Molecular orbital (MO) diagram for H2O (water), and it's bond order
Drawing the MO diagram of H2O may seem a daunting task at first, as it involves intricate molecular orbital interactions and symmetry considerations.
However, you do not need to worry because in this article we will teach you the Molecular orbital diagram of H2O in the simplest way possible, in addition to calculating its bond order and predicting the bond strength and magnetic behavior of water.
So without any further delay, dive into the article and start reading!
How to draw the molecular orbital (MO) diagram of H2O with its bond order?
As per the molecular orbital theory (MOT) of chemical bonding, after bond formation, the individual atomic orbitals cease to exist. Rather, the atomic orbitals of constituent atoms combine to form a unique set of molecular orbitals (MOs).
The electrons of the participant atoms are thus held in these MOs, belonging to the entire molecule in unison.
The linear combination of atomic orbitals (LCAO) produces two types of molecular orbitals:
Bonding molecular orbitals
Anti-bonding molecular orbitals
The number of MOs produced is exactly equal to the number of atomic orbitals coming together.
A bonding molecular orbital (BMO) is formed by the linear combination of two AOs in the same phase.
Contrarily, an antibonding molecular orbital (ABMO) is produced by the linear combination of two AOs in the opposite phase, counteracting the cohesive forces of the combining nuclei.
This is why, a bonding MO always lies at a lower energy (greater stability) than the parent AOs while an antibonding MO occupies an energy level higher than that of parent AOs (higher instability).
The electrons are filled in these MOs following the three simple rules:
Aufbau Principle: Electrons first occupy the lower energy orbitals followed by their placement in the higher energy molecular orbitals.
Hund’s Rule: The incoming electrons are singly filled in the degenerate MOs before pairing occurs.
Pauli Exclusion Principle: Two electrons placed in the same MO exhibit an opposite spin (clockwise and anticlockwise).
The different numbers of electrons present in the bonding and/or antibonding MOs of a molecule are displayed schematically on an energy level diagram called the molecular orbital (MO) diagram.
H2O is a heteronuclear polyatomic molecule i.e., a molecule containing more than two atoms from more than one type of element, in this case, hydrogen (H) and oxygen (O).
To draw the molecular orbital (MO) diagram of H2O you need to grab a paper and a pencil and follow the steps given below.
Steps for drawing the molecular orbital (MO) diagram of H2O with its bond order
1. Write down the electronic configuration of H2O atoms
The H2O molecule comprises an oxygen (O) atom and two atoms of hydrogen (H).
The electronic configuration of an H-atom is 1s1.
The electronic configuration of an O-atom is 1s2 2s2 2p4.
2 H-atoms and 1 O-atom makes a total of 2(1) + 8 = 10 electrons available to be filled in the Molecular orbital diagram of H2O.
Usually, only the valence electrons are displayed in the MO diagram of a molecule, therefore, it is important to note that there are a total of 8 valence electrons in H2O.
2. Determine whether the molecule is homonuclear or heteronuclear
As discussed above, H2O is a heteronuclear molecule.
Both the H-atoms are degenerate and symmetrically equivalent hence they lie at the same energy level in the H2O MO diagram.
Contrarily, the 1s, 2s, and 2p atomic orbitals of the O-atom are arranged in an ascending energy order. However, all the oxygen AOs lie below the AOs of the H-atoms because an O-atom is more electronegative than an H-atom and possesses a higher effective nuclear charge.
An important point to note is that in the H2O molecule, only those AOs will overlap which possess the right amount of energy, and transform in the same way under a specific set of symmetry operations.
The two 1s AOs of the H-atoms may both be in the same phase (1s1 + 1s2) assigned an a1 symmetry classification or be arranged in an opposite phase (1s1 – 1s2) possessing b1 symmetry.
Among the AOs of oxygen, the 1s atomic orbital has an a1 symmetry, however, it possesses a much lower energy than any of the other orbitals of a1 symmetry. Therefore, it stays in a non-bonding state (1a1).
The 2s and 2pz AOs of the O-atom also possess an a1 symmetry and lie at a compatible energy level w.r.t (1s1 + 1s2). Thus, the linear combination of these three AOs produces three MOs, including a strongly bonding MO (2a1), a weakly bonding MO (2a1‘), and a purely antibonding MO (2a1*).
Conversely, the 2py AO has b2 symmetry and an adequate amount of energy to overlap with (1s1 – 1s2) thus producing two MOs including a BMO (1b2) and an ABMO (1b2*).
