The Alkali Metals
6 marks · NormalRubidium and caesium are Group 1 alkali metals.
State two physical properties that rubidium and caesium have in common with other Group 1 metals.
Describe what you would observe if a small piece of caesium was dropped into water.
Explain, in terms of electronic configuration, why caesium reacts more rapidly with water than sodium.
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) Soft (can be cut with a knife), shiny when freshly cut, low melting points (relative to other metals), good conductors of electricity, low density (some float on water). Any two.
(b) Violent, explosive reaction. Caesium melts, moves rapidly on the surface, fizzing, large amounts of hydrogen gas released, may explode, caesium disappears very quickly. Solution formed is caesium hydroxide.
(c) Caesium atoms have more electron shells than sodium, so their outer electron is further away from the nucleus. The outer electron in caesium is less strongly attracted to the nucleus and more easily lost, making caesium more reactive.
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Marking points
B1 (a) · 1 mark · One correct physical property stated (e.g., soft, shiny when cut, low melting point, low density, good electrical conductor).
B2 (a) · 1 mark · A second distinct correct property.
B3 (b) · 1 mark · Correctly describes at least one observation (explosive/violent reaction, melts, rapid movement, gas given off, etc.).
B4 (b) · 1 mark · A second relevant observation (may explode/disappear, forms caesium hydroxide, etc.).
M1 (c) · 1 mark · Links reactivity to electron configuration: caesium has more electron shells/outer electron is further away from nucleus.
M2 (c) · 1 mark · Explains that outer electron is less strongly attracted and more easily lost, leading to increased reactivity.
Worked solution
a) Any two of the following: soft (can be cut with a knife), shiny (when freshly cut), low density (may float on water), low melting point, good conductor of electricity.
b) Caesium will react explosively with water. You would see it rapidly melt, fizzing and moving quickly on the surface of the water, and it may explode. Hydrogen gas is released vigorously. Caesium disappears very quickly, and a solution of caesium hydroxide is formed.
c) Caesium is lower down Group 1 than sodium, so it has more electron shells. The outer electron in caesium is further from the nucleus, so it is less strongly attracted. This makes it more easily lost, so caesium reacts more rapidly with water than sodium.