Answer:
2
Step-by-step explanation:
For the first 20 elements, the first energy level can hold a maximum of two electrons, the second energy level can hold a maximum of 8 and the third energy level can hold a maximum of 8. Calcium has 20 electrons, so we put two in the first energy level, 8 in the second, 8 in the third and we are left with two for the fourth energy level.
Ca: 2,8,8,2