Question

Question #79d88

Answer 1

`"18 valence electrons"`

Explanation:

The total number of valence electrons in a molecule of acetaldehyde, `"CH"_3"CHO"`, can be calculated by adding the number of valence electrons of each atom that makes up the molecule.

A molecule of acetaldehyde contains

• two atoms of carbon, `2 xx "C"`
• four atoms of hydrogen, `4 xx "H"`
• one atom of oxygen, `1 xx "O"`

Now, you should know that you have

• `"For C: 4 valence electrons"`
• `"For H: 1 valence electron"`
• `"For O: 6 valence electrons"`

This means that the total number of valence electrons present in a molecule of acetaldehyde is equal to

`"no. of e"^(-) = overbrace(2 xx "4 e"^(-))^(color(blue)("from 2 atoms of C")) + overbrace(4 xx "1 e"^(-))^(color(darkorange)("from 4 atoms of H")) + overbrace(1 xx "6 e"^(-))^(color(purple)("from 1 atom of O"))`

`"no. of e"^(-) = "18 e"^(-)`

You can double-check the calculations by looking at the Lewis structure of acetaldehyde, or ethanal.

You know that each single bond is made up of `2` valence electrons and that a double bond is made up of `4` valence electrons.

As you can see, you have `5` single bonds and `1` double bond, which corresponds to a total of

`5 xx "2 e"^(-) + 1 xx "4 e"^(-) = "14 e"^(-)`

The two lone pairs of electrons present on the oxygen atom bring the total number of valence electrons to `18`.