The sequence in which energy sublevels are filled is specified by

Aufbau principle

Explanation: Aufbau principle explains that electrons are fed into orbitals starting at the lowest energy level before filling higher energy levels.

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.