Predicting the properties of francium
3 marks · NormalThe melting point of alkali metals decreases as you move down the group:
| Element | Atomic number | Melting point / °C |
|---|---|---|
| Lithium | 3 | 181 |
| Sodium | 11 | 98 |
| Potassium | 19 | 64 |
| Rubidium | 37 | 39 |
| Caesium | 55 | 29 |
| Francium | 87 | ? |
Use the trend shown in the table to estimate a value for the melting point of francium.
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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22 °C (±2)
Also accept: 21; 22; 23; 24
Common mistakes: Using a value similar to caesium's/ignoring downward curve
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Marking points
M1 · 1 mark · Recognises and uses the downward trend in the melting point as atomic number increases.
M2 · 1 mark · Correctly extends the pattern (extrapolation) towards atomic number 87.
A1 · 1 mark · Gives an answer in the correct range (21°C–24°C or 22°C ±2°C).
Worked solution
The melting points decrease from 181°C (Li) to 29°C (Cs). The decrease slows down between Rb (39°C) and Cs (29°C). So, extrapolating further down the group, the melting point for francium should be a few degrees lower than caesium, around room temperature.
Estimated melting point for francium = 22°C.