Why are elements placed in the same group?
They have the same number of electrons in
their outer shell which causes them to have
similar properties
They have the same number of electrons in
their outer shell
They have similar properties
They are in the same period

Explanation:The chemical elements are arranged in order of increasing atomic number. The horizontal rows are called periods and the vertical columns are called groups. Elements in the same group have similar chemical properties. This is because they have the same number of outer electrons and the same valency

altitude, density, pressure

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

the awnser is group 12