Home / iGCSE Chemistry (0620) Theory (Core):8.5 Noble gases: Exam Style Questions Paper 3

iGCSE Chemistry (0620) Theory (Core):8.5 Noble gases: Exam Style Questions Paper 3- New Syllabus

iGCSE Chemistry Theory (Core) Paper 3 - All Chapters

Question

The symbol for an atom of argon is shown.
\[^{40}_{18}\mathrm{Ar}\]
(a) Complete Table 2.1 to show the number of protons and neutrons in one atom of $^{40}_{18}\mathrm{Ar}$.
(b) Argon is an unreactive monatomic element.
(i) Explain what is meant by the term monatomic.
(ii) Explain why argon is unreactive. Give your answer in terms of electronic configuration.
(c) At room temperature and pressure, argon is a gas.
A sample of argon is placed in a sealed syringe with a freely moving plunger.
Complete Table 2.2 to show the effect, if any, on the volume of argon when:
  • the temperature of the argon is increased
  • the pressure of the argon is increased.
Use the words increases, decreases or no change in your answer.

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 2.2 — Atomic structure and the Periodic Table: Proton number and nucleon number (Part a)
• Topic 8.5 — Noble gases: Monatomic and unreactive nature (Part b)
• Topic 1.1 — Solids, liquids and gases: Effect of temperature and pressure on gas volume (Part c)

▶️ Answer/Explanation

(a)

The atomic number (subscript 18) is the number of protons. The mass number (superscript 40) is the total number of protons and neutrons. Therefore, the number of neutrons is $40 – 18 = 22$.

(b)(i) (contains only) one atom

A monatomic element exists as single, unbonded atoms rather than as molecules composed of two or more atoms (like $\mathrm{O}_2$ or $\mathrm{N}_2$).

(b)(ii) (its atom has) a full outer shell / the outer shell contains 8 electrons

Argon has an electronic configuration of 2,8,8. A full outer electron shell confers high energetic stability, so the atom has no tendency to gain, lose, or share electrons.

(c)

Increasing the temperature increases the kinetic energy of gas particles, causing them to collide more forcefully with the plunger, pushing it out and increasing volume. Increasing external pressure pushes the plunger in, compressing the gas and decreasing its volume.

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