Question

Draw Lewis electron dot diagrams for the following species:

$\left(a\right){\mathbf{AsH}}_{\mathbf{3}}\mathbf{;}\left(b\right)\mathbf{HOCl}\mathbf{;}\left(c\right){\mathbf{KrF}}^{\mathbf{+}}\mathbf{;}\left(d\right){\mathbf{PO}}_{\mathbf{2}}{\mathbf{Cl}}_{\mathbf{2}}^{\mathbf{-}}$(central atom).

Short answer

The Lewis electron dot diagrams of the following species:

Step-by-step solution

Steps for constructing Lewis structure

In order to create the most plausible distribution of electrons among the atoms in a given ion/molecule, one has to follow certain steps:

• First, create the pi-bonds.
• Then assign the remaining electrons to the atoms (as lone pairs) in decreasing order by electronegativity.
• Finally, check for atoms (except H) with less than four valences (if found any: rearrange the neighboring electron pairs to the sufficient number of pi-bonds).

Electron pairs are represented with "double dots" or lines - unpaired electrons always have the corresponding number of dots.

Lewis structure of (a)

(a)

Counting the total of 8 valence electrons:
The arsenic is of group V, an element having 5 valence electrons, and the hydrogen in group I, an element having 1 valence electron.
The red circle represents the "domain" of the arsenic, the eight electrons here correspond to the electron structure of the noble gases which is generally considered stable and favorable.

The hydrogen atoms tend to "have" two electrons (blue circle) when engaging in covalent bonding.

Lewis structure of arsine

Lewis structure of (b)

(b) HOCl

Counting the total of 14 valence electrons:

• The hydrogen is Group I. An element having 1 valence electron
• The oxygen is Group VI. An element having 6 valence electrons
• The chlorine is Group VII. An element having 7 valence electrons

Lewis structure of hypochlorous acid

Lewis structure of (c) 

(c)

Krypton is in group VIII, an element having 8 valence electrons and fluorine is in group VII, an element having 7 valence electrons. 1 electron must be subtracted because of the net positive charge.
The valence electrons of the fluorine are highlighted in red. In this case, the krypton atom only has seven electrons, one less than its "original" value. Thus, the positive charge is located on the krypton atom.

Lewis structure of (d)

Counting the total of 32 valence electrons:

• Phosphorus is Group V, an element having 5 valence electrons
• Oxygen is Group VI, an element having 6 valence electrons
• Chlorine is Group VII, an element having 7 valence electrons
• 1 more electron must be added because of the net negative charge.

Consider the following: phosphorus has four bonds (meaning noble gas structure) but it still has a positive charge because this bond structure corresponds to group IV. P has one electron less, and one of the oxygens has one electron more (cf. two negative charges).

There would be another (yet asymmetrical) structure of this ion when one of the O atoms is double bonded to the P center.