Question

Why can't carbon form ionic compounds? How can they form long chains of carbon.

Answer 1

Ionic compounds are those which are formed by either losing or gaining electrons completely .

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.

so it formes covalent bonds by sharing electrons and it forms long chain