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What are the building blocks of the atom, and how do their charges and masses define each element and isotope?

Protons, neutrons and electrons, their relative charges and masses, proton number, nucleon number, isotopes and the use of the notation for representing nuclides, and specific charge.

A focused answer to AQA A-Level Physics 3.2.1.1, covering protons, neutrons and electrons with their relative charges and masses, proton and nucleon number, isotopes, nuclide notation, and how to calculate specific charge.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. The three constituents
  3. Proton number, nucleon number and isotopes
  4. Nuclide notation
  5. Specific charge
  6. Try this

What this dot point is asking

AQA specification point 3.2.1.1 wants you to know the three constituents of the atom and their relative charges and masses, use proton and nucleon number, explain isotopes, read and write nuclide notation, and calculate specific charge.

The three constituents

Protons and neutrons (together called nucleons) sit in the nucleus; electrons orbit in shells around it. A neutral atom has equal numbers of protons and electrons, so the charges balance. The actual masses are 1.67×10−27 kg1.67 \times 10^{-27} \text{ kg} for a nucleon and 9.11×10−31 kg9.11 \times 10^{-31} \text{ kg} for an electron.

Proton number, nucleon number and isotopes

Isotopes have identical chemical properties (because chemistry depends on the electrons, set by ZZ) but different masses and nuclear stability. Carbon-12 and carbon-14 are isotopes: both have 6 protons, but 6 and 8 neutrons respectively. Some isotopes are stable and some are radioactive, which is why carbon-14 can be used for dating while carbon-12 cannot. The relative atomic mass of an element is the average mass of its isotopes, weighted by their natural abundance, which is why it is rarely a whole number.

Nuclide notation

A nuclide is written with the nucleon number above and the proton number below the chemical symbol, for example carbon-14 is written as 614C^{14}_{6}\text{C}. Here A=14A = 14 and Z=6Z = 6, so there are 66 protons and 14−6=814 - 6 = 8 neutrons. The notation lets you read off the full composition of a nucleus at a glance, and it is essential for balancing nuclear equations.

Specific charge

Try this

Q1. State the number of protons, neutrons and electrons in a neutral atom of 1123Na^{23}_{11}\text{Na}. [2 marks]

  • Cue. 1111 protons, 23−11=1223 - 11 = 12 neutrons, 1111 electrons.

Q2. Explain why the electron has a much larger specific charge than the proton. [2 marks]

  • Cue. They carry the same magnitude of charge, but the electron has a far smaller mass, so Qm\dfrac{Q}{m} is larger.

Q3. State what defines which element an atom is. [1 mark]

  • Cue. Its proton number ZZ.

Exam-style practice questions

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

AQA 20184 marksA nucleus of aluminium-27 has a proton number of 1313. Calculate the specific charge of this nucleus. Take the charge on a proton as 1.60×10−19 C1.60 \times 10^{-19} \text{ C} and the mass of a nucleon as 1.67×10−27 kg1.67 \times 10^{-27} \text{ kg}.
Show worked answer →

The charge is from the 1313 protons: Q=13×1.60×10−19=2.08×10−18 CQ = 13 \times 1.60 \times 10^{-19} = 2.08 \times 10^{-18} \text{ C}.

The mass is from the 2727 nucleons: m=27×1.67×10−27=4.51×10−26 kgm = 27 \times 1.67 \times 10^{-27} = 4.51 \times 10^{-26} \text{ kg}.

Specific charge =Qm=2.08×10−184.51×10−26=4.6×107 C kg−1= \dfrac{Q}{m} = \dfrac{2.08 \times 10^{-18}}{4.51 \times 10^{-26}} = 4.6 \times 10^{7} \text{ C kg}^{-1}.

Markers reward using the proton number for the charge, the nucleon number for the mass, and dividing to get the specific charge.

AQA 20213 marksDefine the term isotope, and explain why the electron has a much larger specific charge than the proton.
Show worked answer →

Isotopes are atoms (or nuclei) of the same element, with the same proton number, but different numbers of neutrons, so they have different nucleon numbers.

The electron and proton carry the same magnitude of charge (1.60×10−19 C1.60 \times 10^{-19} \text{ C}), but the electron has a far smaller mass (about 11840\dfrac{1}{1840} of the proton's). Since specific charge is Qm\dfrac{Q}{m}, the much smaller mass makes the electron's specific charge much larger.

Markers reward a correct definition of isotope (same ZZ, different NN or AA) and linking the larger specific charge to the electron's small mass.

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