For the indicated atom in each of the following molecules or...

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 $\mathrm{HS}^{-}$

Key Concepts

Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms as well as the lone pairs of electrons in a molecule or ion. They are essential for identifying the number of valence electrons, determining bonding patterns, and distributing electrons appropriately to reflect formal charges and electron pair arrangements before applying VSEPR theory.

Bonding and Lone Pairs

Distinguishing between bonding pairs (electrons involved in chemical bonds) and lone pairs (nonbonding electrons) is a key concept when determining molecular geometry. The difference in repulsion between lone pairs and bonding pairs—lone pairs typically requiring more space—affects the final shape of the molecule, making it essential to identify and count each type accurately.

Steric Number

The steric number is the total number of electron domains (both bonding pairs and lone pairs) surrounding a central atom. This number directly influences the electron domain geometry; for example, a steric number of four corresponds to a tetrahedral electron domain geometry, while a steric number of three corresponds to a trigonal planar arrangement.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory is the foundational concept used to predict the three-dimensional arrangement of electron pairs around a central atom. It is based on the idea that electron pairs, both bonding and nonbonding, repel each other and will adopt a spatial configuration that minimizes these repulsions, leading to specific geometric arrangements such as linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral shapes.

Electron Domain Geometry

Electron domain geometry refers to the three-dimensional arrangement of all electron domains (bonding and lone pairs) around a central atom as determined by VSEPR theory. Recognizing the electron domain geometry is crucial for understanding the overall shape of the electron cloud around the atom, which affects both physical and chemical properties.

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