IGCSE Chemistry: Atoms, Elements and Compounds Practice Questions
An atom contains protons and neutrons in a central nucleus with electrons arranged in shells around it. The proton number identifies the element, and the nucleon number is the total number of protons and neutrons.
Topic 2 of Cambridge IGCSE Chemistry 0620 supplies the model used for the rest of the course. The two questions that recur most are working out particle numbers from nuclide notation and explaining why isotopes have identical chemical properties. The questions below cover both, plus the element, compound and mixture distinction.
What you need to know for Atoms, Elements and Compounds
- Subatomic particlesProton: relative charge 1 plus, relative mass 1, found in the nucleus. Neutron: charge 0, relative mass 1, found in the nucleus. Electron: charge 1 minus, relative mass negligible, found in shells around the nucleus.
- Proton number and nucleon numberProton number, also called atomic number, is the number of protons and defines the element. Nucleon number, also called mass number, is the total number of protons and neutrons.
- Finding particle numbersProtons equal the proton number. Neutrons equal nucleon number minus proton number. In a neutral atom electrons equal protons. In an ion, add electrons for a negative charge and subtract for a positive charge.
- IsotopesAtoms of the same element with the same proton number but different numbers of neutrons. They have identical chemical properties because chemical reactions depend on electron arrangement, which is the same.
- Electron shellsThe first shell holds up to 2 electrons and the next shells hold up to 8 for the elements met at IGCSE. Sodium is written 2,8,1.
- Element, compound and mixtureAn element contains only one type of atom. A compound contains two or more elements chemically combined in fixed proportions. A mixture contains two or more substances not chemically combined, which keep their own properties and can be separated physically.
IGCSE Chemistry Atoms, Elements and Compounds questions and answers
4 exam-style questions written to the 0620 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
An atom has a proton number of 17 and a nucleon number of 35. State the number of protons, neutrons and electrons it contains, and give its electronic structure.
Show the worked answer
- Protons equal the proton number, so there are 17 protons.
- Neutrons equal nucleon number minus proton number: 35 minus 17 equals 18 neutrons.
- The atom is neutral, so electrons equal protons: 17 electrons.
- Filling the shells: 2 in the first, 8 in the second, leaving 7 in the third. The structure is 2,8,7, which places the element in Group VII, Period 3. It is chlorine.
Chlorine exists as two isotopes, chlorine-35 and chlorine-37. Explain what is meant by isotopes and explain why the two isotopes have identical chemical properties.
Show the worked answer
- Isotopes are atoms of the same element, so they have the same proton number, but different numbers of neutrons and therefore different nucleon numbers.
- Chlorine-35 has 17 protons and 18 neutrons. Chlorine-37 has 17 protons and 20 neutrons.
- Both have 17 electrons arranged as 2,8,7, because the number of electrons equals the number of protons in a neutral atom.
- Chemical reactions involve only the outer shell electrons. Since both isotopes have the same electron arrangement, they react in exactly the same way. Their physical properties, such as density, differ slightly because of the different masses.
Explain the difference between a compound and a mixture, using air and water as examples.
Show the worked answer
- Water is a compound. Its elements, hydrogen and oxygen, are chemically combined in a fixed ratio of two hydrogen atoms to one oxygen atom.
- The properties of water are completely different from those of hydrogen and oxygen, and it can only be separated by chemical means such as electrolysis.
- Air is a mixture. Its components, mainly nitrogen and oxygen, are not chemically combined.
- Each gas keeps its own properties, the proportions can vary, and the gases can be separated by physical means such as fractional distillation of liquid air.
An ion has a proton number of 12 and a charge of 2 plus. State the number of electrons it contains and give its electronic structure.
Show the worked answer
- The proton number is 12, so the neutral atom has 12 electrons with the structure 2,8,2.
- A 2 plus charge means the atom has lost two electrons, since losing negative electrons leaves a positive ion.
- 12 minus 2 gives 10 electrons.
- Ten electrons fill as 2 in the first shell and 8 in the second, so the structure is 2,8. This is the same arrangement as neon, which is why the ion is stable.
Common mistakes in this topic
- Confusing proton number with nucleon number when reading nuclide notation.
- Saying isotopes have different chemical properties.
- Adding electrons for a positive ion.
- Writing electron shells as 2,8,8,8 for early elements. The pattern taught at IGCSE is 2,8,8 for the first twenty elements.
- Describing a compound as a mixture that is hard to separate.
Exam tips
- Write out the three subatomic particles with charge, mass and location as a table you can reproduce in thirty seconds.
- Use the Periodic Table on your data sheet to look up proton numbers rather than memorising them.
- For ion questions, decide first whether electrons have been lost or gained, then adjust.
- Remember that the number of outer shell electrons gives the group number, and the number of shells gives the period.
- Isotope questions almost always come back to electron arrangement. Say it explicitly.
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Atoms, Elements and Compounds FAQs
What are isotopes?
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, and therefore different nucleon numbers. Chlorine-35 and chlorine-37 both have 17 protons but 18 and 20 neutrons respectively.
Why do isotopes have the same chemical properties?
Chemical reactions involve only the outer shell electrons. Isotopes of an element have the same number of protons and therefore the same number and arrangement of electrons, so they behave identically in chemical reactions. Their physical properties, such as density, differ slightly because of the difference in mass.
How do I work out the number of neutrons in an atom?
Subtract the proton number from the nucleon number. If an atom has a nucleon number of 35 and a proton number of 17, it has 35 minus 17, which is 18 neutrons. The proton number is always the smaller of the two figures.
What is the difference between a compound and a mixture?
In a compound the elements are chemically combined in fixed proportions, the properties are completely different from those of the elements, and separation requires a chemical change. In a mixture the substances are not chemically combined, the proportions can vary, each component keeps its own properties, and separation is possible by physical means.
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Written to the published Cambridge IGCSE Chemistry (0620) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.