What subatomic particle is in constant movement around the nucleus forming a cloud like appearance​

Electron

Explanation:

Electrons are negatively charged particles. They travel in paths, called orbits, within the electron cloud surrounding the nucleus. Electrons are constantly moving.

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.