Question

Draw an electron-dot structure for acetonitrile, \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{~N},\) which contains a carbon-nitrogen triple bond. How many electrons does the nitrogen atom have in its outer shell? How many are bonding, and how many are nonbonding?

Short answer

Nitrogen has 8 electrons; 6 are bonding, and 2 are nonbonding.

Step-by-step solution

Determine Valence Electrons

Start by calculating the total number of valence electrons in acetonitrile ( C2H3N). Carbon has 4 valence electrons, hydrogen has 1, and nitrogen has 5. The total number is the sum: (2 × 4) + (3 × 1) + 5 = 16 valence electrons.

Arrange Atoms

Place the carbon atoms connected to each other and hydrogen atoms bonded to one of the carbon atoms. Place the nitrogen atom bonded to the other carbon atom.

Assign Electrons for Bonds

Use pairs of electrons to form single bonds between each C-H, and use a triple bond between the carbon and nitrogen atoms. Each bond uses 2 electrons, so 2 × 3 for C-H bonds and 6 for C≡N bond use up 12 valence electrons out of 16.

Complete with Remaining Electrons

With 4 electrons remaining, place them as lone pairs on the nitrogen atom to complete the octet site. Nitrogen will have 2 lone pairs, as acetonitrile's Lewis structure should show.

Count Electrons Around Nitrogen

The nitrogen atom is involved in a triple bond with carbon, so it has 6 bonding electrons. The 2 lone pairs are nonbonding, constituting 2 pairs × 2 = 4 nonbonding electrons around it.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons in an atom and they play a crucial role in chemical bonding. They are the electrons that participate in forming bonds between atoms in molecules.
To find the total number of valence electrons in a molecule like acetonitrile (C\(_2\)H\(_3\)N), we need to look at each element individually.

• Carbon, with an atomic number of 6, has 4 valence electrons.
• Hydrogen, with an atomic number of 1, has 1 valence electron.
• Nitrogen, with an atomic number of 7, has 5 valence electrons.

By adding up the valence electrons of all the atoms in acetonitrile, we arrive at a total of 16 valence electrons used to form bonds and complete octets.

Lewis Structure

The Lewis structure helps us visualize and understand how atoms share electrons in a molecule. It is a representation showing the arrangement of valence electrons around the atoms in a molecule, and it includes both bonding and nonbonding electrons.
In acetonitrile, we start by arranging the atoms: the two carbon atoms are connected to each other, with one carbon bonded to three hydrogen atoms and the other connected to the nitrogen atom. The carbon-nitrogen triple bond is a key feature here. By carefully placing the 16 electrons around these atoms, with bonds between C-H and a triple bond between C≡N, we create a stable structure developed through the correct allocation of valence electrons.

Carbon-Nitrogen Triple Bond

A carbon-nitrogen triple bond is represented by three shared pairs of electrons between a carbon and a nitrogen atom. It is a strong bond that significantly affects the molecule’s properties and reactivity.
In the Lewis structure for acetonitrile, the triple bond accounts for 6 of the molecule's valence electrons, as each bond contributes 2 electrons. This bonding arrangement is crucial for the stability and overall electron configuration of the molecule. The triple bond gives the molecule a linear geometry around the carbon and nitrogen atoms involved, and it creates a high degree of electron density between them, contributing to the molecule's characteristic properties.

Nonbonding Electrons

Nonbonding electrons, also known as lone pairs, are valence electrons that are not involved in chemical bonding. Instead, they reside as pairs on atoms and can influence molecular shape and reactivity.
In the case of acetonitrile, once the bonding pairs are accounted for, we are left with 4 nonbonding electrons. Nitrogen, having a full octet, holds these electrons as two lone pairs. These nonbonding electrons are significant because they can participate in various chemical reactions, affect the molecule's polarity, and contribute to its geometric arrangement.

Bonding Electrons

Bonding electrons are the valence electrons shared between atoms to form chemical bonds. They are key to forming the molecular structure and determining the molecule’s characteristics.
In acetonitrile, the bonding electrons are distributed between the C-H bonds and the C≡N triple bond. This totals up to 12 bonding electrons in the molecule.

• 3 pairs of bonding electrons are shared between the carbon and hydrogen atoms (3 C-H bonds).
• Another 3 pairs form the carbon-nitrogen triple bond (C≡N).

These bonding interactions stabilize the molecule and allow for the proper electron distribution for a full octet around each atom involved in bonding.