Why do solids have a definite shape and a definite volume?

Those are different gas lawshere is a brief description of themcharles's law states that volume and temperature are directly proportional to each other as long as pressure is held constant.  boyle's law asserts that pressure and volume are inversely proportional to each other at fixed temperature.gay-lussac's law introduces a direct proportionality between temperature and pressure as long as it is at a constant volume.    source:

So the main goal for the elements is stability which is a complete/full valence shell and typically the magic number for a stable element is 8 valence electron but not always. so elements to the left of the periodic table tend to be way less stable then those on the right, so the elements on the left are much more reactive.