Question

which of the following is the smallest in volume? nucleus of oxygen atom, water molcule, protron, ice crystal, electron cloud of an oxygen atom

Answer 1

volume is directly proportional to mass

so the one with the smallest mass has the smallest volume

does that help?

Answer 2

You're probably mean to realize that you can make a logical arrangement of these things, by thinking about how some are made of others, viz.:

1) An ice crystal is made of water molecules, so an ice crystal must be larger than a water molecule.

2) A water molecule is made of hydrogen and oxygen atoms, so a water molecule must be larger than an oxygen atom.

3) An oxygen atom is made of an oxygen nucleus and an oxygen electron cloud. so the oxygen atom must be bigger than both.

4) The electron cloud in an oxygen atom surrounds the oxygen nucleus on all sides, so it must be bigger.

5) An oxygen nucleus is made of protons and neutrons, so it must be bigger than either a proton or neutron.

@lgbasallote, while the volume is proportional to the mass, the proportionality factor (the density) varies enormously, so this is not a useful fact in this context. In fact, the electron cloud of an oxygen atom has a far smaller mass than the oxygen nucleus, but is much bigger -- because it is far less dense.