Problem 167 - Chemistry: An Atoms-Focused Approach

Question

Which of the following molecules and ions contains an atom with an expanded valence shell? (a) \(\mathrm{Cl}_{2} ;\) (b) \(\mathrm{ClF}_{3}\); (c) \(\mathrm{CII}_{3} ;\) (d) \(\mathrm{ClO}^{-}\)

Step-by-Step Solution

Answer

(a) Cl2, (b) ClF3, (c) ClI3, or (d) ClO−. Answer: (b) ClF3, (c) ClI3, and (d) ClO− contain an atom with an expanded valence shell.

Step 1: Determine the Central Atom in Each Molecule/ion

In each molecule/ion, we must first figure out which atom is considered the central atom: (a) Cl2 - Two Cl atoms are bonded together (b) ClF3 - Cl is the central atom, bound to 3 F atoms (c) ClI3 - Cl is the central atom, bound to 3 I atoms (d) ClO- - Cl is the central atom, bound to 1 O atom

Step 2: Check for Expanded Valence Shell

Now, let's determine if there are any atoms with an expanded valence shell among these molecules/ions: (a) Cl2 - The two Cl atoms each have 7 valence electrons, and they share a covalent bond, which means that each Cl atom has 8 electrons in its valence shell. Therefore, Cl in Cl2 does not have an expanded valence shell. (b) ClF3 - The Cl atom has 7 valence electrons, and it forms bonds with 3 F atoms, each sharing an electron pair. So, Cl in ClF3 has 3 additional electrons, which make a total of 10 valence electrons. Hence, Cl in ClF3 has an expanded valence shell. (c) ClI3 - The Cl atom has 7 valence electrons, and it forms bonds with 3 I atoms, each sharing an electron pair. So, Cl in ClI3 has 3 additional electrons, which results in a total of 10 valence electrons. Hence, Cl in ClI3 has an expanded valence shell. (d) ClO− - The Cl atom has 7 valence electrons, and it forms a bond with an O atom, sharing an electron pair. Additionally, there is one more negative charge (electron) brought by the ion, which means Cl in ClO− has 9 valence electrons. Thus, Cl in ClO− has an expanded valence shell.

Step 3: Find the Molecule/ion with Expanded Valence Shell

From Step 2, we find that molecules/ions (b) ClF3, (c) ClI3, and (d) ClO− have atoms with expanded valence shells.

Key Concepts

Central Atom

In molecules and ions, the central atom is the one to which other atoms bond directly. Understanding the role of the central atom helps in determining the geometry and properties of the entire molecule. In the context of the given exercise, the central atom is essential in identifying whether an expanded valence shell exists.
Let's examine the provided examples:

• In \(\mathrm{Cl}_{2}\), both chlorine (Cl) atoms are equivalent, and no single atom can be deemed central as they form a simple diatomic molecule.
• For \(\mathrm{ClF}_{3}\), chlorine (Cl) is the central atom to which three fluorine (F) atoms connect. Cl is the atom around which the molecule's structure is organized.
• In \(\mathrm{ClI}_{3}\), chlorine (Cl) again acts as the central atom, bonded to three iodine (I) atoms. This configuration is crucial for understanding its valence expansion.
• Similarly, in \(\mathrm{ClO}^{-}\), chlorine (Cl) is central, this time with an attachment to an oxygen (O) atom.

In most complex molecules and ions, the central atom often has higher electronegativity or possesses more electrons, allowing it to bond with multiple other atoms.

Valence Electrons

Valence electrons are the outermost electrons of an atom. These electrons play a key role in chemical bonding, as they are involved in the formation of bonds with other atoms. Knowing the number of valence electrons can help predict how atoms will bond together.
In the molecules and ions provided, the number of valence electrons can indicate whether an atom like chlorine (Cl) has expanded its valence shell:

• Chlorine naturally has 7 valence electrons. This is a starting point for counting what happens when it bonds with other atoms.
• For each additional atom it bonds with, chlorine shares a pair of electrons, effectively increasing its total count of valence electrons.
• Sometimes, a central atom can hold more electrons than the typical 8, which is called an expanded valence shell. This is common with elements in the third period and beyond, like sulfur and phosphorus, and in this case, chlorine.

The different bonding scenarios demonstrate how chlorine can expand its valence shell beyond the typical octet, accommodating additional electrons brought by bonding.

Chemical Bonding

Chemical bonding is what holds atoms together in molecules and compounds. Different types of bonds, like covalent, ionic, and metallic bonds, are central to the concept of chemical bonding. In the examples studied here, we explore covalent bonding as the primary type.
In molecules like \(\mathrm{ClF}_{3}\), \(\mathrm{ClI}_{3}\), and \(\mathrm{ClO}^{-}\), we see different covalent interactions.

• Covalent bonds involve the sharing of electron pairs between atoms. This type of bond primarily occurs in nonmetals, as seen with Cl.
• The strength and number of these bonds play a vital role in whether an atom can expand its valence shell. For instance, \(\mathrm{ClF}_{3}\) forms stable bonds where Cl shares electrons with three fluoride atoms, resulting in ten total valence electrons.
• An expanded valence shell in Cl occurs because chlorine can utilize vacant d orbitals to accommodate more electrons. This is applicable especially with elements like chlorine that are in period three of the periodic table or below.

Through understanding chemical bonding, we gain insight into the molecular structures and how atoms like chlorine manage to surpass the octet rule, expanding their valence shells in certain compounds.