Why do electronegativity charts often give no values to the group 18 elements?

Because they have complete electronic configuration

Explanation:

Group 18 elements are known as the noble or inert gases. The elements in this group are non-reactive because they have complete electronic shell configuration.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.