In general, melting points decrease down a group and increase across a period.
Melting points are the amount of energies needed to break the bonds so that a solid substance turns into liquid phase. In general melting point decrease down the group because the valence shell keep increasing and the outer electrons do not have a strong bond with the nucleus. And the melting point increases across the period because as the valency increases the bonds become stronger.
The periodic table is classified into groups and periods. The vertical columns are called the groups, while the horizontal rows are the periods.
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