Final answer:
The formula for the monoatomic cation for calcium is Ca²+, representing a calcium ion that has lost two electrons and carries a +2 charge.
Step-by-step explanation:
The formula for the monoatomic cation for calcium is Ca²+. When calcium (with atomic number Z = 20) loses two electrons to form a cation, it has a +2 charge due to having two more protons than electrons. This resulting ion is represented as Ca²+ and is a monoatomic cation because it consists of a single atom with a positive charge. This is a fundamental concept when learning about ionic compounds and how to balance the overall charge.