Question

Write a Lewis structure for \(\mathrm{SbCl}_{5}\). Does this molecule obey the octet rule?

Short answer

SbCl₅ does not obey the octet rule.

Step-by-step solution

Count Total Valence Electrons

To draw the Lewis structure, first count the total number of valence electrons in the molecule. Antimony (Sb) has 5 valence electrons and each chlorine (Cl) has 7 valence electrons. Hence, for \(\mathrm{SbCl}_{5}\), the total is \(5 + 5 \times 7 = 40\) valence electrons.

Write the Symbol for the Central Atom

Typically, the least electronegative atom is the central atom. In \(\mathrm{SbCl}_{5}\), Sb is the central atom.

Surround Central Atom with Chlorine Atoms

Arrange the 5 chlorine atoms around the antimony atom. This ensures symmetry and is common for this molecular geometry.

Distribute the Electrons

Begin by forming single bonds between Sb and each of the Cl atoms. Each bond accounts for 2 electrons, totaling \(5 \times 2 = 10\) electrons used, leaving \(40 - 10 = 30\) electrons.

Complete the Octet for Chlorine

Assign the remaining 30 electrons to fulfill the octets of the chlorine atoms. Each chlorine gets 6 more electrons, totaling 8 valence electrons (from both bonds and lone pairs), consuming all 30 leftover electrons.

Check the Central Atom

Antimony, the central atom, in \(\mathrm{SbCl}_{5}\), forms 5 bonds and thus holds 10 electrons around it. This exceeds the octet rule but is permissible since elements in period 5 or higher can expand their valency.

Answering the Octet Rule Question

The antimony center in \(\mathrm{SbCl}_{5}\) does not obey the octet rule; it carries more than 8 electrons due to expanded octet capacity available in higher periods.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom. They are crucial in determining how atoms interact and bond with each other. In a Lewis structure, these electrons are depicted as dots, representing the potential for bonding:

• In the molecule \( \mathrm{SbCl}_{5} \), antimony (Sb) has 5 valence electrons.
• Chlorine (Cl) atoms each have 7 valence electrons.

This totals to 40 valence electrons for the entire molecule, calculated by adding antimony's 5 valence electrons to the 35 valence electrons from the five chlorine atoms (7 each). These electrons are distributed in the Lewis structure to showcase the bonds and lone pairs in the molecule.

Octet Rule

The octet rule is a chemical guideline suggesting that atoms tend to form bonds until they are surrounded by 8 valence electrons, resembling the electron configuration of noble gases. This rule, however, has its exceptions:- While chlorine comfortably achieves an octet by sharing electrons, antimony in \( \mathrm{SbCl}_{5} \) violates this rule.- Antimony forms 5 bonds with chlorine, leading to 10 electrons in its shell, which exceeds the typical octet.
This is because antimony is in period 5 of the periodic table, allowing it to expand its valency beyond 8 through the use of d orbitals.

Central Atom

Choosing the central atom is a crucial step in drafting a Lewis structure. Usually, it is the least electronegative element, allowing it to share more electrons. In \( \mathrm{SbCl}_{5} \):
- Antimony (Sb) is chosen as the central atom over chlorine since it is less electronegative.- This choice allows antimony to bond with five chlorines symmetrically, creating a balanced molecule.
The central atom can often accommodate more electrons than the surrounding atoms, particularly in cases where it can expand its valency.

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of atoms in a molecule. It refers to the spatial configuration around the central atom. For \( \mathrm{SbCl}_{5} \):
- The geometry is trigonal bipyramidal.

• This means that three chlorine atoms form an equatorial plane around the central antimony.
• The other two chlorine atoms are positioned axially, one above and the other below this central plane.

This structure results from the five regions of electron density (the five \( \mathrm{Sb} - \mathrm{Cl} \) bonds) around antimony, arranging themselves to minimize repulsions and thus create the most stable form for \( \mathrm{SbCl}_{5} \).