However, the 2px AO of oxygen having a mismatched b1 symmetry does not interact with any other orbital; rather it stays in a non-bonding state (1b1).
Thus, there are a total of 3 bonding MOs, 2 antibonding MOs, and 2 non-bonding MOs in the molecular orbital diagram of H2O.
These molecular orbitals are arranged in an ascending energy order, as shown below.
3. Fill the molecular orbitals of H2O with electrons following the energy and bonding principles
Starting from the lowest energy level, the 2 electrons in the 1s AO of the O-atom retain their position in 1a1 non-bonding MO.
Next, there are a total of 8 electrons to be filled in this MO diagram.
The first two electrons are placed as an electron pair in 2a1 followed by two electrons in 1b2, and two electrons in 2a1‘ while the last two electrons occupy 1b1. However, the antibonding MOs (1b2* and 2a1*) stays empty.
Refer to the figure drawn below.
Thus, the Molecular orbital electronic configuration of H2O is (1a12)(2a12)(1b22)(2a1‘ 2)(1b12).
As per the Molecular orbital diagram of H2O, the non-bonding molecular orbital 1b1 is the highest occupied molecular orbital (HOMO). In contrast, 1b2* is the lowest unoccupied molecular orbital (LUMO).
The electron pairs in the non-bonding MOs (1a1 and 1b1) justify the 2 lone pairs present on the O-atom in the Lewis structure of H2O.
Is H2O diamagnetic or paramagnetic?
The absence of any unpaired electrons in the MO diagram of water (H2O) suggests its diamagnetic nature.
Diamagnetic substances possess no permanent dipole moment value; therefore they get repelled by an external magnetic field.
Bond order of H2O
The bond order formula is:
∴ Bond order = (Nb –Na)/2
Nb = Electrons present in the bonding MOs (Bonding electrons).
∴ Electrons in 2a1 + 1b2 = 2 + 2 = 4
Note: We consider only the electrons present in strongly bonding MOs therefore the electron pair in 2a1‘does not count.
Na= Electrons present in the anti-bonding MOs (Anti-bonding electrons).
∴ Electrons in 1b2* + 2a1*= 0
⇒ Bond order of H2O = (4 – 0)/2 = 2
Bond order > 0 means that H2O is astable molecule.
A bond order of 2 implies that there are two covalent bonds in the molecule, one O-H bond on either side.
What is the molecular orbital (MO) diagram of H2O?
H2O is a heteronuclear polyatomic molecule. Only those AOs of the O-atom and the two H-atoms overlap that possess a comparable energy and proper symmetry.
The molecular orbital (MO) diagram of H2O is shown below.
The Molecular orbital electronic configuration of H2O is (1a12)(2a12)(1b22)(2a1‘ 2)(1b12).
The absence of any unpaired electron in the above diagram suggests its diamagnetic nature.
The bond order of H2O is 2. There are two O-H bonds, one on either side of H2O.
Is H2O paramagnetic or diamagnetic?
H2O is a diamagnetic substance.
How does the MO diagram of H2O suggest its bent shape and structure?
The two bonding MOs of H2O (2a1 + 1b2) do not lie at the same energy level in its molecular orbital diagram.
Therefore, the energies of the two O-H bonds are not equivalent and H2O is an asymmetric molecule, possessing a bent, angular, or V-shape.
How can H2O molecules form hydrogen bonding as per MOT?
Considering two H2O molecules, as per MOT, the HOMO of one H2O interacts with the LUMO of the second H2O molecule, producing a sigma bonding (σ1) and a sigma antibonding (σ1*) MO.
Similarly, the HOMO of the second H2O overlaps with the LUMO of the first H2O to form σ2 and σ2* MOs.
The 4 electrons from the HOMO of both molecules now occupy the new sigma BMOs, at a lower energy level than their parent orbitals, thus facilitating hydrogen bonding and enhanced stability in the structure of water.
Summary
Water (H2O) is a heteronuclear polyatomic molecule, containing three atoms from two different elements.
The MO electronic configuration of H2O is (1a12)(2a12)(1b22)(2a1‘ 2)(1b12).
The absence of any unpaired electron in the Molecular orbital diagram of H2O reveals its diamagnetic nature.
The bond order of H2O is 2.
It is a stable molecule, possessing an asymmetric shape.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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