Question

Why is the boiling point of NH3 is greater than that of PH3??

Answer 1

`NH_3` has hydrogen bonds while `PH_3` has permanent dipole permanent dipole bonds between `PH_3` molecules.

Explanation:

Both have simple molecular structures, however if you recall, nitrogen is more electronegative than phosphorus and is part of the three special elements, Nitrogen,Oxygen and Fluorine that can participate in hydrogen bonding.

As a result, the hydrogen atoms in `NH_3` form a stronger positive dipole as compared to the hydrogen in `PH_3` and thus form a more extreme form of permanent-permanent dipole interaction known as hydrogen bonds which require a lot more energy to overcome.

Answer 2

Nitrogen is more electronegative than phosphorous, to the point where it has hydrogen-bonding interactions.

Explanation:

Although both of these substances are neutrally charged molecules, they are polar because the regions around the hydrogen atoms are slightly positive (δ+) (due to the larger central atom, either nitrogen or phosphorous, pulling more strongly on the shared electron pair) while the unbonded electron pairs are slightly negative (δ-) (due to electrons being negatively charged).

Since opposite charges attract, the negative and positive ends of the molecules are drawn together by dipole-dipole interactions, making the molecules "stick" together and making it harder for them to separate and enter the gaseous state.

Both substances are structured the same way at the molecular level; one central atom with three hydrogen atoms and one unbonded electron pair. However, nitrogen is more electronegative than phosphorous, meaning that it has a stronger tendency to attract electrons. This means that it will pull more strongly on the shared electron pairs with hydrogen atoms, making them slightly more positive, increasing dipole-dipole interactions (making them strong enough to be considered hydrogen-bonds), and increasing the boiling point.

An easy way to compare electronegativity values is to look at the periodic table. Fluorine is the most electronegative element, with elements becoming less electronegative the farther away they get (moving left or moving down from fluorine).