What is inside an atom, and what makes one isotope different from another?
The nuclear model of the atom, the charges and masses of protons, neutrons and electrons, atomic and mass number, and isotopes.
A CCEA GCSE Physics answer on the nuclear model of the atom, the relative charge and mass of protons, neutrons and electrons, atomic number and mass number, nuclide notation, and isotopes.
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What this dot point is asking
CCEA wants you to describe the nuclear model of the atom, state the relative charge and mass of the proton, neutron and electron, define atomic number and mass number, use nuclide notation, and explain isotopes. This is the foundation for all the nuclear topics.
The answer
The nuclear model
The three particles
Atomic number and mass number
In nuclide notation an atom is written with the mass number above and the atomic number below the symbol, for example carbon-14 as .
Isotopes
Worked example: counting particles
Examples in context
- Example 1. Carbon isotopes
- Carbon-12 and carbon-14 are both carbon with 6 protons, but carbon-14 has two extra neutrons. Carbon-14 is radioactive and is used to date ancient organic remains.
- Example 2. Hydrogen isotopes
- Ordinary hydrogen has one proton and no neutrons; deuterium has one neutron and tritium two. All are hydrogen because they each have one proton.
- Example 3. The plum pudding to nuclear model
- Early scientists pictured the atom as a ball of positive charge with electrons dotted in it (the plum pudding model). Scattering experiments, in which a few alpha particles bounced back from a thin gold foil, showed instead that the positive charge and mass are concentrated in a tiny central nucleus, giving the nuclear model we use today.
When you read nuclide notation, remember the larger number (on top) is always the mass number and the smaller (on the bottom) is the atomic number. From these two numbers you can read off the protons (atomic number), neutrons (mass number minus atomic number) and electrons (equal to protons in a neutral atom), which is the basis of many short exam questions.
Try this
Q1. State the relative charge and relative mass of a neutron. [2 marks]
- Cue. Charge 0; relative mass 1.
Q2. An atom has 11 protons and 12 neutrons. State its atomic number and mass number. [2 marks]
- Cue. Atomic number 11; mass number .
Q3. What is the same and what is different between two isotopes of an element? [2 marks]
- Cue. Same number of protons; different number of neutrons.
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 is written as carbon-14 with an atomic number of 6 and a mass number of 14. State the number of protons, neutrons and electrons in a neutral atom of this isotope.Show worked answer →
Protons = atomic number = 6.
Neutrons = mass number minus atomic number = 14 minus 6 = 8.
Electrons = protons in a neutral atom = 6.
Markers reward 6 protons, 8 neutrons and 6 electrons.
CCEA style3 marksExplain what is meant by isotopes, using an example.Show worked answer →
Isotopes are atoms of the same element (so the same number of protons) that have different numbers of neutrons (and so different mass numbers).
Example: carbon-12 and carbon-14 are both carbon (6 protons each) but have 6 and 8 neutrons respectively.
Markers reward: same number of protons (same element), different numbers of neutrons, with a valid example.
Related dot points
- Alpha, beta and gamma radiation and their properties, the random nature of decay, and writing balanced nuclear equations for alpha and beta decay.
A CCEA GCSE Physics answer on the nature and properties of alpha, beta and gamma radiation, the random nature of radioactive decay, and how to balance nuclear equations for alpha and beta decay.
- Half-life as the time for activity to halve, reading half-life from a decay curve, and calculations of remaining activity or mass.
A CCEA GCSE Physics answer on the meaning of half-life, how to read a half-life from a decay curve, and how to calculate the activity or mass of a radioactive source remaining after a number of half-lives.
- Uses of radioactive sources in medicine and industry, the dangers of ionising radiation, irradiation versus contamination, and safety precautions.
A CCEA GCSE Physics answer on the uses of radioactive sources in medicine and industry, how the choice of source depends on its radiation and half-life, the dangers of ionising radiation, and the difference between irradiation and contamination.
- Nuclear fission of heavy nuclei and the chain reaction, the parts of a nuclear reactor, and nuclear fusion in stars.
A CCEA GCSE Physics answer on nuclear fission of heavy nuclei and the chain reaction, the role of fuel rods, control rods and the moderator in a reactor, and nuclear fusion in stars and why it needs extreme conditions.
Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)