Calcium is also in Group 2. It has two electrons in its outer shell. When these electrons are lost, a calcium ion Ca2+ is formed. A calcium ion has the same electronic structure as an argon atom (Ar).
Regarding this, how do you know the charge of an element?
To find the ionic charge of an element you'll need to consult your Periodic Table. On the Periodic Table metals (found on the left of the table) will be positive. Non-metals (found on the right) will be negative. But you need to know the specific ionic charge elments.
How do you know if an element is positive or negative?
That means an atom with a neutral charge is one where the number of electrons is equal to the atomic number. Ions are atoms with extra electrons or missing electrons. When you are missing an electron or two, you have a positive charge. When you have an extra electron or two, you have a negative charge.