What ion will be formed by the selenium atom shown below when it has a stable set of valence electrons?

Se⁻²

Explanation:

Even though the atom is not shown, we can consult the periodic table. Selenium (Se) is in group 6 and period 4, so it has 6 valence electrons. To be stable, the atom must have a noble gas configuration with 8 valence electrons, thus, selenium must gain 2 electrons.

When an atom gain electrons, it is negatively charged, because electrons have a negative charge. So, the ion formed is Se⁻².