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IGCSE Chemistry · 0620

IGCSE Chemistry: Atoms, Elements and Compounds Questions

Atomic structure underpins the whole of Chemistry, and these are among the most predictable marks on the paper. These questions cover isotopes, electron configuration and working out subatomic particles from the mass and atomic numbers.

What you need to know

  • The atomic number is the number of protons; the mass number is protons plus neutrons.
  • In a neutral atom, the number of electrons equals the number of protons.
  • Isotopes are atoms of the same element with the same protons but different numbers of neutrons.
  • Electrons fill shells in the order 2, then 8, then 8 for the first twenty elements.
nucleus electron shells 2, 8, 8
Electrons occupy shells around the nucleus, filling 2 then 8 then 8.

Practice questions with answers

Question 12 marks

Define the term isotopes.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, and therefore different mass numbers.

Mark schemeSame element, same number of protons (1). Different number of neutrons, or different mass number (1).

Common mistakeSaying isotopes have different numbers of protons. The proton number is the same; the neutron number differs.

Exam tipState both the same protons and the different neutrons. One point alone is not enough.

Question 22 marks

Write the electronic configuration of a chlorine atom. (Chlorine has an atomic number of 17)

Chlorine has 17 electrons, arranged as 2, 8, 7.

Mark schemeFirst two shells correct, 2 and 8 (1). Outer shell 7, total 17 (1).

Common mistakeOverfilling the first shell or losing count so the electrons do not total 17.

Exam tipCheck your shells add up to the atomic number before moving on.

Question 33 marks

A sodium atom is written as mass number 23, atomic number 11. State the number of protons, neutrons and electrons it contains.

Protons = 11 (the atomic number). Electrons = 11 (equal to protons in a neutral atom). Neutrons = 23 minus 11 = 12 (mass number minus atomic number).

Mark schemeProtons 11 (1). Electrons 11 (1). Neutrons 12 (1).

Common mistakeUsing the mass number as the number of protons.

Exam tipNeutrons equals mass number minus atomic number. Learn this as a one-line rule.

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Frequently asked questions

How do you find the number of neutrons?

Subtract the atomic number from the mass number. Neutrons equals mass number minus atomic number.

What is the difference between an element and a compound?

An element contains only one type of atom. A compound contains two or more different elements chemically bonded together in fixed proportions.

Why do isotopes have the same chemical properties?

Chemical properties depend on the number and arrangement of electrons. Isotopes have the same number of electrons, so they react in the same way.

How are electrons arranged in shells?

Electrons fill the shell nearest the nucleus first. For the first twenty elements the order is 2, then 8, then 8.

More IGCSE Chemistry practice

StoichiometryElectrochemistryAcids, bases and salts