# How can I calculate the valence electrons of ions?

May 7, 2014

Valence electrons are the electrons contained in the outermost or valence shell of an atom or ion.

#### Explanation:

You determine the valence electrons by using the Periodic Table.

Let's assume you're using a table that has the groups numbered 1-18.

The electron configurations of ions are those of the neutral atoms plus or minus a number of electrons equal to the charge on the ion.

Groups 15 to 17

Write the electronic structure for the neutral atom. Then add electrons to get a total of eight.

E.g., for ${\text{Cl}}^{-}$, we get
$\text{Cl} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6} \textcolor{w h i t e}{l} 3 {s}^{2} 3 {p}^{5}$ but ${\text{Cl}}^{-}$ has one more electron.
$\text{Cl"^"-} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6} \textcolor{w h i t e}{l} 3 {s}^{2} 3 {p}^{6}$

Groups 1, 2, and 13

Write the electronic structure for the neutral atom. Then remove the outermost electrons.

For ${\text{Na}}^{+}$:
$\text{Na} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6} \textcolor{w h i t e}{l} 3 s$, but ${\text{Na}}^{+}$ has one less electron
${\text{Na}}^{+} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6}$

Groups 3 to 12

Remove $s$ electrons before $d$ electrons.

E.g., for $\text{Cr"^"3+}$
$\text{Cr} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6} \textcolor{w h i t e}{l} 3 {s}^{2} 3 {p}^{6} \textcolor{w h i t e}{l} 4 s 3 {d}^{5}$
$\text{Cr"^"3+} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6} \textcolor{w h i t e}{l} 3 {s}^{2} 3 {p}^{6} \textcolor{w h i t e}{l} 3 {d}^{3}$

You remove the $4 s$ electron first, followed by two of the $3 d$ electrons.

EXAMPLES

Write the electron configurations for $\text{O"^"2-}$, $\text{Ca"^"2+}$, and $\text{Zn"^"2+}$.

Solutions

$\text{O} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{4}$
$\text{O"^"2-} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6}$

$\text{Ca} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6} \textcolor{w h i t e}{l} 3 {s}^{2} 3 {p}^{6} \textcolor{w h i t e}{l} 4 {s}^{2}$
$\text{Ca"^"2+} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6} \textcolor{w h i t e}{l} 3 {s}^{2} 3 {p}^{6}$

$\text{Zn} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6} \textcolor{w h i t e}{l} 3 {s}^{2} 3 {p}^{6} \textcolor{w h i t e}{l} 4 {s}^{2} 3 {d}^{10}$
$\text{Zn"^"2+} : 1 {s}^{2} \textcolor{w h i t e}{l} 2 {s}^{2} 2 {p}^{6} \textcolor{w h i t e}{l} 3 {s}^{2} 3 {p}^{6} \textcolor{w h i t e}{l} 3 {d}^{10}$