When a balloon is rubbed on a wool sweater, the balloon builds up a negative electrical charge. When a glass rod is rubbed on silk, the rod builds up a positive electrical charge. Explain why the charged balloon will be attracted to the charged glass rod

The balloon will be attracted to the charged glass rod as there may be a positive energy there. It may also be that it is attracted because they both are charged.

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.