3.0 General Trends in properties of First Row Elements

  d and f Block Elements

• The metallic character of d-block elements is due to their low ionisation enthalpy and presence of vacant $d$-orbitals which favour formation of metallic bonds.

• These metals have hexagonal closed packed (HCP) cubic close packed (CCP) or body centered cubic (BCC) lattice which are the characteristics of true metal.

• Most of these metals are very hard due to presence of a number of unpaired electrons in vacant $d$-sub-shell which are used to form covalent bonds.

• As the no. of unpaired electrons increases hardness of metals also increases (Ex. $Cr$, $Mo$, $W$).

• Some of them are soft metals ($Zn$, $Cd$, $Hg$) due to absence of unpaired $d$-electrons.

• Most of them are hard due to presence of metallic and covalent bond.

• Across the period their melting point and boiling point values first increases, attain maximum value and then steadily decreases as the atomic number increases.

• The melting point and boiling point values of these metals are very high as they have strong metallic bonds with considerable covalent character.

• Melting point and boiling point values of these metals are very high at middle of period due to presence of more number of unpaired d-electrons.

• Beyond ${d^5}$ electrons starts pairing and number of unpaired electron decreases which decreases the melting point and boiling point values.

• $Hg$ is liquid at room temperature (boiling point $234\ K$) probably due to its complicated lattice structure.

• The elements except $Zn$, $Cd$ and $Hg$ are much hard and low volatile due to their high enthalpy of atomization which also increases with unpaired $d$-electrons.

• Osmium is the most dense ($22.6\ g{m^1}{\text{ }}c{m^{ - 3}}$) where as Scandium is lighter with density ($2.99\ g{m^1}{\text{ }}c{m^{ - 3}}$).