Chemistry - Atomic structure - definitions

Matter & materials

Atomic number

The number of protons in the nucleus of the atom of an element.

Mass number

The number of neutrons and protons in an atomic nucleus.

Ion

An atom with a charge made from an atom by the loss or gain of electrons.

Anion

A negatively charged ion; formed when an atom gains electrons.

Cation

A positively charged ion formed when an atom loses electrons.

Orbital

A region of space around the nucleus where there is a 90% chance of finding an electron / The most probable regions in space where electrons that have the specific energy corresponding to the orbital are found.

Core electrons

Electrons that fill the inner energy levels.

Valance electrons

Electrons in the outermost (highest) energy level of an atom.

Valency

The number of bonds an atom can form.

Pauli’s exclusion principle

Only two electrons can occupy an orbital and these must spin in opposite direction

Hund’s rule

No pairing in p-orbitals (placing a second electron in an orbital) before there is not at least one electron in each of them.

Relative atomic mass

The mass of a particle on a scale where an atom of carbon-12 has a mass of

Isotopes

Atoms of the same element having the same number of protons (atomic number), but different numbers of neutrons (mass number).

Group

Vertical column of elements on the periodic table.

Row

Horizontal rows of elements on the periodic table.

Periodicity

The gradual change of physical and chemical properties in the periodic table that repeats itself.

Atomic radius

The distance from the nucleus to the border of the outer orbital.

Ionisation energy

Energy needed per mole to remove an electron from an atom in the gaseous phase.

First ionisation energy

Energy needed to remove the first electron from an atom in the gaseous phase.

Electronegativity

A measure of the tendency of an atom in a molecule to attract bonding electrons/ a pair of shared electrons in a chemical bond.