Question 27 (multiple answer)

Which of the following elements' atoms is likely to form an anion?

(Select all that apply.)

Hint: Look at your periodic table.

A) selenium

B) chlorine

C) magnesium

D) phosphorus

These elements would be A, B, and D.

All elements want to obtain the stable octet, or eight electrons in the outermost electron shell. Those elements with only one or two electrons in their outer shell find it easier to donate them away to reach the octet in a lower shell than to add a bunch on, while other elements that are a couple of electrons short find it easier to adopt some. Adding electrons to an atom makes it an anion, so those prone to accept electrons would be the ones to be anions, and these are the right side of the periodic table.

Selenium is two electrons short of the octet, Chlorine is one, and Phosphorus is three, which means that they can accept electrons to complete the octet, rather than donating six, seven, and five electrons, respectively. Magnesium, however, has two too many electrons, so it would be easier to donate two, rather than accept six. In doing so, it becomes a cation, or a positive ion.