Calcium is an element that belongs to the second group on the periodic table. Calcium has a valency of two and would readily loose both electrons in order to achieve stability when forming bonds.
It has 20 electrons.
Calcium would favor ionic bonds when combining with other atoms and by the virtue of this, it would readily transfer its two outermost electrons. This would make its atom mimic that of Argon-18.
The correct information is given below:
Electron structure of atom: 1S²2S²2P⁶3S²3P⁶4S²
Electron structure of ion: 1S²2S²2P⁶3S²3P⁶
Net ionic charge: +2
1s², 2s², 2p⁴
To determine the filling of electrons in an orbital, we'll have to apply several rules such as
1. Aufbau principle
2. Hunds rule
3. Pauli's exclusion principle.
I won't explain the above rules because I don't want to deviate from the question, but however if you understand them, it'll make questions like this easier to answer.
For oxygen atom
₈O = 1s² 2s² 2p⁴
The filling of the orbital must start with 1s orbital till the space of the two electrons are filled before moving to 2s orbital filling the two space in the shell and then finally the remaining 4 electrons on the 2p orbital.
Assuming the net ionic charge of oxygen in this case is zero (0) and has not accepted electron.
However, oxygen usually accepts 2e⁻(electrons) to attain stability (all orbital strive to attain duplet or octet configuration).
Assuming we have O²⁻, the electron configuration change to 1s² 2s² 2p⁶. This is the state oxygen usually forms compounds in excerpts with group 1 metals where it is able to form super oxide.