Diatomic molecules are composed of only two atoms, of either the same or different chemical elements. Common diatomic molecules include hydrogen (H 2), nitrogen (N 2), oxygen (O 2), and carbon monoxide (CO).Seven elements exist as homonuclear diatomic molecules at room temperature: H 2, N 2, O 2, F 2, Cl 2, Br 2, and I 2.The bond in a homonuclear diatomic molecule is non-polar due to the. A) i) Helium does not form compounds like Xenon due to high ionization enthalpy. Ii) is a stronger acid than because of higher oxidation state of Cl in HClO 4 than in HOCl iii) Sulphur is a polyatomic solid whereas Oxygen is a diatomic gas because oxygen can form pπ.

Bond Order

In the Lewis electron structures, the number of electron pairs holding two atoms together was called the bond order. Within the molecular orbital approach, bond order is defined as one-half the net number of bonding electrons:

[ text{bond order}=dfrac{text{number of bonding electrons} - text{number of antibonding electrons}}{2} label{9.8.1} ]

To calculate the bond order of (H_2), we know that the (σ_{1s}) (bonding) molecular orbital contains two electrons, while the ( sigma _{1s}^{star } ) (antibonding) molecular orbital is empty. The bond order of (H_2) is therefore

[ text{bond order}=dfrac{2-0}{2}=1 label{9.8.2} ]

This result corresponds to the single covalent bond; double and triple bonds contain four or six electrons, respectively, and correspond to bond orders of 2 and 3.

We can use energy-level diagrams to describe the bonding in other pairs of atoms and ions where n = 1, such as the H₂⁺ ion, the He₂⁺ ion, and the He₂ molecule. Again, we fill the lowest-energy molecular orbitals first while being sure not to violate the Pauli principle or Hund's Rules.

Figure (PageIndex{1a}) shows the energy-level diagram for the H₂⁺ ion, which contains two protons and only one electron. The single electron occupies the (σ_{1s}) bonding molecular orbital, giving a (σ_1s)¹ electron configuration. The number of electrons in an orbital is indicated by a superscript. In this case, the bond order is (via Equation ref{9.8.1})

[text{bond order}=dfrac{1-0}{2}=1/2]

Because the bond order is greater than zero, the H₂⁺ ion should be more stable than an isolated H atom and a proton. We can therefore use a molecular orbital energy-level diagram and the calculated bond order to predict the relative stability of species such as H₂⁺. With a bond order of only 1/2 the bond in H₂⁺ should be weaker than in the H₂ molecule, and the H–H bond should be longer. As shown in Table (PageIndex{1}), these predictions agree with the experimental data.

Figure (PageIndex{1b}) is the molecular orbital energy-level diagram for (ce{He_2^{2+}}). This ion has a total of three valence electrons. Because the first two electrons completely fill the (σ_{1s}) molecular orbital, the Pauli principle states that the third electron must be in the ( sigma _{1s}^{star} ) antibonding orbital, giving a ( left (sigma _{1s} right )^{2}left (sigma _{1s}^{star } right )^{1} ) electron configuration. This electron configuration gives a bond order (via Equation ref{9.8.1}) of

[text{bond order}=dfrac{2-1}{2}=1/2]

As with H₂⁺, the He₂⁺ ion should be stable, but the He–He bond should be weaker and longer than in H₂. In fact, the He₂⁺ ion can be prepared, and its properties are consistent with our predictions (Table (PageIndex{1})).

Example (PageIndex{1}): The (ce{He_2^{2+}}) ion

Use a molecular orbital energy-level diagrams to predict the bond order and stability of the (ce{He_2^{2+}}) ion.

Given: chemical species

Asked for: molecular orbital energy-level diagram, bond order, and stability

Strategy:

1. Combine the two He valence atomic orbitals to produce bonding and antibonding molecular orbitals. Draw the molecular orbital energy-level diagram for the system.
2. Determine the total number of valence electrons in the He₂²⁺ ion. Fill the molecular orbitals in the energy-level diagram beginning with the orbital with the lowest energy. Be sure to obey the Pauli principle and Hund’s rule while doing so.
3. Calculate the bond order and predict whether the species is stable.

Solution:

A Two He 1s atomic orbitals combine to give two molecular orbitals: a (σ_{1s}) bonding orbital at lower energy than the atomic orbitals and a ( sigma _{1s}^{star } ) antibonding orbital at higher energy. The bonding in any diatomic molecule with two He atoms can be described using the following molecular orbital diagram:

B The He₂²⁺ ion has only two valence electrons (two from each He atom minus two for the +2 charge). We can also view He₂²⁺ as being formed from two He⁺ ions, each of which has a single valence electron in the 1s atomic orbital. We can now fill the molecular orbital diagram:

Helium Is Diatomic Molecule

The two electrons occupy the lowest-energy molecular orbital, which is the bonding ((σ_{1s})) orbital, giving a (σ_1s)² electron configuration. To avoid violating the Pauli principle, the electron spins must be paired.

C So the bond order is (via Equation ref{9.8.1})

[ dfrac{2-0}{2} =1 nonumber]

He₂²⁺ is therefore predicted to contain a single He–He bond. Thus it should be a stable species.

Exercise (PageIndex{1}): The (ce{H_2^{2−}}) Ion

Use a molecular orbital energy-level diagram to predict the valence-electron configuration and bond order of the (ce{H_2^{2−}}) ion. Is this a stable species?

(ce{H_2^{2−}}) has a valence electron configuration of ( left (sigma _{1s} right )^{2}left (sigma _{1s}^{star } right )^{2} ) with a bond order of 0. It is therefore predicted to be unstable.

Learning Objective

• Recognize the properties of homonuclear diatomic molecules.

Key Points

• Diatomic molecules are always linear.
• Diatomic molecules have quantized energy levels for rotation and vibration.
• The halogen series contains many homonuclear diatomic molecules.
• Hydrogen, nitrogen, and oxygen are stable homonuclear diatomic molecules.

Terms

• diatomicconsisting of two atoms
• homonuclearhaving atoms of only one element, especially elements of only a single isotope

Diatomic molecules are composed of only two atoms, of either the same or different chemical elements. Common diatomic molecules include hydrogen (H₂), nitrogen (N₂), oxygen (O₂), and carbon monoxide (CO). Seven elements exist as homonuclear diatomic molecules at room temperature: H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂. The bond in a homonuclear diatomic molecule is non-polar due to the electronegativity difference of zero.

Geometry

Is Helium A Diatomic Element

All diatomic molecules are linear, which is the simplest spatial arrangement of atoms.

Energy Levels

It is convenient and common to represent a diatomic molecule as two point masses (the two atoms) connected by a massless spring. The energies involved in the molecule’s various motions can then be broken down into three categories:

Helium Is Diatomic Meaning

• Translational energies (the molecule moving from point A to point B)
• Rotational energies (the molecule spinning about its axis)
• Vibrational energies (the molecules vibrating in a variety of ways)

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Helium Is Diatomic Form

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