Why is PCl5 and acceptable compound, but NF5 is not?

1s2 2s2 2p3 is the configuration of Nitrogen. Its valance shell is 2 and there are 5 valance electrons which can participate in the bond formation out of which there are 3 unpaired electrons and 2 paired electrons. So, the Three unpaired electrons can form Three bonds covalently. But the two paired electrons cannot get separated and form two bonds covalently instead they become a bond pair, rather than being as lone pair, by both participating together in the bond formation. Such type of bond in which both the electrons of the same atom are donated to the vacant orbital of the other atom is called as co-ordinate covalent bond. This type of bond brings a positive charge on the Nitrogen. This bond is considered as single bond. So, summing up together, the maximum number of bonds formed by nitrogen is 4(3 unpaired electrons - 3 bonds. 2 paired electrons - 1 bond). So forming 5 bonds is impossible for nitrogen.

Therefore NCl5 and NF5 are impossible because there are 5 bonds made by nitrogen which is impossible as per the above discussion.

yes

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substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.