The Alkali Metals
6 marks · NormalThe alkali metals show clear trends down Group 1.
Rubidium and caesium are both placed below potassium in Group 1 of the Periodic Table. Predict how the reactivity of caesium compares to that of rubidium and explain your answer.
Describe how the size of alkali metal atoms changes as you move from lithium to caesium in Group 1.
Explain, in terms of electronic configuration, why alkali metals become more reactive as you move down Group 1.
State the charge on an ion formed when a caesium atom reacts.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Caesium is more reactive than rubidium. Down the group, reactivity increases because the outer electron is further from the nucleus and lost more easily. · must show reactivity increases, reason: outer electron more easily lost (distance/weaker attraction)
(b) Atoms become larger / increase in size from lithium to caesium. · must show atoms become larger
(c) Alkali metals have one electron in their outer shell. As you go down the group, there are more electron shells, so the outer electron is further from the nucleus, less strongly attracted, and is lost more easily, making the metals more reactive. · must show more shells, outer electron further from nucleus, less attraction, easier to lose electron
(d) +1 (or 1+); forms Cs+ ions. · must show +1
Common mistakes: Reversing the trend in reactivity; Mixing up cation charges
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States that caesium is more reactive than rubidium.
A1 (a) · 1 mark · Correctly explains that increased reactivity is due to outer electron being further from the nucleus/more easily lost.
B1 (b) · 1 mark · States that atomic size increases down the group.
M2 (c) · 1 mark · Mentions increased number of shells and/or outer electron further from the nucleus as you go down.
A2 (c) · 1 mark · Explains that weaker attraction and easier removal of outer electron causes increased reactivity.
B2 (d) · 1 mark · States charge on caesium ion is +1 or 1+ or gives Cs+.
Worked solution
a) Caesium is more reactive than rubidium because as you go down Group 1, the outer electron is further from the nucleus and is less strongly attracted, so it is more easily lost.
b) Atomic size increases from lithium (smallest) to caesium (largest); atoms get bigger down the group.
c) Each Group 1 metal atom has one electron in its outer shell. As you move down, there are more electron shells, increasing the distance from the nucleus. This reduces the attraction between the nucleus and the outer electron, so it is more easily lost, making the atoms more reactive.
d) A caesium ion, Cs+, has a charge of +1.