Which electrons in a calcium atom in the ground state have the greatest effect on the chemical properties of calcium

Answer is: the two electrons in the fourth shell.

Atomic number of calcium is 20, it means that it has 20 protons and 20 electrons, so atom of calcium is neutral.  

Electron configuration of calcium atom: ₂₀Ca 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²; calcium has two valence electrons (4s²).  

In reactions it lose those two electrons ang form cation with positive charge 2+.

Atomic number is the number of protons, which is characteristic of a chemical element.

Beryllium, magnesium,calcium and strontium are alkaline earth metals. The elements have very similar properties.

Alkaline earth metals have in common an outer s- electron shell (two electrons).

altitude, density, pressure

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

water and alcohols have similar properties because water molecules contain hydroxyl groups that can form hydrogen bonds with other water molecules and with alcohol molecules, and likewise alcohol molecules can form hydrogen bonds with other alcohol molecules as well as with water.