A-Level Chemistry
Compounds, Reactions & Equations
Year 1 Foundation Notes
Inside this topic
Atomic structure, periodic table foundations, bonding, formulae, state symbols, common ions, balancing equations and ionic
equations.
Includes worked examples, common mistakes, 25 original practice questions and answers.
Independent revision resource - not affiliated with or endorsed by any school, college or examination board.
Compounds, Reactions & Equations Page 1
, 1. Atomic structure
All chemistry revolves around atoms. An atom is the smallest particle of an element that retains the chemical properties
of that element.
Particle Relative charge Relative mass Location
Proton +1 1 Nucleus
Neutron 0 1 Nucleus
Electron -1 Very small (about 1/2000) Outside nucleus
The nucleus contains protons and neutrons, so almost all of an atom's mass is concentrated there. The number of
protons defines the element and is called the atomic number.
Every carbon atom contains six protons. An atom with seven protons cannot be carbon: it is nitrogen.
In a neutral atom, number of protons = number of electrons, so the positive and negative charges balance.
Electron arrangements
As a simple starting model, electrons occupy different energy levels (shells) around the nucleus. Familiar examples
include sodium 2,8,1; magnesium 2,8,2; chlorine 2,8,7; and argon 2,8,8.
Outer electrons strongly affect bonding and reactivity. At A-Level, this simple model is developed further into principal
energy levels, sub-shells and orbitals.
Quick check
Why is an atom with 12 protons always magnesium? Because proton number defines the element.
Compounds, Reactions & Equations Page 2
Compounds, Reactions & Equations
Year 1 Foundation Notes
Inside this topic
Atomic structure, periodic table foundations, bonding, formulae, state symbols, common ions, balancing equations and ionic
equations.
Includes worked examples, common mistakes, 25 original practice questions and answers.
Independent revision resource - not affiliated with or endorsed by any school, college or examination board.
Compounds, Reactions & Equations Page 1
, 1. Atomic structure
All chemistry revolves around atoms. An atom is the smallest particle of an element that retains the chemical properties
of that element.
Particle Relative charge Relative mass Location
Proton +1 1 Nucleus
Neutron 0 1 Nucleus
Electron -1 Very small (about 1/2000) Outside nucleus
The nucleus contains protons and neutrons, so almost all of an atom's mass is concentrated there. The number of
protons defines the element and is called the atomic number.
Every carbon atom contains six protons. An atom with seven protons cannot be carbon: it is nitrogen.
In a neutral atom, number of protons = number of electrons, so the positive and negative charges balance.
Electron arrangements
As a simple starting model, electrons occupy different energy levels (shells) around the nucleus. Familiar examples
include sodium 2,8,1; magnesium 2,8,2; chlorine 2,8,7; and argon 2,8,8.
Outer electrons strongly affect bonding and reactivity. At A-Level, this simple model is developed further into principal
energy levels, sub-shells and orbitals.
Quick check
Why is an atom with 12 protons always magnesium? Because proton number defines the element.
Compounds, Reactions & Equations Page 2
