Question

Why does the melting point of alkali metals decrease

Answer 1

In the long form of periodic table, the elements are arranged in the order of increasing atomic numbers, and consecutively, atomic number (for neutral atoms) is equal to the count of protons and electrons. Therefore for greater atomic number, the number of shells shall also be increased.

Now if we move down the group in other periodic table, the numbers of shells thus increases, hence the attractive force between the protons and the revolving electrons is dcreased, due to which, the size is increased. Therefore for lesser nuclear charge (attractive forces) the temperature required to change the state would be decreased.

Thus melting and boiling points down the group for any sub- family, decreased.