Atoms, Ions & Reactions

Atom

The smallest particle of an element that still has the properties of that element.

Proton

Positively charged particle found in the nucleus. Number of protons = atomic number.

Neutron

Neutral particle found in the nucleus. Mass number = protons + neutrons.

Electron

Negatively charged particle that orbits the nucleus in shells / energy levels.

Nucleus

Central part of the atom containing protons and neutrons.

Atomic number

Number of protons in an atom (equals number of electrons in a neutral atom).

Mass number

Total number of protons + neutrons in the nucleus.

Electron arrangement / configuration

The way electrons are arranged in shells (e.g. sodium 2,8,1).

Valence electrons

Electrons in the outermost shell; they take part in bonding.

Ion

A charged atom or group of atoms formed by gaining or losing electrons.

Cation

A positive ion formed when a metal atom loses one or more electrons.

Anion

A negative ion formed when a non-metal atom gains one or more electrons.

Ionic compound

Compound made of positive and negative ions held together by electrostatic attraction. Overall charge is zero.

Ionic formula

Shows the simplest ratio of ions so that total positive charge equals total negative charge.

Reactants

Substances present at the start of a chemical reaction (left of the arrow).

Products

New substances formed in a chemical reaction (right of the arrow).

Word equation

A chemical reaction written using the names of the substances.

Symbol equation

A chemical reaction written using chemical formulae.

Balanced equation

An equation with the same number of each type of atom on both sides (conservation of mass).

Conservation of mass / matter

Atoms are not created or destroyed in a chemical reaction; they are only rearranged.