Metallic character of Carbon group elements

Metallic character of Carbon group elements

Carbon and silicon are non-metals, germanium is a metalloid while tin and lead are metals. Thus metallic character increases on descending the group since ionization energy decreases on descending the group.

Hydrides

All of these elements form covalent hydrides though the number of hydrides and the ease with which these are formed decreases from carbon to lead. Carbon gives a vast number of hydrides (alkanes), silicon and germanium (silanes and germanes) whereas stannane (SnH4) and plumbane (PbH4) are the only hydrides of tin and lead are known.

Unlike alkanes, silanes are strong reducing agents, explode in chlorine and are readily hydrolysed by alkaline solutions. The difference is probably due to the difference in electronegativity between C and Si resulting in difference between C-H and Si-H linkages.

Halides

All these elements give tetrahalides. Tetrachlorides are usually fuming liquids at ordinary temperature. Carbon tetrahalide resists hydrolysis. This is because due to the absence of d-orbitals. Maximum covalency of carbon is only four and there is no possibility of formation of coordinate linkages with H2O, which could lead to hydrolysis.

Tetrahalides of rest of the elements undergo hydrolysis. For example

SiX₄ + 2H₂O  SiO₂ + 4HX

Carbon, silicon and germanium form trihalides of the type MHX3. Lead and tin do not form trihalides. Silicon, germanium, tin and lead form dihalides.

Chlorides

1.     The chlorides are all simple molecular substances with tetrahedral molecules.

2.     The stability of the chlorides decreases down the group and the +2 oxidation state becomes more stable than the +4 state. Only tin and lead form chlorides in which their oxidation state is +2, the other chlorides existing solely in the +4 state. Tin(II) chloride is a solid that is soluble in water, giving a solution which conducts electricity. It is also soluble in organic solvents. Its melting point is 246deg C. Lead(II) chloride is also a solid. It is sparingly soluble in water. The chlorides of the group 14 elements in their +4 oxidation state illustrate further the change in character of the elements from non-metal to metal down the group and giving a solution which conducts electricity, and melts at 501deg C. These observations suggest that tin(II) chloride has both covalent and ionic character, while lead(II) chloride is predominantly ionic.

3.     All the chlorides with +4 oxidation state are readily hydrolysed by water, except tetrachloromethane (CCl4).

Carbides

Compounds of carbon with less electronegative elements (eg. metals, Be, B, Si etc.) are called carbides. These are of three main types.

                   i.            Ionic or salt-like eg. acetylides, methanides, allylides

                ii.            Interstitial or metallic eg. WC and

              iii.            Covalent eg. B₄C, SiC.

All the three types of carbides are prepared by heating the element or its oxide with carbon or a hydrocarbon to a high temperature.

2Be + C -- > Be₂C

CaO + 3C -- > CaC2 + CO

SiO2 + 3C -- > SiC + 2CO

Oxides

1.     The oxides show a marked trend in structure from the molecules of carbondioxide to giant structures intermediate between ionic and covalent lower down the group.

2.     The +2 oxidation state is the more stable state in the case of leadoxide, and lead (IV) oxide decomposes on heating giving lead(II) oxide, a solid that melts at 886degC. The structure of lead(II) oxide is predominantly ionic.

3.     The oxides at the top of the group (CO2 and SiO2) have an acidic nature, the carbonate ion CO3^2- being produced easily in dilute aqueous solutions. The ease of formation of oxoanions (SiO3^2-, GeO3^2-etc.) decreases down the group as the acidic character decreases. The oxides of germanium, tin and lead are amphoteric, reacting to form simple salts with acids.