What is inside an atom, and how do we describe and count its particles?
The structure of the atom in terms of protons, neutrons and electrons, atomic number and mass number, isotopes, and the calculation of relative atomic mass from isotopic abundances.
A CCEA GCSE Chemistry answer on atomic structure, covering the masses and charges of protons, neutrons and electrons, atomic number and mass number, how to read symbols, isotopes, and how to calculate relative atomic mass from isotopic abundances.
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What this dot point is asking
CCEA wants you to describe the atom in terms of protons, neutrons and electrons with their masses and charges, use atomic number and mass number to count the particles in any atom or ion, explain what isotopes are, and calculate a relative atomic mass from isotopic abundances.
The three subatomic particles
Almost all the mass of an atom is in the nucleus, because electrons are about 1840 times lighter than protons. A neutral atom has equal numbers of protons and electrons, so the positive and negative charges cancel.
Atomic number and mass number
From these two numbers you can find every particle count:
- Number of protons = atomic number.
- Number of electrons = atomic number (in a neutral atom).
- Number of neutrons = mass number minus atomic number.
For an ion, the proton number stays the same but the electron count changes: a positive ion has lost electrons and a negative ion has gained them. For example has 11 protons but 10 electrons.
Isotopes
Because they have the same number of protons and electrons, isotopes have identical chemical properties, since chemistry depends on the electrons. They differ only in mass, which affects physical properties such as density slightly. Carbon-12 and carbon-14 are isotopes: both have 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8.
Relative atomic mass
To calculate it, multiply each isotope mass by its percentage abundance, add the results and divide by 100.
Examples in context
Example 1. Radioactive dating. Carbon-14 is a naturally occurring isotope of carbon used to date once-living material. Because it has the same chemistry as ordinary carbon-12, living things take it up normally, but it decays after death, so its falling proportion gives an age. This relies directly on the idea that isotopes share chemistry but differ in their nuclei.
Example 2. Why the Periodic Table uses averages. Chlorine is listed as 35.5 even though no single chlorine atom has that mass. The figure is the weighted mean of chlorine-35 and chlorine-37, which is why relative atomic masses are decimals. Chemists use these averages because reactions involve huge numbers of atoms in their natural isotopic mix.
Try this
Q1. State what is meant by the term isotopes. [2 marks]
- Cue. Atoms of the same element with the same number of protons but different numbers of neutrons.
Q2. An atom has 17 protons, 18 neutrons and 17 electrons. State its atomic number and mass number. [2 marks]
- Cue. Atomic number 17; mass number 35 (17 plus 18).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20184 marksAn atom of magnesium is written as Mg with mass number 24 and atomic number 12. State the number of protons, neutrons and electrons it contains, and explain how you worked out the number of neutrons.Show worked answer →
Markers want the three numbers and the method for the neutrons.
The atomic number is 12, so there are 12 protons. A neutral atom has equal numbers of protons and electrons, so there are 12 electrons.
The number of neutrons is the mass number minus the atomic number, because the mass number counts protons plus neutrons. So neutrons , giving 12 neutrons.
Markers reward 12 protons, 12 electrons, 12 neutrons, and the explicit working neutrons equals mass number minus atomic number.
CCEA 20213 marksChlorine has two isotopes, chlorine-35 (75 percent) and chlorine-37 (25 percent). Calculate the relative atomic mass of chlorine.Show worked answer →
The marks are for a correct weighted-mean method and answer.
Relative atomic mass is the weighted average of the isotope masses, using the percentage abundances as weights:
Working out the top line: and , giving . Dividing by 100 gives .
Markers reward multiplying each isotope mass by its abundance, adding them and dividing by 100, and the final answer of 35.5 (no units).
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)