Explain why a liquid can flow whilst a solid retains its shape.

A solid will retain its shape; the particles are not free to move around. Liquid matter is made of more loosely packed particles. It would take the shape of its container. Particles can move about within a liquid,  they are packed densely enough that volume is maintained.

Explanation:

the number of valence electrons for each molecule or ion is shown beneath the structure. remember hydrogen will not have more than two electrons. this hydrogen is part of a covalent bond (sharing two electrons). a double bond here would cause hydrogen to share four electrons with phosphorus.