What is inside an atom, and how do isotopes give an element its relative atomic mass?
Describe sub-atomic particles and their relative masses and charges, work out particle numbers from atomic and mass number, define isotopes, and calculate relative atomic mass.
A focused answer to WJEC GCSE Chemistry topic 1.2, covering protons, neutrons and electrons and their relative masses and charges, atomic number and mass number, working out particle numbers for atoms and ions, isotopes, and calculating relative atomic mass.
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What this topic is asking
WJEC topic 1.2 wants you to describe the structure of the atom - the nucleus of protons and neutrons surrounded by electrons - and the relative masses and charges of these particles. You must work out the number of each particle in an atom or ion from the atomic number and mass number, define isotopes, and calculate relative atomic mass from isotopic abundances.
The sub-atomic particles
The relative masses and charges are:
- Proton: relative mass , relative charge , in the nucleus.
- Neutron: relative mass , relative charge , in the nucleus.
- Electron: relative mass negligible (about ), relative charge , in shells around the nucleus.
Almost all the mass of the atom is in the nucleus, because electrons have negligible mass. An atom is neutral overall because it has equal numbers of protons and electrons.
Atomic number and mass number
For sodium, atomic number and mass number : there are 11 protons, 11 electrons and neutrons.
Working out the particles in an ion
An ion forms when an atom loses or gains electrons, so it has a charge. The number of protons and neutrons does not change - only the electrons do.
- A positive ion (cation) has lost electrons, so it has fewer electrons than protons. For example, has 11 protons but only 10 electrons.
- A negative ion (anion) has gained electrons, so it has more electrons than protons. For example, has 17 protons and 18 electrons.
Isotopes
Because isotopes have the same number of electrons, they have identical chemical properties (chemistry depends on the electrons). They differ only slightly in physical properties such as density. For example, chlorine-35 and chlorine-37 both have 17 protons and 17 electrons, but chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons.
Relative atomic mass
The relative atomic mass () of an element is the weighted mean mass of its atoms, taking into account the masses of the isotopes and how common each one is (its abundance):
Because it is an average, the relative atomic mass is usually not a whole number (chlorine is ). Relative masses are measured compared with carbon-12 and have no units.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC sample4 marksAn atom has a mass number of 39 and an atomic number of 19. State the number of protons, neutrons and electrons, and explain how you found the number of neutrons.Show worked answer →
A Unit 1.2 structured question. Reward: the atomic number 19 equals the number of protons, so there are 19 protons. A neutral atom has equal protons and electrons, so there are 19 electrons. The number of neutrons is the mass number minus the atomic number: neutrons. Markers credit 19 protons, 19 electrons, 20 neutrons, and the explanation that neutrons = mass number minus atomic number. A common slip is to subtract the wrong way round or to forget protons and electrons are equal in a neutral atom.
WJEC sample3 marksChlorine exists as chlorine-35 (75 percent) and chlorine-37 (25 percent). Calculate the relative atomic mass of chlorine.Show worked answer →
A Unit 1.2 calculation. Reward: the relative atomic mass is the weighted mean of the isotope masses: . Markers credit the weighted-mean method (multiplying each mass by its abundance) and the answer 35.5. A common error is to take a simple average of 35 and 37 (giving 36) instead of weighting by abundance.
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Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)