To assess the given statements regarding BF3, PF5, BF4, and NH3, let’s analyze each molecule in terms of the octet rule.
Statement I: BF3 and PF5 are examples of violations of the octet rule.
BF3 is a common example of a molecule that does not follow the octet rule. Boron has only six electrons in its valence shell when bonded with three fluorine atoms. Since it is an electron-deficient compound, it often seeks to bond with other atoms to complete its octet. On the other hand, PF5 (phosphorus pentafluoride) does indeed exceed the octet rule, as phosphorus has five bonds and thus has ten electrons in the valence shell. This is permissible because phosphorus is in the third period of the periodic table and can utilize d-orbitals for bonding. Therefore, this statement is correct.
Statement II: BF4 and NH3 are examples of violations of the octet rule.
BF4 (tetrafluoroborate) is a molecule where boron forms four bonds with fluorine atoms, resulting in an expanded valence shell with eight electrons. Consequently, it adheres to the octet rule, making this part of the statement incorrect. On the other hand, NH3 (ammonia) has nitrogen bonded to three hydrogen atoms, and it has a lone pair of electrons, giving it a complete octet. Thus, NH3 does follow the octet rule. Hence, this portion of the statement is also incorrect.
In summary, only Statement I is correct. BF3 does not reach an octet, while PF5 exceeds it, while both BF4 and NH3 follow the octet rule.