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p-Block elements

The Boron Family & Its Physical and Chemical Properties

  • Group 14 elements (carbon family): C, Si, Ge, Sn, Pb 

  • Carbon is the 17th most abundant element in the earth’s crust (by mass).

  • In elemental state, carbon is available as coal, graphite and diamond.

  • In combined state, it exists as carbonates, hydrogen carbonates and carbon dioxide in air (0.03%)

  • Two stable, naturally occurring isotopes: 12C and 13C

  • 14C is a radioactive isotope used for radiocarbon dating.

  • Silicon is the second most abundant element in the earth’s crust (27.7% by mass).

  • Silicon is the important component of ceramics, glass and cement.

  • Germanium is present only in traces.

  • Tin exists as cassiterite (SnO2) and lead as galena (PbS).

  • Germanium and silicon (in ultra pure form) are used for making transistors and semiconductor devices.

Atomic Properties of the Elements of Carbon Family

  • Valence shell electronic configuration is ns2 np2. 

  • Covalent radius increases from C to Si; after that there is a small increase from Si to Pb.

  • First ionisation enthalpy of group 14 members is higher than that of group 13 elements.

  • Electronegativity: The elements of this group are slightly more electronegative than the elements of group 13.

Physical Properties of the Elements of Carbon Family

  • All the elements are solids.

  • C is non-metal; Si and Ge are metalloids; Sn and Pb are soft metals.

  • Melting and boiling points of these elements are higher than those of group 13 elements.

Chemical Properties

  • Oxidation States:

  • Common oxidation states are +4 and +2.

  • Compounds in +4 state are generally covalent. (Since sum of the first four ionisation enthalpies is very high)

  • C and Si mostly show +4 oxidation state.

  • Ge in +4 state, forms stable compounds, and in +2 state, forms only a few compounds.

  • Sn forms compounds in both +2 and +4 oxidation states.

  • Compounds of lead in +2 state are stable and in +4 state are strong oxidising agents.

  • Due to the presence of d-orbitals in Si, Ge, Sn and Pb, these elements can exceed covalence more than 4. Thus, the halides of these elements undergo hydrolysis and have tendency to form complexes by accepting electron pairs from donor species.

Examples − (central atom is sp3d2)

  • Carbon cannot exceed its covalence by more than 4 (due to the absence of d-orbitals)

  • Reactivity towards Oxygen:

  • All members form two types of oxides, MO (monoxide) and MO2 (dioxide).

  • Oxides with higher oxidation states of elements (CO2, SiO2 and GeO2) are more acidic than those in lower oxidation states. SnO2 and PbO2 are amphoteric.

  • Among monoxides, CO is neutral; GeO is acidic; SnO and PbO are amphoteric.

  • Reactivity towards Water:

  • C, Si and Ge are not affected by water.

  • Sn reacts with steam to form dioxide and dihydrogen gas.

  • Due to the formation of a protective oxide film, lead is unaffected by water.

  • Reactivity towards Halogens:

  • Form halides of formula MX2 and MX4 (X = F, Cl, Br, I)

  • Most of the MX4 are covalent in nature. (Exceptions − SnF4 and PbF4 are ionic in nature)

  • The central metal atom in the covalent halides of the form MX4 undergoes sp3 hybridisation, and the molecule is tetrahedral in shape.

  • Most tetrachlorides are easily hydrolysed by water because of the presence of d-orbital in the central metal atom. d-orbital can accommodate the lone pair of electrons from the oxygen atom of a water molecule.

  • For example −

Question: Why does PbI4 not exist?

Answer: It does not exist because the Pb-I bond formed initially during the reaction does not release enough energy to un-pair 6s2 electrons, to have four un-paired electrons around the lead atom.

Question: Why does exist while does not?

Answer: Six larg…

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