"Main group elements that are non-metals usually gain one or more electrons to form anions, which have a negative charge."
Explanation
Non-metal elements- all of which are in the main group in block p- are on the right hand side of a modern periodic table. Depending on their group number, they contain between one to four less electrons than the next noble gas element in their valence shells but in most cases over four electrons away from the noble gas in the last period.
Non-metal elements would thus by the octet rule prefer to gain one or more electrons to form a complete valence shell. It takes less energy for them to achieve a noble gas electron configuration by gaining electrons than losing ones.
Electrons are negatively-charged. An atom would acquire a negative charge after gaining extra electrons as it becomes ionized.
The non-metal atom would thus form an anion after gaining extra electrons.
