-
465
Democritus
He was known as the person who came up with the atomic theory (theory that atoms exist) This is Democritus' atomic theory exactly:
1.All matter consists of invisible particles called atoms.
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms are homogenous.
5. Atoms differ in size, shape, mass, position, and arrangement.
->Solids are made of small, pointy atoms.
->Liquids are made of large, round atoms.
->Oils are made of very fine, small atoms that can easily slip past -
Period: 465 to
Timeline On Atomic Thoery
-
John Dalton
Discovered that matter is composed of tiny invisible particles called atoms. He also discovered that all atoms of one element are identical but are different from the atoms of other element. Evidence: Law of mass conservation (1789)
Law of constant Composition (1799) -
Joseph J Thompson
He discovered that atoms consist of a large sphere of uniform positive charge embedded with smaller negatively charged particles (corpuscles). Developed the “plum pudding” model of the divisible atom. That the total positive charge of the sphere equals the total negative of the corpuscles. -
Ernest Rutherford
He was known to propose a nuclear model of the atom in which:
a very small positively charged nucleus containing most of the mass of the atom
a very large volume around the nucleus in which electrons move
a nucleus containing positively charged protons
a number of protons equal to the number of electrons -
NIels Bohr
Proposed a planetary' model for the hydrogen atom:
Electrons move around the nucleus in fixed orbits (like planets around the sun). An electron in a particular orbit has constant energy.
An electron can absorb energy and move to a higher energy orbit of larger radius. (excited electrons)
An excited electron can fall back to its original orbit by emitting energy as radiation.
Electrons can only exist in certain discrete energy levels. -
James Chadwick
James chadwick was noted to discovering the existence of the nucleus inside of an atom.He established that atomic number is determined by the numbers of protons in an atom. He also discovered the fourth subatomic particle,the neutron.