Question

when an atom loses an electron, it becomes...
positively charged.
a different element.
negatively charged.
an atom with a lower mass.

Explanation:

An atom has equal numbers of protons (positive charge) and electrons (negative charge). When it loses an electron, there are more protons than electrons. So, it has a net positive charge. Losing an electron doesn't change the element (that's determined by protons), doesn't make it negatively charged (since electron loss reduces negative charge), and electron mass is negligible compared to proton/neutron mass, so mass change is negligible.

Answer:

positively charged.