An ion that consists of 7 protons, 9 neutrons, and 10 electrons has a net charge of A) 2–
B) 2+
C) 3+
D) 3–
The answer is D. Why the answer is not A but D? Can you explain?

altitude, density, pressure

air gets less dense the higher the altitude and also the air pressure is less as altitude rises.

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.