In the reaction of calcium with chlorine, which atom is oxidized? ca + cl2 -> cacl2

Redox reactions consist of reduction and oxidation reactions
oxidation reactions are the giving out of electrons, the oxidation number of the species giving out the electrons increases during oxidation 
reduction reactions are taking in electrons and the oxidation number of the species taking in electrons reduces after reduction
in the reaction Ca + Cl₂ --> CaCl₂
oxidation numbers of Ca in Ca - 0 and CaCl₂ - +2
oxidation numbers of Cl in Cl₂ - 0 and CaCl₂ - -1
oxidation number of Ca has increased from 0 to +2 therefore Ca is oxidised

The air is going to come out because of the low pressure.