To determine the correct formula unit for a compound made from calcium (Ca) and oxygen (O), we must consider how these elements form ions and how they balance their charges to form a neutral compound.
1. Identifying the charges of the ions:
- Calcium (Ca) is an element in Group 2 of the periodic table. It typically loses two electrons to form a Ca^2+ ion.
- Oxygen (O) is an element in Group 16 of the periodic table. It typically gains two electrons to form an O^2- ion.
2. Balancing the ionic charges:
- Calcium forms a Ca^2+ ion, which has a charge of +2.
- Oxygen forms an O^2- ion, which has a charge of -2.
- To create a neutral compound, the total positive charge must balance the total negative charge.
3. Forming the compound:
- Since Ca^2+ has a +2 charge and O^2- has a -2 charge, one Ca^2+ ion will balance with one O^2- ion. This results in the neutral compound CaO.
Thus, the correct formula unit for a compound made from calcium and oxygen is "CaO because the total positive charge on Ca is two."
