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

For each ion, count total electrons including charges, place phosphorus centrally bonded to oxygens, visualize single/double bonds, and indicate charges distinctly.

Step-by-step solution

Understanding the Electron Dot Formula

The electron dot formula represents the valence electrons for each atom in a molecule or ion. To draw it for polyatomic ions, count the total number of valence electrons, adjusting for any charges on the ion. For negatively charged ions, add one electron per negative charge to the total count.

Electron Dot Formula for \(\mathrm{PO}_{4}^{3-}\)

Count the valence electrons: Phosphorus has 5 valence electrons and each Oxygen has 6, totaling to 24 from atoms. Add 3 for the 3- charge to get 27 electrons. Distribute these as dots around the atoms and ensure each oxygen has a complete octet while phosphorus forms bonds with each oxygen.

Structural Formula for \(\mathrm{PO}_{4}^{3-}\)

The structural formula shows the actual connectivity between atoms. Phosphorus is central, single-bonded to four oxygens. Also, show the 3- charge on the ion. There is usually one double bond in resonance forms, but the simple structure includes all single bonds and indicates the resonance equivalence.

Electron Dot and Structural Formula for \(\mathrm{HPO}_{4}^{2-}\)

Count: Phosphorus (5) + Oxygen (6) x 4 = 24 valence electrons. Add 1 for the 2- charge and 1 for hydrogen, totaling to 26. Draw phosphorus center, with three single-bonded oxygens (each having 3 lone pairs), one double-bonded oxygen, and one hydrogen attached to any oxygen. Indicate the 2- charge.

Electron Dot and Structural Formula for \(\mathrm{PO}_{3}^{3-}\)

Tally electrons: Phosphorus (5) + Oxygen (6) x 3 = 18, add 3 for the charge, to give 21. Phosphorus in the center links with each of the three oxygens. Assign two oxygens two lone pairs each, the third oxygen makes a double bond with phosphorus. Show a 3- charge distributed across the ion.

Electron Dot and Structural Formula for \(\mathrm{HPO}_{3}^{2-}\)

Determine electrons: Phosphorus (5) + Oxygen (6) x 3 = 18, plus 1 for hydrogen, and 2 for the charge, totalling to 21. Place phosphorus at the center, bonded with three oxygens (one single, one double, one bonded with hydrogen) and indicate the 2- charge functionality.

Key concepts

Electron Dot Formula

The electron dot formula, often called a Lewis dot structure, is a simple way to represent the valence electrons of atoms within a molecule or ion. It helps in visualizing the electrons as dots placed around the atoms. When dealing with polyatomic ions, it's essential to account for all valence electrons, incorporating the effect of any positive or negative charges. For instance:

• Add electrons for each negative charge, which indicates an extra electron.
• Subtract electrons for each positive charge, indicating fewer electrons available.

For example, when creating the electron dot formula for the \[ \mathrm{PO}_{4}^{3-} \] ion, we start by counting the valence electrons of phosphorus and oxygen. Phosphorus has 5 valence electrons, and each oxygen, being typically present in compounds as an element from group 16, provides 6. Combined with the addition due to the 3- charge, we ensure each atom achieves a stable electron arrangement.

Structural Formula

While the electron dot formula gives insight into electron arrangement, the structural formula focuses on the actual connectivity among atoms within a molecule or ion. This representation highlights how atoms are bonded by lines, which depict the bonds between them. For example, in the case of the \[ \mathrm{PO}_{4}^{3-} \] ion:

• Phosphorus is centrally bonded to four oxygen atoms, forming single bonds.
• Though resonance forms can exist, a simpler representation shows all single bonds, capturing the symmetric distribution among atoms.

This approach prioritizes highlighting the spatial layout and connection without delving into electron counts or lone pairs excessively.

Valence Electrons

Valence electrons, the electrons located in the outermost shell of an atom, are crucial for determining how atoms will bond with each other. These are the electrons that participate in chemical bonding and influence the formation of molecules and ions.When working with ions such as \[ \mathrm{HPO}_{4}^{2-} \]:

• Count the electrons from each atom's outer shell, and consider additional electrons from negative charges.
• For a neutral phosphorus atom, there are 5 valence electrons, yet when combined with oxygen atoms and a 2- charge, more electrons come into play based on the ion's charge.

By focusing on valence electrons, one can predict molecule formation, the nature of bonds (single, double), and ion stability.

Lewis Structures

Lewis structures, closely related to electron dot formulas, provide a complete depiction of all valence electrons in a molecule, visualized as dots around the contributing atoms. They also show how these electrons participate in forming bonds such as single, double, and sometimes triple bonds.Crafting a Lewis structure involves:

• Assigning electrons to account for neutral charges or incorporating extra ones for negatively charged ions.
• Ensuring octet fulfillment whenever possible, particularly for nonmetals like oxygen.

In constructing the Lewis structure for the \[ \mathrm{PO}_{3}^{3-} \] ion, phosphorus will form bonds with each of the three oxygen atoms, positioning electrons to satisfy the octet rule through bonding and lone pairs. A critical aspect is also acknowledging possible resonance structures, which can further stabilize the ion through electron delocalization.