther the electron from the nucleus, the greater its energy. Electrons can jump between orbits and can’t be in-between them. Each orbit can hold a maximum amount of electrons.
Chadwick’s Theory (1932) Nucleus of an atom contains protons (positive charge) and neutrons (no charge) which have equal mass. Electrons (negative charge) circle around the nucleus. Neutral atom has the same number of protons as electrons.