STUDENT SPACE · TOPIC 2

Proton number and isotopes

Use atomic symbols to count particles and recognise isotopes.

The essentials

Proton number Z identifies the element. Mass number A is the number of protons plus neutrons.
Neutrons = A − Z. A neutral atom has Z electrons.
Isotopes have the same proton number but different neutron numbers.

Try each question before opening its hint or answer. Use paper for calculations and diagrams.

Two neutral atoms each have 8 protons. One has 8 neutrons and the other has 9. How are they related?

Need a hint for Question 1?

Compare proton numbers first, then neutron numbers.

Check Question 1

They are isotopes of oxygen. They have the same proton number but different neutron numbers, giving mass numbers 16 and 17.

A neutral aluminium-27 atom has proton number 13. Give its numbers of protons, neutrons and electrons.

Need a hint for Question 2?

Use neutrons = mass number − proton number.

Check Question 2

13 protons, 14 neutrons and 13 electrons. Neutrons = 27 − 13 = 14.

Atoms P and Q have (Z = 6, A = 12) and (Z = 6, A = 14). Atom R has (Z = 7, A = 14). Which pair are isotopes? Explain.

Need a hint for Question 3?

Compare proton numbers first, then neutron numbers.

Check Question 3

P and Q. Both have 6 protons, but P has 6 neutrons and Q has 8. R is a different element.

Use the matching chapter worksheet. Attempt independently, compare your reasoning with your partner, then bring unresolved questions to your teacher.

Change the number of neutrons and compare isotope mixtures.