Question

For the indicated atom in each of the following molecules or ions, give the number and arrangement of the electron pairs around that atom. a. \(\mathrm{S}\) in \(\mathrm{SO}_{3}^{2-}\) b. \(\mathrm{S}\) in \(\mathrm{HSO}_{3}^{-}\) c. \(\mathrm{S}\) in \(\mathrm{HS}^{-}\)

Short answer

a. In $\mathrm{SO}_{3}^{2-}$, sulfur (S) has 4 electron pairs in a tetrahedral arrangement (3 S-O bonds and 1 lone pair of electrons). b. In $\mathrm{HSO}_{3}^{-}$, sulfur (S) has 4 electron pairs in a tetrahedral arrangement (2 S-O bonds, 1 S-H bond, and 1 lone pair of electrons). c. In $\mathrm{HS}^{-}$, sulfur (S) has 3 electron pairs in a trigonal planar arrangement (1 S-H bond and 2 lone pairs of electrons).

Step-by-step solution

Determine valence electrons for Sulfur

Sulfur belongs to group 6 in the periodic table, which means that it has 6 valence electrons.

Step 2(a): Count bonding and non-bonding electrons for S in SO32-

In the ion SO32-, there are three bonds between S and O, and each bond has two bonding electrons. Additionally, there is a charge of -2, which means that there are two extra non-bonding electrons on sulfur. Bonding Electrons: 3 (bonds) × 2 (electrons/bond) = 6 electrons Non-Bonding Electrons: 2 electrons

Step 3(a): Calculate electron pairs for S in SO32-

Adding bonding electrons and non-bonding electrons, we get 6 + 2 = 8 electrons. Dividing this by 2, we obtain 4 electron pairs.

Step 4(a): Describe the arrangement of electron pairs for S in SO32-

In SO32-, the electron pairs' arrangement around the sulfur atom is a tetrahedral arrangement, with three S-O bonds and one lone pair of electrons.

Step 2(b): Count bonding and non-bonding electrons for S in HSO3-

In the ion HSO3-, there are two S-O bonds and one S-H bond, and each bond has two bonding electrons. Additionally, there is a charge of -1, which means that there is an extra non-bonding electron on sulfur. Bonding Electrons: 3 (bonds) × 2 (electrons/bond) = 6 electrons Non-Bonding Electrons: 2 electrons (one lone pair)

Step 3(b): Calculate electron pairs for S in HSO3-

Adding bonding electrons and non-bonding electrons, we get 6 + 2 = 8 electrons. Dividing this by 2, we obtain 4 electron pairs.

Step 4(b): Describe the arrangement of electron pairs for S in HSO3-

In HSO3-, the electron pairs' arrangement around the sulfur atom is a tetrahedral arrangement, with two S-O bonds, one S-H bond, and one lone pair of electrons.

Step 2(c): Count bonding and non-bonding electrons for S in HS-

In the ion HS-, there is one bond between S and H, and each bond has two bonding electrons. Additionally, there is a charge of -1, which means that there is an extra non-bonding electron on sulfur. Bonding Electrons: 1 (bond) × 2 (electrons/bond) = 2 electrons Non-Bonding Electrons: 4 electrons (two lone pairs, due to extra electron and 5 remaining valence electrons from Sulfur)

Step 3(c): Calculate electron pairs for S in HS-

Adding bonding electrons and non-bonding electrons, we get 2 + 4 = 6 electrons. Dividing this by 2, we obtain 3 electron pairs.

Step 4(c): Describe the arrangement of electron pairs for S in HS-

In HS-, the electron pairs' arrangement around the sulfur atom is a trigonal planar arrangement, with one S-H bond and two lone pairs of electrons.

Key concepts

Electron Pair Geometry

Electron pair geometry is a fundamental concept in VSEPR (Valence Shell Electron Pair Repulsion) theory, which helps predict the 3D shape of molecules. The geometry depends on the number of electron pairs—both bonding and lone pairs—around a central atom. According to VSEPR theory, electron pairs repel each other and try to stay as far apart as possible to minimize repulsion. This repulsion leads to specific, predictable geometries.

• If a central atom has 4 electron pairs surrounding it, like in the sulfate ion (SO$_3^{2-}$) and bisulfite ion (HSO$_3^-$), the electron pair geometry will likely be tetrahedral.
• In the case of HS$^-$, where sulfur is surrounded by 3 electron pairs, the electron pair geometry will be trigonal planar.

Understanding electron pair geometry is crucial for predicting molecular properties, such as polarity and reactivity.

Bonding Electrons

Bonding electrons are the electrons involved in chemical bonds, connecting atoms together. Each bond usually consists of two electrons which are shared between atoms. The number of bonds directly affects the geometry around an atom, influencing molecular structure and properties.

• In the sulfate ion (SO$_3^{2-}$), sulfur forms 3 S-O bonds, using 6 electrons, contributing to a tetrahedral orientation.
• In bisulfite (HSO$_3^-$), sulfur is bonded to 2 oxygen atoms and 1 hydrogen, with 6 bonding electrons, also leading to a tetrahedral arrangement.
• In hydrogen sulfide ion (HS$^-$), sulfur has 1 S-H bond, accounting for 2 bonding electrons, affecting its trigonal planar structure.

These bonding electrons are significant as they determine the molecule's shape, which influences the molecule's physical and chemical characteristics.

Lone Pairs

Lone pairs are the valence electrons that are not involved in bonding and remain unshared on the central atom. They play a crucial role in defining the molecular geometry by exerting repulsion forces similar to bonding pairs, but often more strongly. Lone pairs affect bond angles and the overall geometry of the molecule.

• In the sulfate ion (SO$_3^{2-}$), there is one lone pair on sulfur, causing slight distortion to the ideal tetrahedral shape.
• For bisulfite (HSO$_3^-$), sulfur has one lone pair, shaping a tetrahedral arrangement with altered angles compared to a perfect geometry.
• In the HS$^-$ ion, the sulfur has two lone pairs. This results in increased repulsion compared to bonded pairs, leading to a trigonal planar arrangement.

By affecting molecular shape, lone pairs influence various physical and chemical properties, such as boiling point, solubility, and angle measurements.