Question

(a) What is the oxidation state of \(\mathrm{P}\) in \(\mathrm{PO}_{4}{ }^{3-}\) and of \(\mathrm{N}^{-}\)in \(\mathrm{NO}_{3}{ }^{-}\)? (b) Why doesn't N form a stable \(\mathrm{NO}_{4}^{3-}\). ion analogous to \(\mathrm{P}\) ?

Short answer

The oxidation state of P in PO₄³⁻ is +5 and the oxidation state of N in NO₃⁻ is also +5. Nitrogen does not form a stable NO₄³⁻ ion because it has a maximum oxidation state of +5 due to its electronic configuration, which only allows for forming five covalent bonds using the electrons in the 2s and 2p orbitals.

Step-by-step solution

1. Determine the oxidation state of P in PO₄³⁻

To find the oxidation state of P, we can use the following rules: - The oxidation state of oxygen is usually -2. - The sum of the oxidation states of all atoms in a polyatomic ion equals the charge of the ion. In the PO₄³⁻ ion, there are four oxygen atoms, each with an oxidation state of -2. Therefore, the total negative charge contributed by the oxygen atoms is -8. Since the overall charge of the phosphate ion (PO₄³⁻) is -3, we can calculate the oxidation state of P as follows: Oxidation state of P + (-8) = -3 Oxidation state of P = -3 + 8 Oxidation state of P = +5

2. Determine the oxidation state of N in NO₃⁻

Similarly, we can use the rules mentioned above to find the oxidation state of N in NO₃⁻. There are three oxygen atoms, each with an oxidation state of -2, so the total negative charge contributed by oxygen atoms is -6. The overall charge of the nitrate ion (NO₃⁻) is -1. Thus, we can calculate the oxidation state of N as follows: Oxidation state of N + (-6) = -1 Oxidation state of N = -1 + 6 Oxidation state of N = +5

3. Explain why N doesn't form a stable NO₄³⁻ ion

In order to form a stable NO₄³⁻ ion, nitrogen should be in an oxidation state of +7 since there would be four oxygen atoms (-2 each) contributing to a total charge of -8. However, nitrogen has a maximum oxidation state of +5 due to the fact that it has only five valence electrons to lose, share or gain in a chemical bond. The formation of the NO₄³⁻ ion would require nitrogen to lose, share or gain seven electrons, which is not possible for nitrogen due to its electronic configuration. Nitrogen has 2 electrons in the 1s orbital and 5 electrons in the 2s and 2p orbitals. It can only form a maximum of 5 covalent bonds using the electrons in the 2s and 2p orbitals. Therefore, nitrogen doesn't form a stable NO₄³⁻ ion analogous to phosphorus.

Key concepts

Oxidation State Determination

Understanding the oxidation state of an element within a compound or ion is essential for interpreting the substance's chemical behavior. It indicates the degree of oxidation (loss of electrons) or reduction (gain of electrons) of an atom in a compound. Determining oxidation states is a straightforward process involving a set of rules.

To determine the oxidation state, Generally, start by applying known states such as those of alkali metals (+1) and alkaline earth metals (+2), and the common -2 state of oxygen. We then balance the remaining atoms to ensure the sum of oxidation states equals the charge on the ion or zero for a neutral compound. For polyatomic ions like \(\mathrm{PO}_4^{3-}\) and \(\mathrm{NO}_3^-\), recall that the sum of the oxidation states must equal the overall charge of the ion. In these examples, the oxidation states of phosphorus and nitrogen can be calculated by considering the known oxidation state of oxygen and the charge on the ion itself.

Practical Application in Calculating Oxidation States

Using the rules mentioned earlier, we calculated in Step 1 that the oxidation state of phosphorus in \(\mathrm{PO}_4^{3-}\) is +5. Similarly, in Step 2, the nitrogen in \(\mathrm{NO}_3^-\) also has an oxidation state of +5. These calculations allow us to predict and understand reactivity, bonding, and color changes in reactions involving these ions.

Valence Electrons and Bonding

The valence electrons are the outermost electrons of an atom and are primarily responsible for an element's chemical properties. These electrons participate in bond formation, either by being shared with another atom to form a covalent bond or by being transferred to or from another atom, resulting in ionic bonds.

For instance, nitrogen typically has five valence electrons, which can form a maximum of five covalent bonds, corresponding to its oxidation state of +5 in the nitrate ion. Likewise, phosphorus, which can have an oxidation state up to +5, uses its valence electrons to form bonds in the phosphate ion \(\mathrm{PO}_4^{3-}\).

Bonding Limitations Due to Valence Electrons

As seen in the exercise, nitrogen cannot exceed an oxidation state of +5 and therefore cannot form a stable \(\mathrm{NO}_4^{3-}\) ion. This is because nitrogen's valence electrons are limited to the five it has available for bonding in the second energy level (2s and 2p orbitals), setting a boundary for the number of covalent bonds it can form.

Stability of Polyatomic Ions

The stability of polyatomic ions is influenced by the balance between the repulsive forces among like-charged electrons and the attractive forces between electrons and protons. The distribution of oxidation states among the atoms within an ion helps maintain stability. In a stable polyatomic ion, each atom achieves a state akin to the noble gas configuration, and the oxidation states of each atom must balance out to equal the overall charge of the ion.

Electron Configurations and Stability

The stability of \(\mathrm{PO}_4^{3-}\) can be attributed to the achievement of a noble gas electron configuration for phosphorus when it has an oxidation state of +5. On the other hand, the hypothetical \(\mathrm{NO}_4^{3-}\) ion would demand nitrogen to reach an oxidation state of +7 to balance the negative charges from four oxygen atoms - a state that nitrogen cannot achieve given its electron availability, hence destabilizing the ion. Evidently, the stability of polyatomic ions is not merely about achieving charge balance but also concerning the structural limitations imposed by the electronic configurations of their constituent atoms.