● 400 B.C.- billiard ball shaped
○ Asked: Could matter be divided into smaller and smaller pieces forever or is there a limit?
○ Theory: Matter couldn’t be divided any smaller, eventually the smallest piece possible will be achieved
○ “Atomos” - Smallest piece
○ Atoms are infinite in number
○ His theory was ignored for nearly 2000 years
○ All elements are composed of atoms. Atoms are indivisible and indestructible ○ Atoms of the same element are alike, atoms of different elements are different.
○ Compounds are formed by the joining of two or more atoms
○ Foundation of modern chemistry
● Thomson’s Plum Pudding Model
○ Atoms were made from positively charged substance with smaller, negatively charged particles
○ Studied the passage of an electric current through a gas. As the current passed through the gas, it gave off rays of negative charge
○ Concluded: negative charges came from WITHIN, particles smaller than the atom existed, the atom was divisible, “corpuscles” - electrons,
● Rutherford’s Gold Foil Experiment
○ Fired a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)
○ Most of the “bullets” passed through the gold atoms, but some bounced away as if hitting something solid
○ Concluded: Atoms were mostly open space, the atom had a small dense positively charged solid center called a nucleus, and that the nucleus is tiny, and that most of the positive charges were in the nucleus, with the negative electrons scattered throughout the atom
● Bohr Model
○ Placed each electron in a specific energy level
○ Electrons move in orbits around the nucleus and that the energy levels were placed a certain distance.
■ 1st - 2 electrons
■ 2nd/3rd - 8