hydrogen bonding 啫係點

hydrogen bonding is not ionic bond nor covalent bond.
You may regard Hydrogenbond as Van der waals' force, and is stronger than
the other Van der waals' force i.e. dipole attraction

There is no involvement in sharing electron in H-bond. In Hydrogen bond,
Y---HX
the orbital holding lone pair of electronegative atom Y overlap with the empty
orbital of hydrogen atom. You also have to understand, Y , being electronegative atom, carries partial negative charge, and H, hydrogen carries partial postive charge. + form bond with -. Hydrogen bond is raised from this.

water molecule cannot form 4 hydrogen bond, but the coordination number
of water is 4.

Covalent bonds - the elements SHARE PAIRS of electrons.
Ionic bonds - ions (elements with different no. of protons & electrons - thus have either positive (+) or negative (-) charge) bond as they have different charges.
As one knows "same charge repell, opposite charges attract"

As the name "hydrogen bond" implies, one part of the bond involves a hydrogen atom. The hydrogen must be attached to a strongly electronegative heteroatom, such as oxygen, nitrogen or fluorine, which is called the hydrogen-bond donor. This electronegative element attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a positive partial charge. Because of the small size of hydrogen relative to other atoms and molecules, the resulting charge, though only partial, nevertheless represents a large charge density. A hydrogen bond results when this strong positive charge density attracts a lone pair of electrons on another heteroatom, which becomes the hydrogen-bond acceptor.

The hydrogen bond is not like a simple attraction between point charges, however. It possesses some degree of orientational preference, and can be shown to have some of the characteristics of a covalent bond. This covalency tends to be more extreme when acceptors bind hydrogens from more electronegative donors.