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)

a rain shadow is a dry region of land on the side that is protected against the prevailing winds of a mountain range. the prevailing winds are the winds that occur most of the time in a particular place of the earth

prevailing winds carry air toward the mountain range. as the air rises up over a mountain range, the air cools, water vapor condenses, and clouds form. on this side of the mountains, called the windward side, precipitation falls in the form of  rain  or snow. this dry air produces a  rain shadow.

in the analysis of systems that involve fluid flow, we frequently encounter the combination of properties u and pv.