Question

State the reason why carbon can neither form C4+ cations nor C4- anions but forms covalent compound. Also state the reason to explain why covalent compounds: (i) are bad conductors of electricity. (ii) have low melting and boiling points.

Answer 1

Now carbon is at the borderline, basically it has a choice whether
 1. it wants to gain 4 electrons or 2. lose the same in order ro acquire noble gas configuration.
1st case In the former case its small nucleus wont be able to handle 4 more electrons i.e 10.
2nd case Since we know energy is always required to knock out an electron from an element (Like always ;)) , the sum of four ionisation enthalpies (energy required to knock out the most loosely bound electron from a gaseous atom) would be very high.And therefore carbon chooses none of them and decides to share its electrons to complete its octet and hence follows octet rule.

THE TYPE OF BOND IN IONIC COMPOUNDS IS IONIC BOND AND IN CARBON IS COVALENT BOND.

Since carbon is not an ionic compound it do not conduct electricity in an any state.
THEY HAVE LOW MELTING AND BOILING POINTS BECAUSE OF WEAK ATTRACTION.

Answer 2

♤ Carbon can't lose 4 electron to form C^4+ ions as very high energy is required to remove 4 electrons.

♤ Carbon can not gain 4 electron to form C^4- ions as 6 protons can not hold 10 ♤ electrons and Energy needs to be supplied to overcome repulsion.

♤ Carbon can share 4 electron to form covalent compounds.

♤ Carbons do not conduct electricity as they are non - polar and do not form ions.

♤ They have low melting points and boiling points due to weak van der Waal's force of attraction between molecules