Question

when an atom gains an electron it becomes...
an atom with a heavier mass.
a different element.
negatively charged.
positively charged.

Explanation:

An atom has equal numbers of protons (positively charged) and electrons (negatively charged), giving it a neutral charge. When an atom gains an electron, it now has more negative charges (electrons) than positive charges (protons). So, it becomes negatively charged.

• Mass of an atom is mainly due to protons and neutrons. Gaining an electron (which has negligible mass compared to protons and neutrons) doesn't make it a "heavier - mass" atom in a significant sense relevant to this question.
• The identity of an element is determined by the number of protons. Gaining an electron doesn't change the number of protons, so it's still the same element.
• Since it gains a negative charge (electron), it can't be positively charged.

Answer:

negatively charged.