Chemistry

Atom

The smallest particle of an element that retains the properties of that element.

Dalton’s model

Atoms are solid, indivisible spheres; atoms of the same element are identical.

Thomson’s model

'Plum pudding' model — atoms contain negatively charged electrons embedded in a positive sphere.

Rutherford’s model

Atoms have a small, dense, positively charged nucleus surrounded by mostly empty space.

Bohr’s model

Electrons orbit the nucleus in fixed energy levels (shells).

Chadwick’s discovery

Identified the neutron, a neutral particle in the nucleus.

Atomic number (Z)

Number of protons in an atom; defines the element.

Mass number (A)

Total number of protons and neutrons in an atom’s nucleus.

Element symbol

One- or two-letter abbreviation (e.g., Fe for iron).

IUPAC notation

^A_ZX — shows element symbol X, atomic number Z, and mass number A.

Relative atomic mass (Ar)

The weighted average mass of an element’s atoms compared to 1/12 the mass of a carbon-12 atom.

Isotopes

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

Isotopic composition

The percentage abundance of each isotope of an element.

Chemical properties

Determined by electron configuration, so isotopes react the same chemically.

Physical properties

Vary between isotopes (e.g., density, melting point).

Ionisation

The process of removing or adding electrons to form charged particles (ions).

Mass spectrometer

Instrument that measures the mass-to-charge ratio of ions.

Mass spectrum

Graph showing relative abundance of isotopes based on their masses.

Relative abundance

The percentage of each isotope present in a sample.

Nucleus

Central core containing protons and neutrons.

Proton

Positively charged particle, mass ≈ 1 amu.

Neutron

Neutral particle, mass ≈ 1 amu.

Electron

Negatively charged particle with negligible mass.

Electron configuration

Arrangement of electrons in energy levels (e.g., 2,8,8,2).

Energy levels (shells)

Fixed regions around the nucleus where electrons reside.

Electrostatic attraction

Force between positive nucleus and negative electrons.

Flame test

Heating a sample in a flame causes electrons to jump energy levels; emitted light colour identifies the element.

Emission spectrum

Coloured lines produced by electrons releasing energy as they return to lower levels.

Absorption spectrum (AAS)

Measures the light absorbed when electrons move to higher energy levels; used to determine element concentration.

Periodic table

Arrangement of elements in order of increasing atomic number.

Group

Vertical column in the periodic table; elements in a group share similar chemical properties.

Period

Horizontal row; shows gradual changes in properties across the row.

Atomic radius

Distance from the nucleus to the outermost electrons; decreases across a period, increases down a group.

Valency

The combining power of an atom — number of electrons lost, gained, or shared in bonding.

First ionisation energy

Energy required to remove the most loosely bound electron from one mole of gaseous atoms.

Electronegativity

The ability of an atom to attract electrons in a bond.