Unit 01: Atomic Structure Vocabulary

Vocabulary terms and definitions from the Cambridge International AS Level Chemistry Unit 01 lecture notes on Atomic Structure.

Nucleus

A very small, dense area at the center of an atom containing protons and neutrons.

Atomic Number

The total number of protons in an atom, also known as the proton number.

Mass Number

The total number of protons and neutrons in an atom, also known as the nucleon number.

Cation

A positively-charged ion formed when an atom loses one or more electrons.

Anion

A negatively-charged ion formed when an atom gains one or more electrons.

Isotopes

Atoms of the same element with the same number of protons but a different number of neutrons.

Radioisotopes

Unstable isotopes that are radioactive.

Principal Quantum Shell ($n$)

An energy level outside the nucleus in which electrons are arranged, numbered according to distance from the nucleus.

Atomic Orbital

A region of space around the nucleus where the probability of finding a particular electron is maximum ($>95\%$).

Degenerate Orbitals

Orbitals having the same energy, such as $p_x$, $p_y$, and $p_z$.

Aufbau Principle

A rule stating that in the ground state of an atom, electrons must occupy orbitals in order of increasing energy.

Pauli's Exclusion Principle

A rule stating that an orbital can accommodate a maximum of two electrons, which must have opposite spins.

Hund's Rule

A rule stating that in a set of degenerate orbitals, electrons must occupy the orbitals singly first before pairing.

Electronic Configuration

A description of how electrons in an atom or ion are arranged in their shells, sub-shells, and orbitals.

First Ionisation Energy ($\Delta H_{I.E.1}$)

The energy needed to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of gaseous $1+$ ions.

Second Ionisation Energy ($\Delta H_{I.E.2}$)

The energy needed to remove one electron from each gaseous $1+$ ion in one mole of ions to form one mole of gaseous $2+$ ions.

Shielding Effect

The process where electrons in full inner shells repel outer electrons and prevent the full nuclear charge from being felt by the outer electrons, also called screening.

Spin-Pair Repulsion

The extra repulsion experienced between a pair of electrons occupying the same orbital, resulting in less energy required to remove an electron.