Problem 54 - General Chemistry: Principles and Modern Applications

Question

Write plausible Lewis structures for the following free radicals: (a) \(\cdot \mathrm{C}_{2} \mathrm{H}_{5} ;\) (b) \(\mathrm{HO}_{2}\) "; (c) ClO?.

Step-by-Step Solution

Answer

The Lewis structures are as follows: (a) For \(\cdot C_{2}H_{5}\) : \(C - C - H\) with single bonds between the atoms and an unpaired electron on the Carbon atom. There are three hydrogens bonded to one Carbon atom and two hydrogens bonded to the other. (b) For \(HO_{2}\) : \(O - O - H\) with single bonds between the atoms and an unpaired electron on the Oxygen atom. (c) For \(ClO\) : \(Cl - O\) with a single bond between the atoms and an unpaired electron on the Cl atom.

Step 1: Identification of central atom

Identify the central atom for each of the molecules. The central atom is usually the one that has the most bonding capacity which is \(\mathrm{C}\) for \(\cdot\mathrm{C}_{2}\mathrm{H}_{5}\), \(\mathrm{O}\) for \(\mathrm{HO}_{2}\), and \(\mathrm{Cl}\) for \(\mathrm{ClO}\).

Step 2: Determination of total valence electrons

Determine the total number of valence electrons for each molecule. \(C_{2}H_{5}\) has 14 valence electron, \(HO_{2}\) has 13 valence electrons, and \(ClO\) has 13 valence electron.

Step 3: Draw skeletal structure and placing electrons

Draw the skeleton of the molecule and place the electrons. Here, hydrogen atoms are always terminal. Also remember that each bond uses 2 electrons.

Step 4: Place remaining electrons

After placing bonding electrons, place the remaining electrons as lone pairs on each atom, starting with the outer atoms first. Remember atoms follow the octet rule (except Hydrogen follows a duet), which means they should have 8 electrons in their outer shell. Atomic hydrogen has one electron in its 1s orbital, it will always share its one electron to achieve stable configuration of Helium.

Step 5: Placement of unpaired electron

Place the unpaired electron on the central atom to form the free radical. In these structures, an unpaired electron must be present because radicals have an odd number of valence electrons.

Key Concepts

Free Radicals

Free radicals are a unique and important concept in chemistry. These species have an unpaired electron in their structure, making them highly reactive. They often appear in chemical reactions, especially those involving breaking or forming bonds. Because they contain an unpaired electron, free radicals usually have an odd number of total valence electrons.
This unpaired electron gives free radicals their characteristic reactivity, as they often seek out other atoms or molecules to pair with. This can lead to a variety of reactions, including polymerizations or even the affecting of biological molecules in living organisms. Understanding free radicals is essential for comprehending a wide range of chemical processes.

Valence Electrons

Valence electrons are the outermost electrons in an atom and play a crucial role in bonding and chemical reactions. They determine how atoms will bond with each other to form molecules. Knowing the number of valence electrons helps predict the behavior and chemical properties of an element.
The number of valence electrons in an atom can be determined by looking at its group number on the periodic table for main group elements. For example, in the exercise provided,

• Ethyl radical ( \( \cdot \mathrm{C}_2 \mathrm{H}_5 \)) has 14 valence electrons.
• Hydroperoxyl radical (HO extsubscript{2}) has 13 valence electrons,
• and the Chlorine monoxide radical ( ClO?) also has 13 valence electrons.

Understanding valence electrons is imperative for constructing valid Lewis structures, as they show how electrons are shared or transferred between atoms.

Central Atom

In a molecule, the central atom is typically the atom that forms the most bonds with other atoms. To identify the central atom, look for the element with the highest bonding capacity or the lowest electronegativity, excluding hydrogen usually as it is terminal.
For instance, in

• The ethyl radical ( \( \cdot \mathrm{C}_2 \mathrm{H}_5 \)), carbon is the central atom.
• In the hydroperoxyl radical (HO extsubscript{2}),oxygen acts as the central atom.
• For the chlorine monoxide radical (ClO?),chlorine functions as the central atom.

Identifying the central atom correctly is a fundamental step in drawing Lewis structures because it guides how the rest of the molecule is arranged.

Octet Rule

The octet rule is a key principle in chemistry that dictates that atoms tend to bond in such a way that each atom has eight electrons in its valence shell, acquiring a stable noble gas electron configuration. This rule applies mainly to main-group elements such as carbon, nitrogen, oxygen, and the halogens.
When drawing Lewis structures, the octet rule is vital for determining how many bonds an atom can form. Exceptions to this rule include hydrogen, which follows the duet rule and is stable with two electrons, and

• elements in higher periods of the periodic table
• can exceed the octet due to the availability of d orbitals.

For radicals, achieving the octet can be a bit tricky due to the odd number of valence electrons; therefore, one electron remains unpaired, contributing to the radical's reactivity. Understanding the octet rule enables more accurate prediction and representation of molecular structures.