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Atomic Theory
Atomic theory is the study of the nature of atoms and how they combine to form all types of
matter.
The philosophers of ancient Greece reasoned that the basic forms of matter were fire, water,
earth, and air. In ancient China, the elements were thought to be fire, water, wood, metal, and
earth.
The History of Atomic Theory: https://www.youtube.com/watch?v=thnDxFdkzZs
Timeline:
Democritus (440 BC):
- believed that matter is made up of tiny particles that can't be broken down any further
John Dalton (1766 - 1844):
- all matter is made of small, indivisible particles called atoms
- all the atoms of an element are identical in properties such as size and mass
- atoms of different elements have different properties
- atoms of different elements can combine in specific ways to form new substances
- devised a series of element symbols
J. J. Thompson (1856 - 1940):
- experimented with electric currents in cathode ray tubes
- discovered electrons
- the plum pudding model
Ernest Rutherford (1891 - 1937):
- shot positively charged particles at a very thin foil of pure gold
- most of the high speed particles went through the foil, but some reflected
- the nucleus was discovered
James Chadwick (1891 - 1974):
- student of Rutherford, refined the concept of the nucleus
- nucleus contains both neutral particles and positively charged particles
Niels Bohr (1885 - 1962):
- Bohr Model: electrons surround the nucleus in specific energy levels, called shells
- electrons jump between shells by gaining or losing energy
- each shell can contain only a specific number of electrons
The Quantum Mechanical Model
-the most accurate and advanced model of the atom, used today by physicists and chemists
- electrons do not exist as tiny points inside an atom
- electrons exist in specific energy levels, surrounding the nucleus as a cloud
All elements are composed of atoms, and one atom is the smallest unit of any element. Atoms
All elements are composed of atoms, and one atom is the smallest unit of any element. Atoms
themselves are made of subatomic particles: protons (p), neutrons (n), and electrons (e).
Relative mass compares the mass of an object to the mass of another object. An electron is
the lightest subatomic particle of the three, and so it is assigned a relative mass of 1. Protons
and neutrons are much heavier, with relative masses of 1836 and 1837, respectively.