Final answer:
A non-metal, particularly a halogen, is a very reactive element that commonly forms -1 anions by gaining one electron to achieve a complete outer shell.
Step-by-step explanation:
A very reactive element that often forms -1 anions is a non-metal, specifically found within group 17 of the periodic table, known as the halogens. The halogens include elements like fluorine, chlorine, bromine, iodine, and astatine. They have one less electron than the nearest noble gases and are thus very reactive as they seek to gain one electron to complete their outer shell and achieve a more stable electron configuration.
When aluminum and carbon react to form an ionic compound, aluminum (Al), being a metal, loses electrons and forms a cation with a charge of 3+ (Al3+), and carbon (C), being a non-metal, gains electrons to form an anion with a charge of 4- (C4-). Therefore, aluminum forms the cation and carbon forms the anion in this reaction.