Hello,
The definition of temperature is such that it can be thought of as a direct measure of the level of molecular motion (a simplification) i.e. hot objects contain molecules that exhibit more motion than those in a cooler object (in general...again, a valid simplification). Think of molecular motion as vibrational and translational i.e. each molecules vibrates as it moves (translates) around.
The density of a gas is simply the mass per unit volume, which is affected by temperature. A higher temperature means more molecular motion (by definition). More molecular motion means, when they hit things, the molecule's impart more force which in turn means the pressure exerted by the gas increases - this causes the gas to exert more force on the walls of its container and, assuming it has enoughy give, cause the container (and therefore the volume occupied by the gas) to expand. Same mass in a bigger volume means lower density.
As density is just mass per unit volume you can see that if you imagine a certain number of gas molecules then it is the distance between them in a certain region which determines the density i.e. if they're far apart then there'll be less in a given volume therefire there'll be less mass in a given volume i.e. the density is lower.
Even if the individual molecules expanded by any appreciable degree (which they don't really) e.g. they doubled in size, you can picture how this would have no real impact on the density: there would still be more or less the same number in a given volume (say 1m^3), they'd just be a "stretched" a bit more - you can see this by just sketching a box with some molecules in then strecthing each one - the number of molecules is, for all intents and purposes, the same. This means the mass in the volume is unchanged which means the density is unaffected.
Thus it's the temperature (= molecular motion) of the substance that affects density, not any expansion/contraction of the individual molecules (which is practically non-existent anyway).
Hope this helps,
V1R
