Question

Both ‘H’ and ‘F’ are monovalent in covalent bonding,

phosphorus forms PF5

but no PH5

exists. Explain.​

Answer 1

Answer:

Sol. Phosphorus has five electrons in the valence shell (3s23p3

)

as well as it contains vacant 3d 3d orbitals, so there is

scope for pentavalency. However, the 3d orbital of

phosphorus is much larger is size than its 3s and 3p-

orbitals, and do not usually hybridize. When highly

electronegative atoms like F approach to P, its 3d-orbitals,

and do not usually hybridize. When highly

electronegative atoms like F approach to P, its 3d-orbitals

hybridize with 3s and 3p orbitals and enable P to undergo

sp3d hybridization to form PF5

, PCl5

etc. Hydrogen being

very less electronegative, cannot contract 3d orbitals of

phosphorus to the level where it can participate in

hybridization with 3s and 3p orbitals. Hence, with

hydrogen, only s and p orbitals of P hybridize and no

more than its three electrons can take part in sharing,

showing trivalency only.

Answer 2

Answer:

H atoms need only two electrons to fill the 1s subshell. Each H atom starts with a single electron in its valence shell:

H-H

The two H atoms can share their electrons:

H-H-2

We can use circles to show that each H atom has two electrons around the nucleus, completely filling each atom’s valence shell:

H-H-3

Because each H atom has a filled valence shell, this bond is stable, and we have made a diatomic hydrogen molecule. (This explains why hydrogen is one of the diatomic elements.) For simplicity’s sake, it is not unusual to represent the covalent bond with a dash, instead of with two dots:

H–H

Because two atoms are sharing one pair of electrons, this covalent bond is called a single bond.

As another example, consider fluorine. F atoms have seven electrons in their valence shell:

F-F

These two atoms can do the same thing that the H atoms did; they share their unpaired electrons to make a covalent bond.

F-F-2

Note that each F atom has a complete octet around it now:

F-F-3

We can also write this using a dash to represent the shared electron pair:

F-F-4

There are two different types of electrons in the fluorine diatomic molecule. The bonding electron pair makes the covalent bond. Each F atom has three other pairs of electrons that do not participate in the bonding; they are called lone electron pairs. Each F atom has one bonding pair and three lone pairs of electrons.

Covalent bonds can be made between different elements as well. One example is HF. Each atom starts out with an odd number of electrons in its valence shell:

H-F

The two atoms can share their unpaired electrons to make a covalent bond:

H-F-2

We note that the H atom has a full valence shell with two electrons, while the F atom has a complete octet of electrons.

Explanation:

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