What is the trend in melting points across a period?

The melting points generally increase across a period, reaching a peak at the group 4 elements, then decrease.

As you move across a period in the periodic table from left to right, the melting points of the elements generally increase, peaking at the group 4 elements (such as carbon in period 2 and silicon in period 3). After this peak, the melting points then decrease towards the end of the period. This trend is due to the changes in the structure and bonding of the elements across a period.

At the start of a period, the elements are metals with a metallic bonding. The strength of the metallic bonds increases across the period because the number of delocalised electrons increases. This results in a higher melting point. The group 4 elements, such as carbon and silicon, have giant covalent structures, where a lot of energy is required to break the strong covalent bonds, hence they have very high melting points.

After the group 4 elements, the structure changes to simple molecular, with the exception of the group 0 elements which are monatomic. The forces between these molecules are weak van der Waals forces or dipole-dipole interactions, which are much easier to break. Therefore, the melting points decrease. The group 0 elements have very low melting points due to their weak van der Waals forces between the atoms.

In summary, the trend in melting points across a period can be explained by the changes in the type of structure and bonding of the elements. The melting points increase with the strength of the metallic bonds, peak at the group 4 elements with giant covalent structures, and then decrease due to the presence of weak intermolecular forces in simple molecular structures.