What makes a hydrocarbon saturated?

Explanation:

It's easier to answer what makes it unsaturated.

H - C ≡ C - H

This is Acetylene. It has three shared bonds between the two carbons. That many bonds makes it unsaturated.

     H   H

H - C - C - H

     H   H

Draw lines from 1 H on top to the Carbon underneath Do the same with the other hydrogen. Make the two Hydrogens on the bottom do the same thing.

The point is that Carbon has 4 bonds. 1 bond goes to the other Carbon. The other 3 take on Hydrogen. That molecule is saturated: nothing can be added in its natural state.

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.