A Note about Angles

The structure of a molecule determines the angles to a very large degree. In other words, HNO is definitely bent, not linear, even though there are only two atoms. 

However, it would be dishonest to say that shape doesn't have some bearing on the angles. 

Think of structure as the rough focus and shape as the fine adjustment, making slight, but measurable changes in the angles.

The question of course is why. The answer lies in the space occupied by electron pairs that are lone pairs compared to those in bonds. Specifically, a bonded pair of electrons is pulled by nuclei from both ends and, as a result, ends up longer and thinner than a lone pair which is only attached at one end. 

As a result, any lone pairs in a structure will occupy a little extra space and will compress the angles between any bonded atoms, as shown below:

So, in HNO, the measured angle of the shape (the angle between the atoms) is a little bit less than the typical trigonal planar angle, certainly nowhere near as small as the angles in a tetrahedron, but not quite the full 120^o expected from the structure. 

A decent approximation is that the angles between the atoms lose 2 to 2.5^o for every lone pair in a structure. Thus the angle in HNO is about 118^o (120-2), the angle in NH3 is about 107^o (109.5-2.5) and the angle in water (with 2 lone pairs) is about 104.5^o (109.5 – 5).

In addition, just to be confusing, angles can also depend to some extent on the energy level of the electrons involved in bonding (the valence electrons). Since higher valence levels involve larger orbitals, the repulsion is decreased and the angles are closer to the original 90 degrees of the p orbitals from which the bonds were formed. This idea will NOT be explored here,as it is beyond the scope of this text.