Question

What happens to the melting point of the d-block elements across a period?

Answer 1

To determine the trend in change of melting point of d-block elements across a period we first need to understand what is melting point. The melting point of a substance is the temperature at which the substance changes its state from solid-state to liquid state.

Complete answer:
Now, various factors are affecting the melting point of a substance. Three of the main factors are the size and composition of the molecule, the number of impurities in a sample compound, and the forces of attraction.
We know that in the modern periodic table, "the element's physical and chemical properties are the periodic function of their atomic numbers."
d-block elements are those which have the valence electron in the d-orbital of the atom. They exist in the middle of the modern periodic table and start in the 4th period ranging from groups 3 to 12.
Now, d-block elements usually have high melting points due to the covalent bonding formed by the empty or partially filled d-orbitals and unpaired electrons.
Since electrons get paired in the d-orbital after ${{d}^{5}}$ configuration, the melting point of elements in the d-block increases from ${{d}^{1}}$ configuration to ${{d}^{5}}$ configuration and then starts to decrease.
The highest melting point in their respective periods is possessed by group 6 elements chromium (Cr), molybdenum (Mo), and Tungsten (W).
The lowest melting point in their respective periods is possessed by group 12 elements zinc (Zn), cadmium (Cd), and mercury (Hg).
Mercury (Hg) is the only metal that exists as a liquid at room temperature as the outer s-electrons are less involved in metallic bonding due to lanthanide contractions.

Note:
It should be noted that due to weak metallic bonding due to half-filled configuration, the group 7 elements magnesium (Mn) and technetium (Tc) have not only an abnormally low melting point but also a low boiling point.