Predicting the properties of francium
4 marks · NormalThe graph below shows the melting points of some alkali metals against their atomic numbers.
Describe the trend shown by the graph.
Estimate the melting point of francium (atomic number 87) using the graph.
Explain how you used the data to make your estimate.
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) Melting point decreases as atomic number increases.
(b) 22 °C
(c) By drawing a line of best fit through the data points, the graph curves downward and levels out. At atomic number 87 (francium), the predicted melting point is slightly below caesium’s, so it is about 22°C. · must show Line of best fit/extrapolation, Observes curve levels off
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly describes trend: melting point decreases as atomic number increases.
A1 (b) · 1 mark · Estimate of francium’s melting point in the range 21–25°C.
M1 (c) · 1 mark · Mentions drawing/measuring from line of best fit or curve for extrapolation.
E1 (c) · 1 mark · Explains the curve levels off so the melting point for francium is only slightly lower than caesium’s.
Worked solution
a) The melting point decreases as the atomic number increases.
b) Estimated melting point for francium: 22°C.
c) The graph curves downwards and levels off, so I extended the curve smoothly to atomic number 87. The predicted value is a little below caesium’s value, so I estimated about 22°C.