material science

Atomic number (Z)

For a chemical element, the number of protons within the atomic nucleus.

Isotopes

Atoms of the same element that have different atomic masses.

Atomic weight (A)

The weighted average of the atomic masses of an atom's naturally occurring isotopes. It may be expressed in terms of atomic mass units (on an atomic basis), or the mass per mole of atoms.

Atomic mass unit (amu)

A measure of atomic mass; defined as 1/12 of the mass of an atom of carbon-12 (^{12}C).

Mole

The quantity of a substance corresponding to 6.022 \times 10^{23} atoms or molecules.

Quantum mechanics

A branch of physics that deals with atomic and subatomic systems; it allows only discrete values of energy. By contrast, for classical mechanics, continuous energy values are permissible.

Bohr atomic model

An early atomic model in which electrons are assumed to revolve around the nucleus in discrete orbitals.

Wave mechanical model

An atomic model in which electrons are treated as being wavelike.

Quantum number

A set of 4 numbers, the values of which are used to label possible electron states. Three of the quantum numbers are integers that specify the size, shape, and spatial orientation of an electron's probability density; the fourth number designates spin orientation.

Electron state (level)

One of a set of discrete, quantized energies that are allowed for electrons. In the atomic case each state is specified by 4 quantum numbers.

Pauli exclusion principle

The postulate that for an individual atom, at most 2 electrons, which necessarily have opposite spins, can occupy the same state.

Ground state

A normally filled electron energy state from which an electron excitation may occur.

Electron configuration

For an atom, the manner in which possible electron states are filled with electrons.

Valence electrons

The electrons in the outermost occupied electron shell, which participate in interatomic bonding.

Periodic table

The arrangement of the chemical elements with increasing atomic number according to the periodic variation in electron structure. Nonmetallic elements are positioned at the far right-hand side of the table.

Electropositive

For an atom, having a tendency to release valence electrons. Also, a term used to describe metallic elements.

Electronegative

For an atom, having a tendency to accept valence electrons. Also used to describe nonmetallic elements.