Role and significance of guassion or normal distribution?

The normal distribution (or Gaussian distribution), also referred as bell curve, is very useful due to the central limit theorem. Normal distribution states which are average of random variables converge in distribution to the normal and are normally distributed when the number of random variables is large.

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.