What is the electron configuration of Ca?
Long-hand version: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 Short-hand version: [Ar] 4s^2 Note: The "^" symbol means the the following number is in the form of a superscript.
The element calcium, or Ca, is located in group 2, period 4 of the periodic table. Therefore, its electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2. This can also be written as [Ar] 4s2, because the beginning of that electron configuration is the same as argon's. The 42 you mentioned in your question is referring to a specific isotope of calcium and has nothing to with the electron configuration.