PH2- Lewis structure

PH2 (phosphanide) has one phosphorus atom and two hydrogen atoms.

In PH2 Lewis structure, there are two single bonds around the phosphorus atom, with two hydrogen atoms attached to it, and on the phosphorus atom, there are two lone pairs.

Also, there is a negative (-1) charge on the phosphorus atom.

Contents

Steps

Here’s how you can easily draw the PH2 Lewis structure step by step:

#1 Draw a rough skeleton structure
#2 Mention lone pairs on the atoms
#3 If needed, mention formal charges on the atoms

Now, let’s take a closer look at each step mentioned above.

#1 Draw a rough skeleton structure

• First, determine the total number of valence electrons

In the periodic table, phosphorus lies in group 15, and hydrogen lies in group 1.

Hence, phosphorus has five valence electrons and hydrogen has one valence electron.

Since PH2 has one phosphorus atom and two hydrogen atoms, so…

Valence electrons of one phosphorus atom = 5 × 1 = 5
Valence electrons of two hydrogen atoms = 1 × 2 = 2

Now the PH2 has a negative (-1) charge, so we have to add one more electron.

So the total valence electrons = 5 + 2 + 1 = 8

• Second, find the total electron pairs

We have a total of 8 valence electrons. And when we divide this value by two, we get the value of total electron pairs.

Total electron pairs = total valence electrons ÷ 2

So the total electron pairs = 8 ÷ 2 = 4

• Third, determine the central atom

Here hydrogen can not be the central atom. Because the central atom is bonded with at least two other atoms, and hydrogen has only one electron in its last shell, so it can not make more than one bond.

Hence, here we have to assume that the central atom is phosphorus.

Therefore, place phosphorus in the center and hydrogens on either side.

• And finally, draw the rough sketch

#2 Mention lone pairs on the atoms

Here, we have a total of 4 electron pairs. And two P — H bonds are already marked. So we have to only mark the remaining two electron pairs as lone pairs on the sketch.

Also remember that phosphorus is a period 3 element, so it can keep more than 8 electrons in its last shell. And hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell.

Always start to mark the lone pairs from outside atoms. Here, the outside atoms are hydrogens. But no need to mark on hydrogen, because each hydrogen has already two electrons.

So for phosphorus, there are two lone pairs.

Mark the lone pair on the sketch as follows:

#3 If needed, mention formal charges on the atoms

Use the following formula to calculate the formal charges on atoms:

Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons

For phosphorus atom, formal charge = 5 – 4 – ½ (4) = -1

For each hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0

Here, the phosphorus atom has a charge, so mark it on the sketch as follows:

In the above structure, you can see that the central atom (phosphorus) forms an octet. And the outside atoms (hydrogens) also form a duet. Hence, the octet rule and duet rule are satisfied.

Now there is still a negative (-1) charge on the phosphorus atom.

This is okay, because the structure with a negative charge on the most electronegative atom is the best Lewis structure. And in this case, the most electronegative element is phosphorus.

Therefore, this structure is the most stable Lewis structure of PH2.

And since the PH2 has a negative (-1) charge, mention that charge on the Lewis structure by drawing brackets as follows: