Describe the structure of nuclei of isotopes using the terms atomic…

The idea

Every nucleus contains protons and neutrons. The atomic (proton) number Z counts the protons and fixes the element; the mass (nucleon) number A counts protons plus neutrons. Isotopes of one element share Z but differ in neutron number, so they differ in A. Nuclear symbol notation places A as a leading superscript and Z as a leading subscript before the element symbol, so carbon-13 with six protons and seven neutrons is written with 13 above and 6 below the C. In a multiple-choice question, check both numbers against the particle counts before choosing: protons give the lower figure, protons plus neutrons give the upper figure.

What earns the marks

• Identifies the atomic (proton) number as the number of protons in the nucleus and so the value written as the lower leading figure.
• Identifies the mass (nucleon) number as the total number of protons plus neutrons and so the value written as the upper leading figure.
• States that isotopes of an element have the same proton number but different neutron numbers, giving different mass numbers.
• Selects the symbol in which the mass number appears as a superscript and the proton number as a subscript before the element symbol.
• Calculates the neutron number by subtracting the proton number from the mass number for a stated isotope.

Common mistakes and how to put them right

• Confuses the two leading figures and places the proton number above the element symbol.

  Put it right: Keep the mass number above and the proton number below.

• Counts only protons when finding the mass number.

  Put it right: Add protons and neutrons together.

• Assumes isotopes differ in proton number.

  Put it right: Keep the proton number the same and vary the neutrons.

• Treats the neutron number as the upper figure.

  Put it right: Derive neutrons by subtracting protons from the mass number.

Exam tips

• Read the particle counts in the question stem first, then match them to the upper and lower figures in each option.
• Eliminate any option in which the lower figure differs between two isotopes of the same element.
• Check the position of each number relative to the element symbol before committing to an answer.