Question

:: Elements A, B, C, D have same number of valence electrons. Their melting points are 458°K, 3700K, 525°K and 245ºK respectively. Arrange the elements in increasing order of their atomic numbers ​

Answer 1

Answer:

Correct option is A)

The melting point of period three elements increases from sodium to silicon and decreases from silicon to argon. In general, melting point increases across a period up to group 14, then decreases from group 14 to group 18.

The melting point of group 1 and 2 metals decreases down the groups, due to a decrease in charge density (as atomic radius increases and charge stays constant, as the increase in shielding from more inner electron shells is more significant than the increase in proton number) meaning that the delocalised electrons are further away from the metal cations, leading to weaker electrostatic forces of attraction that require less energy to break down, causing a decrease in melting point

Na 97.8 883 0.97

K 63.7 759 0.86

Rb 39.5 688 1.53

Thus As the metal atoms increase in size, they got farther apart from each other, so their melting and boiling points decrease. The strength of dispersion forces among neutral atoms decreases with increasing distance between the atoms.