Nitrogen forms $N^{3-}$ ions. Using the periodic table, how many electrons are in an $N^{3-}$ ion?

What the periodic table tells you

The periodic table gives nitrogen’s atomic number, which equals the number of protons. For a neutral atom, the number of electrons equals the number of protons.

Start with neutral nitrogen

Nitrogen has atomic number 7, so:

• Protons $= 7$
• Neutral electrons $= 7$

Adjust for the $3-$ charge

A $3-$ charge means the ion has gained 3 electrons compared with the neutral atom:

$$\text{electrons in }N^{3-} = 7 + 3 = 10$$

Quick check

Negative charge means extra electrons, so the electron count should be bigger than 7. Getting 10 matches that idea.