Question

Energy is required to remove two electrons from Ca to form \(\mathrm{Ca}^{2+}\) and is required to add two electrons to \(\mathrm{O}\) to form \(\mathrm{O}^{2-}\). Why, then, is \(\mathrm{CaO}\) stable relative to the free elements?

Short answer

The stability of \(\mathrm{CaO}\) relative to the free elements \(\mathrm{Ca}\) and \(\mathrm{O}\) is due to the combination of the energy released during electron transfer and the strong electrostatic attraction between the oppositely charged ions in the ionic bond. Although energy is needed to remove two electrons from \(\mathrm{Ca}\) and add them to \(\mathrm{O}\), the overall energy change is negative, leading to the stability of the formed compound.

Step-by-step solution

Understanding the formation of \(\mathrm{CaO}\)

First, we need to understand how \(\mathrm{CaO}\) is formed. Calcium, \(\mathrm{Ca}\), loses two electrons to become \(\mathrm{Ca}^{2+}\), and oxygen, \(\mathrm{O}\), gains two electrons to become \(\mathrm{O}^{2-}\). This is an example of an ionic bond, where oppositely charged ions (cations and anions) attract each other and form a stable compound.

Ionization energy and electron affinity

To remove two electrons from \(\mathrm{Ca}\) and add them to \(\mathrm{O}\), energy is required. Removing electrons from an atom requires energy called ionization energy, and adding electrons to another atom will typically release energy called electron affinity. Each element has different ionization energies and electron affinities. Removing two electrons from \(\mathrm{Ca}\) involves the first and second ionization energies, and adding two electrons to \(\mathrm{O}\) involves the first and second electron affinities.

Energy changes in the formation of \(\mathrm{CaO}\)

The formation of \(\mathrm{CaO}\) involves two key energy changes: (1) the energy required to remove two electrons from \(\mathrm{Ca}\), and (2) the energy released when adding two electrons to \(\mathrm{O}\). If the energy released in step 2 is greater than the energy required in step 1, the overall energy change will be negative, meaning the formation of \(\mathrm{CaO}\) is exothermic. This means excess energy is released into the environment and is a driving force for the formation of the stable compound.

Electrostatic attraction between ions

Another important factor contributing to the stability of \(\mathrm{CaO}\) is the electrostatic attraction between the oppositely charged ions – \(\mathrm{Ca}^{2+}\) and \(\mathrm{O}^{2-}\). This attraction is quite strong due to the high charges of the ions, leading to a stable ionic bond between them.

Explanation for the stability of \(\mathrm{CaO}\)

In summary, the stability of \(\mathrm{CaO}\) relative to the free elements \(\mathrm{Ca}\) and \(\mathrm{O}\) is due to the combination of the energy released when the electrons are transferred and the strong electrostatic attraction between the oppositely charged ions in the ionic bond. Even though energy is required to remove two electrons from \(\mathrm{Ca}\) and add them to \(\mathrm{O}\), the overall energy change is negative, leading to the stability of the formed compound.