Relative electrical charges of subatomic particles: The number of protons in an atom of an element is its atomic number. All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.

The idea

The atomic number (proton number) is the defining property of an element. It equals the number of protons in the nucleus of one atom. Every atom of a given element has the same proton count, so the atomic number is constant for that element. Atoms of different elements have different proton counts, so their atomic numbers differ. For example, every hydrogen atom has 1 proton, so its atomic number is 1; every carbon atom has 6 protons, so its atomic number is 6. In a neutral atom, the number of electrons equals the number of protons, so atomic number also tells you the electron count. The proton count never changes in chemical reactions; it only changes in nuclear processes.

What earns the marks

• State that the atomic number is the number of protons in an atom.
• Explain that all atoms of the same element have the same number of protons, so the atomic number is fixed for that element.
• Explain that atoms of different elements have different numbers of protons, so their atomic numbers are different.
• Use a specific element as an example, such as hydrogen with 1 proton or carbon with 6 protons, to show the link between proton number and atomic number.
• Recognise that in a neutral atom the number of electrons equals the number of protons, so atomic number also gives the electron count.

Common mistakes and how to put them right

• Confusing atomic number with mass number.

  Put it right: Atomic number counts protons only, while mass number counts protons plus neutrons.

• Thinking that atoms of the same element can have different numbers of protons.

  Put it right: All atoms of an element have the same proton count; differing neutron numbers produce isotopes, not different elements.

• Assuming the atomic number equals the number of neutrons.

  Put it right: The atomic number is the proton count; the neutron number is found by subtracting the atomic number from the mass number.

Exam tips

• Read the question carefully to check whether it asks for the atomic number, the mass number or the number of neutrons, and use the correct definition.
• When given a symbol such as ¹²C or ¹⁶O, use the lower number as the atomic number and the upper number as the mass number.
• Link the proton count to the element identity in your answer, for example by stating that 6 protons means the atom is carbon.