How many electron shells would be completely filled by a neutral atom of calcium?
Three. The fourth shell would contain two out of a possible eight electrons.
The first 3 energy levels are filled, the 4s and 4p and 4d sublevels are filled, and the 5s and 5p sublevels are also filled. So only the first three energy levels are completely filled. The fourth and fifth energy levels are partly filled. The electron configuration is 1s22s22p63s23p63d104s24p64d105s25p6 or [Kr]4d105s25p6.