Question

Write the electron dot formula and draw the structural formula for each of the following polyatomic ions: (a) \(\mathrm{PO}_{4}^{3-}\) (b) \(\mathrm{HPO}_{4}^{2-}\) (c) \(\mathrm{PO}_{3}^{3-}\) (d) \(\mathrm{HPO}_{3}^{2-}\)

Short answer

The electron dot and structural formulas depict connectivity and electron distribution of ions, with octet completion and bearing charges appropriately.

Step-by-step solution

Understanding the Lewis Structures

To draw Lewis structures for polyatomic ions, identify the total number of valence electrons in the molecule. Add electrons for negative charges and subtract electrons for positive charges.

PO₄³⁻ Ion

The phosphate ion has one phosphorus atom (5 valence electrons) and four oxygen atoms (6 valence electrons each), adding up to 24 electrons. Adding three more for the 3- charge gives a total of 32 electrons. Start by placing P in the center and distributing electrons to O atoms, ensuring full octets on each oxygen. Note the resonance structures.

Structural Formula for PO₄³⁻

In the structural formula, draw P single-bonded to all four O atoms. Three O atoms are single-bonded with a negative charge on them, while one O forms a double bond to satisfy the formal charge balance with total -3 charge.

HPO₄²⁻ Ion

For HPO₄²⁻, include a hydrogen atom bonded to one oxygen. The total electron count is 5 (P) + 4x6 (O) + 1 (H) + 2 (for the charge) = 32 electrons. Distribute electrons ensuring full octets and adjust for the -2 charge with one oxygen forming a double bond.

Structural Formula for HPO₄²⁻

Draw P in the center with single bonds to three O atoms and one H bonded to one O. One O is double-bonded to P to balance the formal charges, leaving each single-bonded O with a negative charge.

PO₃³⁻ Ion

For the PO₃³⁻ ion, we have 5 (P) + 3x6 (O) + 3 (for charge) = 26 electrons. Central P is bonded to each O, one with a double bond while the rest bear a negative charge arranged in resonance.

Structural Formula for PO₃³⁻

Here, draw P in the center, single-bonded to two O atoms with negative charges, and double-bonded to one oxygen atom for charge balance and resonance depiction.

HPO₃²⁻ Ion

Construct HPO₃²⁻ by adding a hydrogen atom to one of the oxygen atoms of PO₃³⁻. Balance a total of 26 electrons: 5 (P) + 3x6 (O) + 1 (H) + 2 (charge adjustment). Ensure all atoms, particularly oxygen, have a full octet without extra charge on the structure due to suitable resonance formation.

Structural Formula for HPO₃²⁻

The structural formula should have P single-bonded to two O atoms (one of which bears H), and one O atom double-bonded. The other O atoms carry negative charges adjusted via resonance.

Key concepts

Polyatomic Ions

Polyatomic ions are fascinating chemical species made up of more than one atom. These atoms are covalently bonded together, but the entire ion bears a net charge. This charge results from an imbalance between the total number of protons and electrons in the ion.
Examples of polyatomic ions include those used in the exercise like phosphate (\(\mathrm{PO}_{4}^{3-}\)) and hydrogen phosphate (\(\mathrm{HPO}_{4}^{2-}\)).

• Polyatomic ions can carry positive or negative charges, although negative ions are often more common.
• The charge of a polyatomic ion is illustrated using a superscript number and sign after the chemical formula, indicating how many more electrons than protons are in the structure.
• These ions often participate in forming salts and other compounds through ionic bonding with metals or other oppositely charged ions.

Understanding polyatomic ions is crucial for predicting the chemical behavior of many compounds, especially when they react or dissolve in water.

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. They play a crucial role in chemical bonding because they are the electrons involved in forming bonds with other atoms. In the context of polyatomic ions, collecting and counting valence electrons is the first step to constructing their Lewis structures.
Take, for instance, the phosphate ion (\(\mathrm{PO}_{4}^{3-}\)). Here’s how one determines its electron count:

• First, know that phosphorus has 5 valence electrons.
• Each oxygen atom brings 6 valence electrons, summing to 24 electrons from four oxygens.
• The ion's 3- charge suggests an additional 3 electrons to this total, making 32 valence electrons in zero net charge configuration.

Knowing how to calculate and distribute these electrons is vital for drawing accurate Lewis and structural formulas for any polyatomic ion.

Octet Rule

The octet rule is a fundamental concept in chemistry stating that atoms tend to form bonds to achieve a full set of eight valence electrons, resembling the electron configuration of noble gases, which are notably stable. This rule is pivotal in constructing Lewis structures, as it helps predict how atoms will share or trade electrons to stabilize themselves.
When creating the electron dot formula for polyatomic ions:

• The octet rule guides the distribution of available electrons around each atom.
• In our phosphate ion (\(\mathrm{PO}_{4}^{3-}\)) example, phosphorus forms bonds with four oxygen atoms. Each oxygen seeks to fulfill its octet, and in doing so, resonance structures are often used to depict all conceivable electron distributions.
• While aiming for octets, sometimes expanded octets occur, especially with elements capable of holding more than eight electrons, such as phosphorus.

This rule, while not without exceptions, serves as an essential guideline for predicting the molecular configuration and reactivity of compounds.