The number of electrons for the Ca atom are 20 electrons. the first two electrons will go in the 1s orbital. Since 1s can only hold two electrons the next 2 electrons for Calcium go in the 2s orbital. The next six electrons will go in the 2p orbital. The p orbital can hold up to six electrons. The 2p orbital will be filled with six electrons and then put the next two electrons in the 3s. Next 6 electrons are placed ion the 3p orbital. Remaining 2 electron will be placed in 4s
Therefore, the Calcium electron configuration will be 1s22s22p63s23p64s2.