I have two big problems in my mind about hydrogen bond for many years. I have checked many textbooks but all of them fails to explain what I don't understand. 1. To form a hydrogen bond (particular strong attraction) Why it is necessary for a hydrogen atom to attach to a highly electronegative atom and form bond with the lone pair of electrons of another electronegative atom?? 2. For HF (Hydrogen Fluoride), it is said that it can form two hydrogen bond for each molecule, while for H2O (water), it is said that it can form four hydrogen bond for each molecule. Why?? I really don't understand why HF could only form 2 Hydrogen bond after studying the problem for a very long time. I think there are three lone pairs of electrons in F atom, so it can possibly form three hydrogen bond with neighboring HF molecule, while H atom in HF can form another hydrogen bond so it results in a total of four hydrogen bonds formed for each molecule instead of two which is said by most textbooks. But I know I should be wrong, but I really can't understand. Please explain it to me. Thanks very much!!