Question

A phosphite ion, \(\mathrm{PO}_{3}{ }^{3-},\) can bond to an oxygen atom to form the phosphate ion. Draw the electron dot formula for \(\mathrm{PO}_{4}^{3-}\) and label a coordinate covalent bond.

Short answer

32 valence electrons are used to form \( \mathrm{PO}_{4}^{3-} \); one oxygen donates a lone pair for a coordinate bond.

Step-by-step solution

Calculate Total Valence Electrons

First, count the total number of valence electrons available for the ion \( \mathrm{PO}_{4}^{3-} \). Phosphorus (P) has 5 valence electrons and each Oxygen (O) has 6 valence electrons. Additionally, the 3- charge indicates the addition of three extra electrons. Total valence electrons: \(5 + 4 \times 6 + 3 = 32\) electrons.

Arrange Atoms

Place the Phosphorus atom in the center because it is less electronegative compared to Oxygen. Arrange the four Oxygen atoms around the central Phosphorus atom to form a tetrahedral structure.

Connect Atoms with Single Bonds

Draw a single bond between the central Phosphorus atom and each of the four surrounding Oxygen atoms. Each single bond uses 2 electrons, so with four single bonds, 8 electrons are used (\(4 \times 2 = 8\)). This leaves \(32 - 8 = 24\) electrons remaining.

Distribute Remaining Electrons

Distribute the remaining 24 electrons to complete the octet for each Oxygen atom. Start by placing 6 additional electrons on each Oxygen, because Phosphorus can have expanded octets. Now all 32 electrons are accounted for.

Form a Coordinate Covalent Bond

To emphasize the formation of a coordinate covalent bond, note that one of the Oxygen atoms forms a double bond with Phosphorus by sharing its lone pair of electrons. Mark this double bond as the coordinate bond since the entire lone pair comes from the Oxygen atom.

Key concepts

Electron Dot Formula

An electron dot formula, also known as a Lewis dot structure, visually represents how atoms in a molecule or ion share electrons to form chemical bonds. In the phosphate ion, \(\mathrm{PO}_{4}^{3-}\), it helps us visualize the bonds between the central phosphorus atom and surrounding oxygen atoms. Each pair of dots represents a pair of shared electrons, typically forming a single or double bond.

For \(\mathrm{PO}_{4}^{3-}\), the central phosphorus atom is surrounded by four oxygen atoms. The electron dot formula shows bonds between phosphorus and each oxygen atom, with one of these bonds highlighted as a coordinate covalent bond. This visualization aids in understanding the molecular geometry and electron sharing in the ion.

Valence Electrons

Valence electrons are the outermost electrons of an atom and they are crucial in chemical bonding. They dictate how an atom will interact with others. For phosphorus, the valence electron number is 5, and for each oxygen atom, it is 6. In the phosphate ion \(\mathrm{PO}_{4}^{3-}\), having a 3- charge means there are additional electrons to consider.

Let’s calculate this:

• Phosphorus (P): 5 valence electrons
• Oxygen (O): \(4 \times 6 = 24\) valence electrons
• Additional from the 3- charge: 3 electrons

Adding these, we have \(5 + 24 + 3 = 32\) valence electrons in total. Understanding this calculation helps you distribute electrons correctly across the molecule, ensuring each atom achieves a stable electron configuration.

Coordinate Covalent Bond

A coordinate covalent bond is a unique type of chemical bond where both electrons in the bond originate from the same atom. In the context of \(\mathrm{PO}_{4}^{3-}\), one of the oxygen atoms uses its lone pair to form a double bond with the phosphorus atom.

This bond is noteworthy because it emphasizes how atoms can share electrons unevenly. It's represented in the electron dot formula by marking the bond distinctly, usually with an arrow pointing from the donor atom (oxygen) toward the recipient atom (phosphorus). This depiction helps clarify that the pair of electrons used in the bond donation originates solely from the oxygen atom.

Expanded Octet

The concept of an expanded octet refers to certain atoms, like phosphorus, being able to hold more than eight electrons in their valence shell. This is possible because these atoms, typically found in or beyond period three of the periodic table, have access to d orbitals.

In \(\mathrm{PO}_{4}^{3-}\), the phosphorus atom forms one regular single bond and three double bonds, summing up more than eight electrons around it. This behavior might seem counter-intuitive given the "octet rule," but it's a real and well-observed phenomenon in chemistry.

• Helps in accommodating additional electrons when forming molecules.
• ChuChemistrycal reactions can involve heavier, non-metal elements.

Understanding expanded octets is crucial for explaining the structure and reactivity of many polyatomic ions and molecules.