What is an atom made of, and what are isotopes?
The structure of the atom in terms of protons, neutrons and electrons, their relative charges and masses, atomic number and mass number, isotopes, and calculating relative atomic mass from isotopic abundances.
A focused CCEA GCSE Double Award Science (Chemistry Unit C1) answer on atomic structure, covering protons, neutrons and electrons with their charges and masses, atomic number and mass number, isotopes, and calculating relative atomic mass from isotopic abundances.
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What this dot point is asking
CCEA Double Award wants the sub-atomic particles with their charges and masses, how atomic number and mass number describe an atom, what isotopes are, and how to calculate relative atomic mass from the abundances of isotopes. The particle counts and the relative-atomic-mass sum are reliable marks.
The sub-atomic particles
Almost all the mass of an atom is in the nucleus, because the electrons are so light. An atom is neutral overall because the number of protons equals the number of electrons.
Atomic number and mass number
To find the number of neutrons, subtract the atomic number from the mass number. For example, an atom with mass number 23 and atomic number 11 has 11 protons, 11 electrons and 23 minus 11, which is 12 neutrons.
Isotopes
Because isotopes have the same number of electrons, they have the same chemical properties. They differ only in mass. Chlorine, for example, exists as chlorine-35 and chlorine-37. Both react in exactly the same way because chemistry depends on electrons, not on the number of neutrons in the nucleus.
The history of the atom is useful background: early models pictured the atom as a solid sphere, then as a "plum pudding" of negative electrons in a positive cloud. The nuclear model used today - a tiny dense positive nucleus with electrons in shells around mostly empty space - came from experiments that fired particles at thin metal foil and found that most passed straight through. This is the model the rest of chemistry is built on.
Calculating relative atomic mass
The relative atomic mass is the weighted mean mass of the isotopes, taking their abundances into account.
Examples in context
Example 1. Why the Periodic Table shows 35.5 for chlorine. Chlorine is a mixture of two isotopes, so its quoted relative atomic mass is the weighted average of the two masses, not a whole number. This explains the decimal values seen on the table for many elements.
Example 2. Carbon dating uses isotopes. Carbon has isotopes including carbon-12 and the radioactive carbon-14. They behave the same chemically, but carbon-14 decays over time, which is the basis of carbon dating. This shows isotopes differing in mass and stability but not chemistry.
Try this
Q1. What is the relative charge and mass of a neutron? [2 marks]
- Cue. Charge 0, relative mass 1.
Q2. How do you find the number of neutrons in an atom? [1 mark]
- Cue. Mass number minus atomic number.
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-style3 marksAn atom has atomic number 11 and mass number 23. State the number of protons, neutrons and electrons it contains.Show worked answer →
Use the two numbers for three marks.
Protons equal the atomic number, so 11 protons.
Electrons equal the protons in a neutral atom, so 11 electrons.
Neutrons equal mass number minus atomic number, so 23 minus 11, which is 12 neutrons.
Markers reward 11 protons, 11 electrons and 12 neutrons.
CCEA-style3 marksChlorine has two isotopes: 75 percent chlorine-35 and 25 percent chlorine-37. Calculate its relative atomic mass.Show worked answer →
Use a weighted mean for three marks.
Multiply each mass by its percentage: (35 times 75) plus (37 times 25), which is 2625 plus 925, equals 3550.
Divide by 100: 3550 divided by 100 equals 35.5.
So the relative atomic mass of chlorine is 35.5. Markers reward the weighted-mean method and the answer of 35.5.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)