In PH4+, all the orbitals are used in bond formation where as, in PH3 one lone pair is present. In PH4+, the HPH bond angle is tetrahedrel angle of 109• .but in ph3 the lone pair bond pair repulsion is higher than bond pair - bond pair repulsion .tjus the bond angle becomes less than 109.5•

**Bond Angle in PH3**

Phosphine (PH3) is a molecule composed of one phosphorus atom and three hydrogen atoms. The bond angle in PH3 is approximately 93.5 degrees.

**Bond Angle in PH4**

Phosphonium (PH4) is a cation composed of one phosphorus atom and four hydrogen atoms. The bond angle in PH4 is approximately 109.5 degrees.

**Explanation**

The difference in the bond angles between PH3 and PH4 can be attributed to the electronic and steric effects.

**Electronic Effects:**

In PH3, the phosphorus atom has a lone pair of electrons, which causes electron-electron repulsion. As a result, the bond angles in PH3 are slightly compressed, leading to a smaller bond angle of 93.5 degrees. The lone pair occupies more space compared to the bonded pairs, resulting in a smaller bond angle.

On the other hand, in PH4, all four hydrogen atoms are bonded to the phosphorus atom without any lone pairs. This absence of a lone pair reduces electron-electron repulsion, allowing for a larger bond angle of 109.5 degrees. The bonded pairs are spread out evenly around the phosphorus atom, resulting in a larger bond angle.

**Steric Effects:**

Steric effects refer to the repulsion between atoms or groups due to their physical size. In PH3, the lone pair of electrons on the phosphorus atom creates additional repulsion with the hydrogen atoms. This repulsion causes the hydrogen atoms to be pushed closer together, resulting in a smaller bond angle.

In PH4, there are no lone pairs present on the phosphorus atom. As a result, there is no additional steric repulsion between the hydrogen atoms, allowing them to be further apart and resulting in a larger bond angle of 109.5 degrees.

Overall, the presence of a lone pair of electrons in PH3 causes additional electron-electron repulsion and steric repulsion, resulting in a smaller bond angle compared to PH4, which lacks a lone pair.