TOPIC 1: atomic theory

Atomic Number (Z)

Atomic Number (Z)

The number of protons in the nucleus of an atom of an element. Represented by the symbol Z. Also called the proton number.

Aufbau Principle

The principle stating that electrons occupy the lowest energy sub-shells first.

Diatomic molecule

A molecule consisting of two atoms.

Electron

A negatively-charged fundamental particle found in energy levels around the nucleus of an atom. The mass of an electron is 1/1840 the mass of a proton or neutron.

Electronic Configuration

The number of electrons in each sub-shell in each quantum shell in an atom.

First Ionisation Energy (1IE)

The amount of energy required to remove one mole of electrons from one mole of gaseous atoms.

Hund’s Rule

The rule stating that single electrons fill all empty orbitals within a sub-shell before they start to form pairs in orbitals.

Ion

A charged atom or molecule.

Ionisation

The stage of mass spectrometry during which the atoms of the sample lose electrons to become positive ions.

Isotopes

Atoms of the same element with different numbers of neutrons, and hence different masses.

Mass Number (A)

The number of protons plus the number of neutrons in an atom. Represented by the symbol A.

Mass Spectrometry

An accurate instrumental technique that can be used to determine the relative isotopic mass and the relative abundance for each isotope of an element in a sample.

Mass-to-Charge Ratio (m/z)

The mass of an ion divided by its charge.

Molecular Ion Peak (M⁺)

The peak with the highest m/z in a mass spectrum.

Neutron

A neutral particle found in the nucleus of an atom. Has a relative mass of 1.

Orbital

A region within a sub-shell in which there is a high probability of finding an electron. All orbitals hold a maximum of two electrons each with opposite spins.

Pauli Exclusion Principle

The principle stating that each orbital may hold a maximum of two electrons each of opposite spins.

Period

A row in the Periodic Table.

Periodicity

The regularly repeating pattern of atomic, physical, and chemical properties with increasing atomic number.

Proton

A positively-charged particle found in the nucleus of an atom. Has a relative mass of 1.

Proton Number (Z)

The number of protons in the nucleus of an atom of an element. Represented by the symbol Z. Also called the atomic number.

Quantum Shell

Also just ‘shell’. The energy levels which an electron can occupy in an atom.

Relative Atomic Mass (Aᵣ)

The average mass of an atom of an element, taking into account the natural abundances of its isotopes, relative to 1/12 the mass of an atom of carbon-12.

Relative Formula Mass

The sum of the relative atomic masses of the atoms in the numbers shown in the formula.

Relative Isotopic Mass

The mass of an atom of a particular isotope of an element relative to 1/12 the mass of an atom of carbon-12.

Relative Molecular Mass (Mᵣ)

The weighted average of the masses of the molecules of a substance relative to 1/12 the mass of an atom of carbon-12.

Spin

A fundamental property of an electron. Electrons can either spin up (↑) or spin down (↓).

Subatomic Particles

The particles of which an atom consists: protons, neutrons and electrons.

Sub-Shell

A division of a shell, represented by the letter s, p, d or f, which corresponds to an increase in energy. Different shells contain different numbers of sub-shells.

Successive Ionization Energies

The amounts of energy required to remove successive moles of electrons from the positive ions created by first ionization of an atom.