Exercise 3 solution

Melting points (Halogen article): chlorine -101.5 C, iodine 113.7 C.
Estimate for bromine = (-101.5 + 113.7) / 2 = 6.1 C   (Python: 6.1)
Page value for bromine = -7.3 C
Difference = 6.1 - (-7.3) = 13.4 C too high (Python: 13.4)

Why it is only a rough guide: the average assumes the property changes in equal
steps down the group. The data do not guarantee that. The same method on the
alkali metals, using lithium (180.54) and potassium (63.38) to estimate sodium,
gives 121.96 C against the page value of 97.72 C, about 24.2 C too high.
An estimate from two neighbours shows roughly where a value lies, but it is not a
measurement. Mendeleev's predictions were likewise estimates from neighbours
(for example ekasilicon predicted at atomic mass 72, observed 72.630 for germanium).