Jan 13, 2016

See explanation.

#### Explanation:

In order to be able to draw the Lewis structure for a given molecule, you need to know how many valence electrons it contains.

The total number of valence electrons available for a molecule is determine by taking into account the valance electrons each atom that is part of that molecule brings to the table.

A hydrogen sulfide molecule, $\text{H"_2"S}$, contains two hydrogen atoms and one sulfur atom. This means that the molecule will have a total of $8$ valence electrons

• one from each of the two hydrogen atoms
• six from the sulfur atom

The sulfur atom will act as a central atom, since it can form more than one bond. The hydrogen atoms will bond to the central atom via single bonds.

These bonds will account for $4$ of the molecule's $8$ valence electrons. The remaining $4$ valence electrons will be placed as lone pairs on the sulfur atom.

This will ensure that the central atom has a *complete octet.

The exact same approach is used for nitrogen trichloride, ${\text{NCl}}_{3}$. A molecule of nitrogen trichloride contains one nitrogen atom and three chlorine atoms.

This means that you get a total of $26$ valence electrons

• five from the nitrogen atom
• seven from each of the three chlorine atoms

Nitrogen will act as central atom here. The three chlorine atoms will bond to the central atom via single bonds. These bonds will account for $6$ of the molecule's $26$ valence electrons.

Each chlorine atom will complete its octet by adding $6$ other valence electron as lone pairs. These lone pairs will account for another $3 \times 6 = 18$ valence electrons.

The remaining $2$ valence electrons will be placed as lone pairs on the nitrogen atom, ensuring its complete octet.