Question

What would be the subscripts for the formula unit Ca_O_?

Answer 1

Answer:

CaO    ( So subscripts are 1,1)

Explanation:

In order to determine the possible subscripts of the element/ molecule you need to see the charges that each element has  

In this case  

Ca i.e. calcium has a charge of +2. Charges are present on the element because of the unequal number of electrons. The elements which have a positive charge are called cations

O i.e. Oxygen has a charge of -2 and they are also present due to the uneven number of electrons. The elements which have a negative charge are called anions.

Both the charges +2 of Ca and -2 of O will cancel out and will result in CaO

Answer 2

Answer: The subscripts for the formula unit is $Ca_{1}O_{1}$

Explanation:

It is known that electronic distribution of calcium is 2, 8, 8, 2. So, it loses two electrons to gain stability and thus acquires a +2 charge.

Similarly, electronic distribution of oxygen is 2, 8, 6 and in order to gain stability it accepts two electrons. Therefore, oxygen acquires a charge of -2.

Now, since both calcium and oxygen have a charge of 2, so by cross multiplication they cancel out the charge and thus, the formula for calcium oxide is CaO.