When electrons are added to the outermost shell of a carbon atom, it forms

When electrons are added to the outermost shell of a carbon atom, it forms the carbide anion (C⁴⁻).

Explanation:

Carbon is in Group 14 in the Periodic Table so it has 4 valence electrons in its valence shell. According to the Octet Rule, all elements tend to gain, lose or share electrons to have 8 electrons in its valence shell, and have the same electron configuration of the closest noble gas (a more stable element). Since C already has 4 electrons, it will tend to gain 4 electrons more to share the electron configuration of Neon. When C gains 4 electrons it forms the carbide anion, whose formula is C⁴⁻. "4-" is the charge resulting from gaining 4 electrons (negative charges)