Do atoms of one element change into atoms of another element during chemical reactions

Nope

Chemical reactions occur when atoms are separated, joined or rearranged.

However, atoms of one element are never changed into atoms of another element during a chemical reaction.

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.

there is none because earth is stationary and flat.