Explaining the trend in the reactivity of the halogens
3 marks · NormalIodine (I2) is a solid at room temperature, and its melting point is 114 °C. Its colour as a solid is purple-black. Astatine (At) is the element below iodine in Group 7 and has not been isolated, but its properties can be predicted from group trends.
Predict the following properties of astatine (At2) based on your knowledge of halogen group trends:
State the physical state (solid, liquid or gas) in which astatine would exist at room temperature.
Estimate whether the melting point of astatine would be higher or lower than that of iodine. Explain your answer.
State and explain what colour (compared to iodine) you would expect astatine to be as a solid.
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) Solid · must show State: solid
(b) Higher melting point than iodine, because melting point increases down the group. · must show Trend: higher melting point down group, Explanation: melting point increases going down Group 7
(c) Darker in colour than iodine (very dark grey or black), because colour gets darker down the group. · must show Colour: darker than iodine, Explanation: colour darkens down group
Common mistakes: Stating astatine is a gas; Saying melting point decreases down the group; Believing colour gets lighter
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Marking points
B1 (a) · 1 mark · Identifies that astatine is a solid at room temperature.
B1 (b) · 1 mark · States melting point of astatine is higher than iodine and gives correct explanation (melting point increases down group).
B1 (c) · 1 mark · States astatine is darker than iodine and gives correct explanation (colour darkens down group).
Worked solution
a) Astatine would be a solid at room temperature. This is predicted from the trend that the state of Group 7 elements changes from gas (chlorine) to liquid (bromine) to solid (iodine), and astatine is below iodine.
b) Astatine's melting point would be higher than that of iodine. Down the group, melting points increase due to stronger intermolecular forces as atomic mass increases.
c) Astatine would be a darker colour than iodine (very dark grey or black). The colour of the halogens gets darker going down the group.