Each atom of protium has one proton, no neutrons, and one electron. each atom of deuterium has one proton, two neutrons, and one electron. are these the same or different elements? why? asap you guys!

These are the same element.  

Explanation:

H = 1p + 1e

D = 1p + 1e + 1n

Protium (H) and deuterium (D) are isotopes of the same element (H).

If two atoms have the same number of protons, they are the same element.

If they have the same number of protons but different numbers of neutrons, they are isotopes.

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.