Why is there a difference in the H–X–H bond angles in CH4 andNH3?

The difference in the H–X–H bond angles in CH4 and NH3 can be explained by the concept of electron pair repulsion theory. According to this theory, electron pairs in a molecule repel each other and try to stay as far apart as possible to minimize repulsion.

In CH4 (methane), there are four hydrogen atoms bonded to a central carbon atom. The carbon atom is surrounded by four electron pairs, which are arranged in a tetrahedral shape. The bond angles between the hydrogen atoms in CH4 are all approximately 109.5 degrees. This is because the electron pairs repel each other equally, causing the hydrogen atoms to be positioned as far apart as possible.

In NH3 (ammonia), there are three hydrogen atoms bonded to a central nitrogen atom. The nitrogen atom is also surrounded by a lone pair of electrons. The presence of the lone pair affects the bond angles in NH3. The lone pair occupies more space than a bonding pair, causing greater repulsion. As a result, the bond angles in NH3 are slightly less than 109.5 degrees. The actual bond angle in NH3 is approximately 107 degrees.

Therefore, the difference in the H–X–H bond angles in CH4 and NH3 is due to the presence of a lone pair on the central atom in NH3, which causes greater electron pair repulsion and slightly decreases the bond angles compared to CH4.