Question

Of the following species, the one with a triple covalent bond is (a) \(\mathrm{NO}_{3}^{-} ;\) (b) \(\mathrm{CN}^{-} ;\) (c) \(\mathrm{CO}_{2} ;\) (d) \(\mathrm{AlCl}_{3}\).

Short answer

Among the given species, the molecule \(\mathrm{CN}^{-}\) is the one with a triple covalent bond.

Step-by-step solution

Examine NO3^-

NO3^- or Nitrate ion consists of one Nitrogen atom and three Oxygen atoms. It forms a resonance structure. In this case, there is one single and one double bond between Nitrogen and Oxygen atoms that shifts its place with two other oxygen atoms. So, there are no triple covalent bonds in the Nitrate ion.

Examine CN^-

CN^- or Cyanide ion consists of one Carbon atom and one Nitrogen atom. Both of these atoms are nonmetals. They share three pairs of electrons between them to make a triple bond. Hence, CN^- has a triple covalent bond.

Examine CO2

CO2 or Carbon dioxide consists of one Carbon atom and two Oxygen atoms. While there are double bonds between the Carbon atom and each of the Oxygen atoms, it does not have a triple bond.

Examine AlCl3

AlCl3 or Aluminium chloride consists of one Aluminium atom and three Chlorine atoms. The Aluminium atom forms three bonds, one with each Chlorine atom. Hence, no triple bond exists in Aluminium chloride.

Key concepts

Chemical Bonding

When we explore the world of chemistry, chemical bonding is foundational to understanding how atoms come together to form molecules. It’s the glue that holds atoms in a molecule together. At its most basic, a chemical bond is the force that connects two or more atoms. There are several types of bonds, including ionic, covalent, and metallic bonds.

Covalent bonds form when two atoms share electrons, allowing each to attain a stable electron configuration. This shared pair of electrons represents a 'single covalent bond'. When two atoms share two pairs, it’s called a 'double covalent bond'. A 'triple covalent bond', which is the focus of our exercise, involves three shared pairs of electrons and is one of the strongest types of chemical bonds. An example from our problem set is the CN⁻ ion, where carbon and nitrogen share three electron pairs, forming a triple bond.

Molecular Geometry

The shape of a molecule, known as its molecular geometry, is critical because it influences the molecule’s physical properties and chemical reactivity. This shape is determined by the arrangement of atoms within a molecule. VSEPR (Valence Shell Electron Pair Repulsion) theory can be used to predict molecular structures based on the repulsion between electron pairs around the central atom.

Considering a molecule with a triple covalent bond such as CN⁻, the structure is linear because the triple bond counts as a single electron group that strongly repels other electron groups. This results in the atoms being as far apart as possible, achieving a linear shape for the molecule. In contrast, the NO₃⁻ ion, which lacks a triple bond, has a planar structure due to the distribution of its electron pairs around the central atom.

Resonance Structures

In chemistry, certain molecules and ions are best represented by more than one valid Lewis structure. These are known as resonance structures. They are not isomers but rather depict the same molecule with electron pairs distributed differently. The actual molecule is a hybrid of these structures and is more stable than any of the individual resonance forms.

The NO₃⁻ ion from our exercise is a classic case of resonance. It consists of a nitrogen atom surrounded by three oxygen atoms. The double bond between nitrogen and one of the oxygen atoms can be placed between nitrogen and any of the three oxygen atoms, leading to three resonance structures. Each structure suggests a different location for the double bond, but in reality, the extra electron density is delocalized and shared equally among all oxygen atoms. Understanding resonance is key to comprehending electron distribution in molecules where a simple, single Lewis structure does not accurately represent reality.