text: Hello Catherine, Your question is a very good one.Now,in the first molecule OF 2, all the valence electrons are used up in bonding and it forms a stable molecule having linear geometry. In the second molecule SCL 2 the same is happening and the molecule is linear in geometry.But, in the last one that is PF 5, the three valence electrons of Phosphorus are used up in bonding with Fluorine electrons,leaving two  electrons of Phosphorus as Lone-pairs,as a  result the molecule has a distorted geometry and the molecule has a Pyramidral shape like that of NH 3. I think I can able to make you understand.Bye.