Athabasca tar sands.
Naturally occurring mixture of bitumen and sand found in Northern Alberta.

Atmolysis.
Refers to separation of a mixture of gases by means of their different rates of diffusion. Usually separation gets effected by allowing the gases to diffuse through the walls of a porons partition or membrane.

Atmosphere.
A unit of pressure equal to 101325 pascals. This is equal to 760.0 mm Hg. The actual atmospheric pressure fluctuates around this value. The unit is usually used for expressing pressures well in excess of standard atmospheric pressure, e.g. in high pressure chemical process.

Atom.
The smallest part of an element which can exist as a stable entity.

Atomic heat.
The number of Joules of heat required to raise the temperature of one male of atoms of an element through 1 Kelvin.

Atomicity.
The number of atoms in a molecule of an element or a compound e.g. the atomicity of hydrogen is 2(H₂) the atomicity of Sulphur dioxide is 3(SO₂) the atomicity of helium is 1 (He).

Atomic mass (amu)
The mass of an individual atom is very small. Mass of hydrogen and oxygen are 1.673×10^-24 and 26.558×10^-24g, respectively. To express masses of atoms in a convenient way, the unit called atomic mass unit (amu) is introduced. Hydrogen being the lightest element was taken as standard and its atomic mass was fixed equal to unity. In 1961, a refinement was made and the unit was changed. The present unit is based on the C-12 isotope which is nearly 12 times as heavy as the hydrogen atom. The atomic mass unit is exactly 1/12 the mass of one 12-C atom.

Atomic nucleus.
A part of an atom-having most of its mass, positively charged. Consists of protons and neutrons. The positive charge of a nucleus is equal to the number of protons in contains, Z.

Atomic number (z).
The number of protons in the nucleus of one of its atom. Atomic number = Mass number – Number of neutrons.