Question

The Lewis structure for hydrogen cyanide is \(\mathrm{H}-\mathrm{C} \equiv \mathrm{N}:\) Draw circles enclosing electrons to show that carbon and nitrogen obey the octet rule.

Short answer

In hydrogen cyanide, circles around carbon and nitrogen show that each atom has a complete octet; carbon with 2 electrons from H-C and 6 from C-N, and nitrogen with 6 from C-N and 2 lone electrons.

Step-by-step solution

Understanding the Octet Rule

The octet rule states that atoms tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. Hydrogen is an exception as it achieves stability with two electrons.

Count Valence Electrons for Carbon and Nitrogen

Carbon has 4 valence electrons and nitrogen has 5 valence electrons. Together with the single valence electron from hydrogen, there are a total of 10 valence electrons to be accounted for in hydrogen cyanide (HCN).

Draw the Lewis Structure for HCN

Place the less electronegative atom (carbon) in the middle. Draw a single bond between hydrogen and carbon, and a triple bond between carbon and nitrogen. This uses 2 electrons for the H-C bond and 6 electrons for the C-N triple bond.

Enclose Electrons to Show Octet Completion

Draw circles around the carbon and nitrogen to enclose their valence electrons. Carbon has 2 electrons from the H-C bond and 6 from the C-N triple bond, totaling 8. Nitrogen has 6 electrons from the C-N triple bond and 2 lone electrons, also totaling 8.

Key concepts

Octet Rule

The octet rule is a principle of chemistry that underpins the stability of atoms when they form molecules. It posits that atoms are most stable when they have eight electrons in their valence shell, mirroring the electron configuration of noble gases.

Hydrogen is a classic exception, reaching stability with a duo of electrons. In the Lewis structure of hydrogen cyanide, both carbon and nitrogen aspire to complete their octet to emulate the noble gases’ stability. To exhibit how each achieves the octet configuration in hydrogen cyanide's structure, we draw circles around their valence electrons. This visual aid shows carbon with its four starting electrons from its own shell, and four it shares with nitrogen and hydrogen, while nitrogen is depicted with its five native and three shared electrons.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are pivotal in determining an atom's ability to bond with others. They are the electrons involved when atoms form chemical bonds. In hydrogen cyanide, the valence electrons are the 'currency' with which carbon and nitrogen fulfill the octet rule.
Carbon holds four valence electrons, while nitrogen has five. These valence electrons, along with the single electron from hydrogen, amount to a sum of ten, which must be considered when drawing the Lewis structure for hydrogen cyanide. The carbon atom ends up sharing two electrons in a bond with hydrogen and six electrons in a triple bond with nitrogen, settling its octet.

Electron Configuration

An atom's electron configuration is the arrangement of electrons in its orbitals. The configuration outlines which orbitals are filled, which affects the atom's chemical properties and bonding characteristics.

In the context of hydrogen cyanide, carbon and nitrogen must adopt an electron configuration that brings them contentment through a complete octet. By bonding with each other and hydrogen, they adjust their configuration in such a way that mimics the stability that noble gases have naturally. The triple bond between carbon and nitrogen is not a coincidence; it's a requisite for both elements to reach an optimal electron configuration.

Chemical Bonding

Chemical bonding involves the joining of atoms to form molecules through the sharing or transferring of electrons. Molecules like hydrogen cyanide are crafted through the subtle art of electron sharing, where atoms bond to achieve greater stability.

The single bond between hydrogen and carbon is a sharing of two electrons – one from each atom. The triple bond between carbon and nitrogen is a more robust affair; it shares six electrons, involving two electrons from carbon and four from nitrogen, thus allowing both to reach their octet and bond securely. These interactions are the weaving threads of the molecule's chemical tapestry.