The Periodic Table: Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: The Periodic Table revision notes

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Questions

1. State what the group number and period number of an element tell you about the arrangement of its atom’s electrons, using an example. [3]

2. Explain why elements in the same group of the periodic table have similar chemical properties. [2]

3. Describe the reaction of potassium with cold water, giving three observations and a word equation. [4]

4. Explain fully why reactivity increases down Group I, referring to atomic radius, shielding and nuclear attraction. [3]

5. Explain why reactivity decreases down Group VII, and why this is the opposite trend to Group I, in terms of gaining versus losing an electron. [4]

6. Chlorine is bubbled through a solution of potassium iodide.

(a) State the observation. [1] (b) Write the word equation. [1] (c) Explain why the reaction occurs. [2]

7. State four properties of transition metals that distinguish them from Group I metals, referring to density, melting point, colour and reactivity. [4]

8. Explain why the noble gases are unreactive. [2]

9. State the appearance at room temperature and pressure of chlorine, bromine and iodine. [3]

10. Iron forms both iron(II) and iron(III) compounds. Explain what this shows about transition elements, and contrast it with Group I metals. [3]

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Answers

1. The group number gives the number of outer-shell electrons [1]; the period number gives the number of occupied electron shells [1]; e.g. sodium (Group I, Period 3) has 1 outer-shell electron and 3 occupied electron shells [1].

2. They have the same number of outer-shell electrons [1], and it is the outer electrons that take part in chemical reactions [1].

3. Observations: it floats on the surface and melts into a ball; it moves rapidly and fizzes; a lilac flame is seen; it disappears — any three [1] [1] [1]. Word equation: potassium + water → potassium hydroxide + hydrogen [1].

4. The outer electron is in a shell further from the nucleus [1]; there is more shielding by inner shells [1]; so the attraction between the nucleus and the outer electron is weaker and it is lost more easily [1].

5. Reactivity decreases down Group VII [1]. A halogen must gain an electron rather than lose one [1]. Further down the group the outer shell is further from the nucleus with more shielding [1], so an incoming electron is attracted less strongly — the same physics producing the opposite trend [1].

6. (a) The solution turns brown [1]. (b) chlorine + potassium iodide → potassium chloride + iodine [1]. (c) Chlorine is more reactive than iodine [1], so it displaces iodine from its compound [1].

7. One mark for each of the four named properties: higher density than Group I metals [1]; higher melting points than Group I metals [1]; they form coloured compounds (Group I compounds are white/colourless) [1]; they are less reactive than Group I metals [1].

8. They have full outer electron shells [1], so they have no tendency to lose, gain or share electrons [1].

9. Chlorine — a pale yellow-green gas [1]. Bromine — a red-brown liquid [1]. Iodine — a grey-black solid [1].

10. It shows that transition elements can have ions with variable oxidation numbers [1] — iron(II) compounds and iron(III) compounds are both stable, ordinary compounds, with the same metal simply forming two different, equally valid ions [1]. This is unlike Group I metals, which only ever form one ion charge (always 1+) [1].

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Where marks are usually lost

• Saying reactivity increases down both Group I and Group VII — remembering the two trends run in opposite directions, and why (losing an electron gets easier down Group I, gaining one gets harder down Group VII), avoids this.
• Giving only one reason for the Group I trend when three are needed.
• Saying noble gases are unreactive “because they are gases” — the reason is a full outer electron shell, not the physical state.
• Confusing group number with period number.
• Giving the wrong physical state for a halogen at room temperature — chlorine is a gas, bromine a liquid, and iodine a solid, and mixing these up loses an easy mark.
• Describing iron(II)/iron(III) as “isotopes” of each other rather than as different ions of the same element with different oxidation numbers.
• Describing a halogen displacement reaction without explaining why it happens — a more reactive halogen displaces a less reactive one from solution, and reactivity is what should be named as the reason, not simply “because it can”.

Displacement reactions — a useful memory aid

The same order that governs halogen reactivity (Cl > Br > I) also happens to match the order of decreasing solubility (and increasingly strong colour) of the silver halide precipitates formed with silver nitrate solution in qualitative analysis — chloride a white precipitate, bromide cream, iodide yellow. This shared order is a useful memory aid when revising Group VII, but the two trends arise from different underlying causes — halogen reactivity does not explain silver halide solubility, even though the two happen to run in the same order.

For condensed recall notes on this topic, see the The Periodic Table revision notes; for the full explanation with additional detail, see the The Periodic Table: Groups and Trends study guide.