What is the difference between speed and average speed?

Speed is speed, or how fast something is going, while velocity is speed or how fast something is going in a specific direction. Average speed is the speeds added up and divided by number of speeds, and you do the same thing to calculate average velocity.

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.