Question

How can I calculate the valence electrons of ions?

Answer 1

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 `"Cl"^-`, we get
`"Cl": 1s^2 color(white)(l)2s^2 2p^6color(white)(l) 3s^2 3p^5` but `"Cl"^-` has one more electron.
∴ `"Cl"^"-": 1s^2 color(white)(l)2s^2 2p^6 color(white)(l)3s^2 3p^6`

Groups 1, 2, and 13

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

For `"Na"^+`:
`"Na": 1s^2 color(white)(l)2s^2 2p^6 color(white)(l)3s`, but `"Na"^+` has one less electron
∴ `"Na"^+: 1s^2 color(white)(l)2s^2 2p^6`

Groups 3 to 12

Remove `s` electrons before `d` electrons.

E.g., for `"Cr"^"3+"`
`"Cr": 1s^2 color(white)(l)2s^2 2p^6 color(white)(l)3s^2 3p^6 color(white)(l)4s 3d^5`
∴ `"Cr"^"3+": 1s^2 color(white)(l)2s^2 2p^6 color(white)(l)3s^2 3p^6 color(white)(l)3d^3`

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

EXAMPLES

Write the electron configurations for `"O"^"2-"`, `"Ca"^"2+"`, and `"Zn"^"2+"`.

Solutions

`"O": 1s^2 color(white)(l)2s^2 2p^4`
`"O"^"2-": 1s^2 color(white)(l)2s^2 2p^6`

`"Ca": 1s^2 color(white)(l)2s^2 2p^6 color(white)(l)3s^2 3p^6 color(white)(l)4s^2`
`"Ca"^"2+": 1s^2color(white)(l) 2s^2 2p^6 color(white)(l)3s^2 3p^6`

`"Zn": 1s^2 color(white)(l)2s^2 2p^6 color(white)(l)3s^2 3p^6 color(white)(l)4s^2 3d^10`
`"Zn"^"2+": 1s^2 color(white)(l)2s^2 2p^6 color(white)(l)3s^2 3p^6 color(white)(l)3d^10`