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What is inside an atom, and how do we describe a nucleus using proton and nucleon numbers?

Describe the nuclear model of the atom and use proton number, nucleon number, and isotopes

A focused answer to the O-Level Physics outcome on atomic structure. The nuclear model with protons, neutrons, and electrons, proton and nucleon numbers, nuclide notation, and what isotopes are.

Generated by Claude Opus 4.87 min answer

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe the nuclear model of the atom, with a tiny dense nucleus of protons and neutrons surrounded by electrons, to use proton number and nucleon number, to read nuclide notation, and to explain what isotopes are. The big idea is that an atom is mostly empty space with its mass and positive charge concentrated in the nucleus.

The answer

The nuclear model

An atom has a tiny, dense, positively charged nucleus at its centre, containing protons and neutrons. Negatively charged electrons move around the nucleus, much further out. The atom is mostly empty space: the nucleus is thousands of times smaller than the atom but holds almost all its mass.

The three particles

Particle Charge Relative mass Location
Proton +1+1 11 Nucleus
Neutron 00 11 Nucleus
Electron −1-1 About 11840\tfrac{1}{1840} Around the nucleus

A neutral atom has equal numbers of protons and electrons, so the positive and negative charges balance.

Proton number and nucleon number

  • The proton number (atomic number) ZZ is the number of protons in the nucleus. It defines the element.
  • The nucleon number (mass number) AA is the total number of protons and neutrons in the nucleus.

The number of neutrons is the difference:

neutrons=A−Z\text{neutrons} = A - Z

Nuclide notation

An atom is written as ZAX^{A}_{Z}\text{X}, where X is the chemical symbol, AA the nucleon number (top), and ZZ the proton number (bottom). For example 1123Na^{23}_{11}\text{Na} has 1111 protons and 23−11=1223 - 11 = 12 neutrons.

Isotopes

Isotopes are atoms of the same element with the same proton number but different nucleon numbers, that is, the same number of protons but different numbers of neutrons. They have identical chemistry (same protons and electrons) but different masses.

Examples in context

Example 1. Carbon isotopes. Carbon exists mainly as 612C^{12}_{6}\text{C} but also as the heavier 614C^{14}_{6}\text{C}, which has two extra neutrons. Both behave the same chemically, so living things take up both, but 14C^{14}\text{C} is radioactive, which is the basis of carbon dating to find the age of ancient remains.

Example 2. Nuclear fuel. Natural uranium is mostly 238U^{238}\text{U} with a small amount of 235U^{235}\text{U}. The two are isotopes with the same chemistry, but only 235U^{235}\text{U} readily undergoes fission, so nuclear fuel must be enriched to increase the proportion of the lighter isotope. Recognising isotopes is essential to understanding nuclear power.

Try this

Q1. State what the proton number and nucleon number of an atom tell you. [2 marks]

  • Cue. The proton number is the number of protons (defining the element); the nucleon number is the total number of protons and neutrons.

Q2. An atom is 1840Ar^{40}_{18}\text{Ar}. State the number of protons and neutrons. [2 marks]

  • Cue. Protons =18= 18; neutrons =40−18=22= 40 - 18 = 22.

Q3. Define an isotope. [2 marks]

  • Cue. Atoms of the same element with the same proton number but different nucleon numbers (same protons, different neutrons).

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksAn atom is written as 1123Na^{23}_{11}\text{Na}. (a) State the proton number and the nucleon number. (b) State the number of protons, neutrons, and electrons in the neutral atom.
Show worked answer →

(a) The proton number (bottom) is 1111; the nucleon number (top) is 2323.

(b) Protons =11= 11 (equal to the proton number). Neutrons =23−11=12= 23 - 11 = 12 (nucleon number minus proton number). Electrons =11= 11, equal to the protons because the atom is neutral.

Markers reward identifying the proton number as 1111 and the nucleon number as 2323, neutrons as the difference (1212), and electrons equal to protons in a neutral atom.

Original4 marks(a) Define an isotope. (b) Chlorine has two isotopes, 1735Cl^{35}_{17}\text{Cl} and 1737Cl^{37}_{17}\text{Cl}. State what is the same and what is different about their atoms.
Show worked answer →

(a) Isotopes are atoms of the same element with the same proton number but different nucleon numbers (the same number of protons but different numbers of neutrons).

(b) Both have 1717 protons (and 1717 electrons), so they are the same element with the same chemistry. They differ in their number of neutrons: 35Cl^{35}\text{Cl} has 35−17=1835 - 17 = 18 neutrons, while 37Cl^{37}\text{Cl} has 37−17=2037 - 17 = 20 neutrons.

Markers reward the definition (same protons, different neutrons), the same proton number, and the different neutron numbers (1818 and 2020).

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