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. \(\mathrm{P}\) in \(\mathrm{PH}_{3}\) b. \(\mathrm{Cl}\) in \(\mathrm{ClO}_{4}^{-}\) c. \(\mathrm{O}\) in \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

a. P in PH₃: 3 electron pairs in a trigonal planar arrangement b. Cl in ClO₄⁻: 4 electron pairs in a tetrahedral arrangement c. O in H₂O: 4 electron pairs (2 bonding and 2 lone pairs) in a tetrahedral arrangement

Step-by-step solution

Counting electron pairs around the central atom

Count the bonding electron pairs and lone pairs around the central atom, as follows: a. P in PH₃: - P has 3 H bonded atoms (3 electron pairs) and no lone pairs. b. Cl in ClO₄⁻: - Cl has 4 O bonded atoms (4 electron pairs) and no lone pairs. c. O in H₂O: - O has 2 H bonded atoms (2 electron pairs) and 2 lone pairs. **Step 2: Determine electron pair geometry**

Determine the electron pair geometry of each central atom

Based on the number of electron pairs around the central atom, determine the electron pair geometry using the VSEPR theory (Valence Shell Electron Pair Repulsion), as follows: a. P in PH₃: - 3 electron pairs - Trigonal planar geometry b. Cl in ClO₄⁻: - 4 electron pairs - Tetrahedral geometry c. O in H₂O: - 4 electron pairs - Tetrahedral geometry **Step 3: Describe the arrangement of electron pairs**

Provide descriptions of the electron pair arrangements

Provide a description of the electron pairs' arrangement and geometry based on the previous step's findings: a. P in PH₃: - The central atom P is surrounded by 3 electron pairs, all involved in P-H bonding. The geometry is trigonal planar. b. Cl in ClO₄⁻: - The central atom Cl is surrounded by 4 electron pairs, all involved in Cl-O bonding. The geometry is tetrahedral. c. O in H₂O: - The central atom O is surrounded by 4 electron pairs, 2 involved in O-H bonding and 2 being lone pairs. The geometry is tetrahedral.

Key concepts

Electron Pair Geometry

In chemistry, understanding electron pair geometry is crucial in predicting how molecules or ions will be shaped. This concept is deeply grounded in VSEPR theory, which stands for Valence Shell Electron Pair Repulsion. Essentially, VSEPR theory posits that electron pairs will arrange themselves around a central atom in such a way that they minimize repulsion between them. Think of it like trying to keep balloons in a bunch from touching each other—they'll naturally spread out as much as possible.

The number of electron pairs determines the electron pair geometry:

• Two electron pairs adopt a linear geometry.
• Three pairs lead to a trigonal planar geometry.
• Four pairs form a tetrahedral shape.
• Five pairs give rise to a trigonal bipyramidal shape.
• Six pairs arrange in an octahedral geometry.

These geometries are fundamental because they help chemists understand how molecules behave and interact with one another.

Electron Pairs

Electron pairs are groups of two electrons that either form a bond or exist as lone pairs. Within VSEPR theory, these pairs are the prime players that determine molecular shape. An electron pair can be either of two types:

• Bonding pairs, which form bonds between atoms, like those in hydrogen (H) and oxygen (O).
• Lone pairs, which do not directly participate in bonding but still affect the shape due to their repulsion with other pairs.

In our exercise, different molecules show various scenarios. For instance, in \(\text{PH}_3\), phosphorus (P) is surrounded by three bonding pairs—all forming P-H bonds. Meanwhile, oxygen (O) in \(\text{H}_2\text{O}\), has two bonding pairs and two lone pairs. Lone pairs create stronger repulsions than bonding pairs, thus they have a significant impact on shaping the molecule.

Central Atom

In VSEPR theory, the central atom is pivotal in determining the structure of a molecule or ion. It is the atom around which other atoms are arranged, and it latches onto electron pairs—either bonding or lone. For example, in \(\text{PH}_3\), phosphorus (P) is the central atom to which three hydrogens are bonded. Similarly, in \(\text{H}_2\text{O}\), oxygen (O) is the core atom bonded to two hydrogens.

The central atom’s identity and the number of electron pairs it has directly influence the molecule's shape. If the central atom has no lone pairs, like chlorine (Cl) in \(\text{ClO}_4^-\), it often forms a symmetrical shape, here tetrahedral, because all pairs are bonding. Understanding the role of the central atom helps in predicting molecular geometry and interactions, aiding in comprehending the vast landscape of chemistry.