Question

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

Short answer

Electron dot and structural formulas differ by bonding arrangement, reflecting electron pairing and resonance.

Step-by-step solution

Count Valence Electrons for PH4+

For the ion \(\mathrm{PH}_4^+\), phosphorus has 5 valence electrons, and each hydrogen has 1 valence electron (for a total of 4 hydrogens, giving 4 electrons). Subtract 1 electron due to the +1 charge, making the total 8 valence electrons.

Draw Electron Dot Formula for PH4+

Arrange the electrons around the \(\mathrm{PH}_4^+\) ion by placing phosphorus in the center with single bonds connected to each of the four hydrogens. Each bond represents two electrons.

Count Valence Electrons for SeO3^2-

For \(\mathrm{SeO}_{3}^{2-}\), selenium has 6 valence electrons, and each oxygen has 6 valence electrons (3 oxygens total, making 18 electrons). Add 2 electrons for the -2 charge, resulting in 26 valence electrons.

Draw Electron Dot and Structural Formula for SeO3^2-

Place selenium in the center and each oxygen around it. Form double bonds with two oxygens and a single bond with one oxygen, and allocate remaining electrons to satisfy the octet rule and accommodate the charge. The structural formula features double bonds to two oxygens and a single bond to one, with electron pairs on selenium and oxygen.

Count Valence Electrons for CO3^2-

For \(\mathrm{CO}_{3}^{2-}\), carbon contributes 4 valence electrons, and each oxygen provides 6 valence electrons (3 oxygens total, making 18 electrons). Add 2 for the -2 charge, resulting in 24 valence electrons.

Draw Electron Dot and Structural Formula for CO3^2-

Position carbon in the center with oxygens surrounding it. Establish one double bond with one oxygen and single bonds with the other two. Distribute remaining electrons as lone pairs to comply with the octet rule. The structural formula includes a mix of single and double bonds to depict resonance structures.

Count Valence Electrons for BO3^3-

For \(\mathrm{BO}_{3}^{3-}\), boron has 3 valence electrons, and each oxygen has 6 valence electrons (3 oxygens total, making 18 electrons). Add 3 electrons for the -3 charge, leading to 24 valence electrons.

Draw Electron Dot and Structural Formula for BO3^3-

Place boron centrally with oxygens radiating outwards. Form single bonds with each oxygen and apply additional electrons as lone pairs for structure stability. A balanced structural formula portrays either single bonds with electron pairs or resonance structures with one double bond at a time.

Key concepts

Electron Dot Formula

The electron dot formula is a way to represent the valence electrons of an atom or ion using dots. Valence electrons are the outermost electrons of an atom that participate in chemical bonding. Understanding how to count these electrons is essential in drawing the electron dot formulas. Here's how you go about creating an electron dot formula:

• Identify the central atom, which is typically the less electronegative element.
• Count all the valence electrons available from the atom and its surrounding atoms. Remember to adjust for any charges, adding electrons for negative charges and subtracting for positive ones.
• Start arranging the electrons around the central atom to describe bonds, usually using single bonds, then distribute remaining electrons as lone pairs to complete octets.
• Each bond, whether single, double, or triple, is shared between two atoms and is represented by pairs of dots or lines.

This setup helps visualize the electron sharing between atoms, crucial for understanding molecular geometry and reactivity.

Structural Formula

The structural formula provides a more visual insight into how atoms are connected in a molecule or ion, specifying the arrangements of atoms and bonds. Unlike the electron dot formula, it often employs lines to represent bonds, which allows a cleaner depiction of a molecule's structure. To draw a structural formula:

• Start with the electron dot formula you prepared.
• Convert pairs of electrons between atoms (which form bonds) into lines. A single line represents a single bond, whereas a double or triple bond is represented with two or three lines, respectively.
• You should depict each atom's connectivity and if required, include lone pairs if they significantly affect molecular structure.
• For polyatomic ions, use brackets and denote charges outside the bracket for clarity.

This representation not only clarifies connectivity but also aids in visualizing resonance and potential molecular interactions, making it a valuable tool in chemical education and practice.

Polyatomic Ions

Polyatomic ions are charged species composed of two or more atoms covalently bonded together. Despite being covalent entities, they have net charges due to an excess or deficiency of electrons, making them crucial players in ionized interactions.Characteristics of polyatomic ions include:

• Stable bonding patterns similar to neutral molecules but possess additional or fewer electrons, giving them a charge, like \(\mathrm{NO}_3^-\) or \(\mathrm{NH}_4^+\).
• They function as units within ionic compounds, interacting ionically due to their overall charges.
• Many polyatomic ions exhibit resonance, wherein certain electron distributions are not fixed to one position, allowing electrons to move over several equivalent positions.

Understanding polyatomic ions' structure helps explain diverse chemical properties and reactivities, essential in fields like analytical chemistry and biochemistry.

Octet Rule

The octet rule is a fundamental principle in chemistry that states atoms generally aim to have eight electrons in their valence shell, achieving a stable electronic configuration similar to noble gases. Key points about the octet rule include:

• Atoms achieve an octet through electrons gained, lost, or shared in bonds. Exceptions exist, such as hydrogen, which reaches stability with just two electrons.
• Molecules adjust their structures, like forming single or multiple bonds, to fulfill the octet rule where possible.
• While many compounds follow the octet rule, some elements, like phosphorus, sulfur, and elements beyond the third period, can have expanded octets that exceed 8 electrons.

This rule is a guiding principle in predicting molecule formations and reactivities, though exceptions remind us of the complexity and variance in chemical bonding.