AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Atomic structure and the periodic table › A simple model of the atom, symbols, relative atomic mass,
Practise Size and mass of atoms. 5 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How big atoms and nuclei are, where the mass of an atom is, mass number and isotopes, and how to work out the numbers of protons, neutrons and electrons in an atom or ion. Particle-counting questions and standard form calculations come up on almost every paper.
Key facts
Radius of an atom ≈ 0.1 nm = 1 × 10−10 m
Radius of nucleus < 1/10 000 of the atom's radius (≈ 1 × 10−14 m)
1 nm = 1 × 10−9 m
Relative mass: proton 1, neutron 1, electron very small
Almost all the mass of an atom is in the nucleus
Mass number = protons + neutrons
Neutrons = mass number − atomic number
Isotopes: same number of protons, different numbers of neutrons
Ions: a positive ion has fewer electrons than protons (Al3+: 3 fewer); a negative ion has more (O2−: 2 more)
Notes
How small atoms are
Atoms are very small: the radius of an atom is about 0.1 nm (1 × 10−10 m). 1 nm = 1 × 10−9 m.
The radius of the nucleus is less than 1/10 000 of the radius of the atom (about 1 × 10−14 m).
To picture the scale: if an atom were as wide as a 100 m sports field, its nucleus would be less than 1 cm across.
Where the mass is
diagram
Relative masses: proton 1, neutron 1, electron very small.
Protons and neutrons are both in the nucleus, so almost all the mass of an atom is in the nucleus.
The mass number is the total number of protons and neutrons in an atom.
An atom can be written as \({}^{23}_{11}\text{Na}\): the top number (23) is the mass number and the bottom number (11) is the atomic number.
Top number: mass number. Bottom number: atomic number.
Counting the particles
Protons = atomic number.
Neutrons = mass number − atomic number. For \({}^{23}_{11}\text{Na}\): 23 − 11 = 12 neutrons.
Electrons in an atom = number of protons.
In an ion, the protons and neutrons are unchanged. A 2+ ion has 2 fewer electrons than protons; a 1− ion has 1 more. For example, Mg2+ (atomic number 12) has 12 protons and 10 electrons.
Isotopes
diagram
Isotopes are atoms of the same element with different numbers of neutrons.
Isotopes have the same atomic number but different mass numbers, e.g. \({}^{35}_{17}\text{Cl}\) has 18 neutrons and \({}^{37}_{17}\text{Cl}\) has 20 neutrons.
Isotopes: same number of protons, different numbers of neutrons.
Isotopes of an element have the same number of electrons, so they react in the same way.
How to answer each type of question
Work out the numbers of protons, neutrons and electrons
3 to 4 marksGrade 5
Protons = atomic number (the bottom number).
Neutrons = mass number − atomic number.
Electrons = protons for an atom; adjust for the charge if it is an ion.
Example. An atom of aluminium is represented as \({}^{27}_{13}\text{Al}\). (a) Give the numbers of protons, neutrons and electrons in this atom. (b) Aluminium forms the ion Al3+. Give the number of electrons in this ion.