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

Ionic compound calcium oxide (\(\mathrm{CaO}\)) is stable because, despite the energy required to remove electrons from calcium and add them to oxygen, the overall process is energetically favorable. The energy gain from the formation of the ionic compound, specifically the lattice energy, generally exceeds the ionization energies and electron affinities associated with transforming the free elements into their respective ions, making \(\mathrm{CaO}\) energetically favorable and more stable compared to its constituent elements.

Step-by-step solution

Understand the process of ion formation from free elements

Calcium (\(\mathrm{Ca}\)) has 2 valence electrons and has a tendency to lose these 2 electrons to achieve a stable electron configuration. When it loses 2 electrons, it forms a calcium ion (\(\mathrm{Ca}^{2+}\)) with a +2 charge. Oxygen (\(\mathrm{O}\)) has 6 valence electrons and has a tendency to gain 2 electrons to achieve a stable electron configuration. When it gains 2 electrons, it forms an oxide ion (\(\mathrm{O}^{2-}\)) with a -2 charge.

Discuss energy requirements for ion formation

To form \(\mathrm{Ca}^{2+}\), we need energy to remove 2 electrons from calcium. The energy required to remove each electron is called ionization energy. In the case of calcium, the total energy needed to remove 2 electrons equals the sum of the first and the second ionization energies. For oxygen, we need to add 2 electrons to form \(\mathrm{O}^{2-}\). The energy released in the process of adding an electron to an atom is called electron affinity. In the case of oxygen, the total energy released when adding 2 electrons equals the sum of the first and the second electron affinities.

Understand ionic bond formation

Now that we have both \(\mathrm{Ca}^{2+}\) and \(\mathrm{O}^{2-}\) ions formed, they can combine to form the ionic compound calcium oxide (\(\mathrm{CaO}\)). The electrostatic attraction between the oppositely charged ions (Coulombic attraction) holds them together in the compound. The energy change due to this attraction is called lattice energy, which contributes to the stability of the compound.

Why is \(\mathrm{CaO}\) stable relative to the free elements?

Although energy is required to remove 2 electrons from calcium and add 2 electrons to oxygen, the overall process is energetically favorable because energy is also released when the ionic compound \(\mathrm{CaO}\) forms. The lattice energy is typically larger than the ionization energies and electron affinities involved, meaning that the formation of the stable ionic compound releases more energy than it consumes. Thus, even though some energy is required to form the ions, the overall energy released during the process compensates for the energy input, making the formation of \(\mathrm{CaO}\) energetically favorable and stable compared to its free elements.

Key concepts

Ionization Energy

Ionization energy is the energy required to remove an electron from an isolated atom or ion. In other words, it's how much energy is needed to strip an atom of its electrons, one by one. The first ionization energy refers to the energy needed to remove the first electron, and it's generally easier to remove this first electron than the ones that follow. The second ionization energy is always higher because it's harder to remove an electron from a positively charged ion than from a neutral atom.

To illustrate, when we look at calcium (Ca), which possesses 2 valence electrons, removing these electrons requires sufficient energy input. This is because electrons are attracted to the protons in the nucleus, and overcoming this attraction needs energy, thus explaining why ionization energy is a pivotal concept in understanding ionic bond formation.

Electron Affinity

Electron affinity, in contrast to ionization energy, is the energy change that occurs when an electron is added to a neutral atom. Atoms like oxygen (O) have a high affinity for electrons, meaning they release energy when they gain an electron. This release of energy can be thought of as the atom's 'happiness' at becoming more stable.

It is important to note that the first electron affinity is typically exothermic (releases energy), but adding additional electrons to an already negative ion requires more energy, making subsequent electron affinities positive or less exothermic. Therefore, understanding electron affinities is vital in explaining why certain elements tend to gain electrons and thus contribute to the formation of stable ionic compounds.

Lattice Energy

Lattice energy is crucial when discussing the stability of ionic compounds. It is the energy released when ions bind together to form a solid crystal lattice. The stronger the charge and the smaller the ion, the greater the lattice energy. This energy is a direct measure of the stability of the crystal lattice structure of ionic compounds.

The strong electrostatic forces between the oppositely charged ions in calcium oxide (CaO) result in a high lattice energy, which means that a lot of energy is released when these ions come together. This significant release of energy is the key reason ionic compounds like CaO are so stable compared to the separate atoms they're formed from.

Chemical Stability

Chemical stability is the tendency of a chemical substance to maintain its original chemical composition without degradation or change. For an ionic compound like calcium oxide (CaO), stability comes from the exothermic nature of its formation. Despite the energy required to ionize calcium and add electrons to oxygen, the overall process releases more energy than consumed, mainly due to the high lattice energy. This means that the end product is more stable energetically than the starting reactants, a principle that drives many chemical reactions towards the formation of ionic bonds.

Valence Electrons

Valence electrons play a fundamental role in chemical reactivity and the formation of bonds. These are the electrons found in the outermost shell of an atom and are involved in forming chemical bonds. For instance, calcium has two valence electrons which it can 'happily' lose to achieve a noble gas configuration and become stable as a Ca2+ ion. Oxygen, on the other hand, has six valence electrons and needs two more to reach the stable configuration of eight valence electrons in its outer shell, becoming an O2- ion. Thus, the transfer of valence electrons from one atom to another is the essence of ionic bond formation.

The movement of valence electrons from metal to nonmetal atoms and the resultant full valence shells are what create the chemically stable situation in ionic compounds. This stability arises as atoms seek the lowest energy state, which typically corresponds to a full outer electron shell, analogous to that of noble gases.