Water (H2O) Lewis dot structure, molecular geometry or
shape, electron geometry, bond angle, formal charge, hybridization
‘’Water is essential for all life forms on Earth ‘’. The fact that this statement never gets overrated makes us think about the chemical nature and properties of water. H2O is the chemical formula for water.
Let’s find out how to draw water (H2O) Lewis structure, what is its molecular geometry or shape, electron geometry, bond angle, formal charges, hybridization, etc. all through this article.
The Lewis structure of H2O represents the oxygen (O) atom present at the center. It is bonded to 2 atoms of hydrogen (H) at the sides. There are a total of 2 lone pairs present in the water (H2O) lewis dot structure. Both lone pairs are on the central oxygen (O) atom.
Let’s go through the following steps one by one and see how we can draw the Lewis structure of H2O without any difficulty.
Steps for drawing the Lewis dot structure of H2O
1. Count the total valence electrons in H2O
The Lewis dot structure of a molecule is a simplified representation of all the valence electrons present in the molecule. Therefore, the first step whenever we want to draw the Lewis structure of a molecule is to find the total valence electrons present in it.
The valence electrons present in the atom of an element can be readily determined by identifying the concerned element from the Periodic Table.
For instance, oxygen (O) present in water is situated in Group VI A of the Periodic Table of elements so it has a total of 6 valence electrons. On the other hand, the hydrogen (H) atom marks its position at the top of the Periodic Table and has a single electron in its valence shell.
∴ The H2O molecule consists of 2 hydrogen atoms and 1 oxygen atom. So, the total valence electrons available for drawing the Lewis dot structure of H2O = 6 + 2(1) = 8 valence electrons.
2. Find the least electronegative atom and place it at the center
The convention is that the least electronegative atom out of all concerned atoms is placed at the center of the Lewis structure. This is because the least electronegative atom easily shares its electrons with other atoms in a molecule.
But in the case of water (H2O), it is important to note that the only two elements involved are hydrogen (H) and Oxygen (O). Hydrogen is less electronegative than oxygen, but we cannot choose it as the central atom.
Rather, hydrogen is always placed at an outer position in the Lewis structure of all the molecules because it can only form a single bond with its adjacent atoms.
Oxygen having 6 valence electrons is more likely to share its electrons with the other atoms. So, it is placed at the center of the Lewis diagram. The 2 H atoms are placed in its surroundings, as shown below.
3. Connect outer atoms with the central atom
In this step, all the outer atoms are joined to the central atom using single straight lines.
In the H2O molecule, the 2 H atoms are the outer atoms while the O atom lies at the center. So, each H atom is joined to the central O atom using straight lines.
If we count the total valence electrons used in the above diagram, we know that each straight line represents a single covalent bond, and each bond is equal to a bonded electron pair (2 electrons).
Hence in the above structure, (2×2) = 4 valence electrons are used out of the total 8 valence electrons initially counted.
Total valence electrons available – electrons used till step 3 = 8-4 = 4 valence electrons.
This means we are still left with 4 valence electrons to be accommodated in the Lewis structure of H2
4. Complete the duplet or octet of outer atoms
There are 2 H atoms present as the outer atoms around the central O atom in the Lewis structure of H2O.
Each hydrogen (H) atom needs a total of 2 valence electrons only in order to achieve a stable duplet electronic configuration.
Each O-H single bond represents 2 electrons placed around an H atom. That means the duplet of each H atom is already complete in the Lewis structure of H2O drawn till this step. So, there is nolonepair around the H atoms in the H2O molecule.
5. Complete the octet of the central atom
Total valence electrons used till step 4 = 2 single bonds + no lone pair around any H atom = 2(2) + 0 = 4 valence electrons.
Total valence electrons – electrons used till step 4 = 8-4 = 4 valence electrons.
∴ Thus, these 4 valence electrons are placed as 2 lone pairs around the central O atom which leads to a complete octet configuration of the O atom in the H2O Lewis structure, as shown below.
The final step is to ensure the stability of this Lewis structure which can be done with the aid of the formal charge concept.
6. Check the stability of Lewis’s structure with the help of the formal charge concept
The less the formal charge on the atoms of a molecule, the better the stability of its Lewis structure.
The formal charge can be calculated using the formula given below.
Zero formal charges present on all the atoms in the Lewis structure of H2O confirm that it is a stable Lewis structure.
The good news is that we have drawn it correctly. Now that you are confident about the Lewis structure of H2O, so let’s step ahead and discuss some other important facts about the shape and geometry of the molecule.
What are the electron and molecular geometry of H2O?
Water (H2O) has a bent, angular, or V-shape and molecular geometry. In contrast to that, the ideal electron geometry of H2O is tetrahedral. It is due to the presence of 2 lone pairs on the central O atom in the H2O molecule that its molecular geometry is different from the ideal electronic geometry.
Lone pair-lone pair repulsions and lone pair-bond pair repulsions significantly affect the geometry of the water molecule and make it occupy an asymmetric bent shape.
Molecular geometry of H2O
The water (H2O) molecule has an asymmetric bent shape or molecule geometry. There are 2 lone pairs of electrons on the central oxygen (O) atom which leads to strong lone pair-lone pair and lone pair-bond pair electronic repulsions. The two lone pairs occupy positions on the molecule as far apart from each other as possible in order to minimize their repulsive effect.
The electronic repulsive effect decreases in the order of lone pair-lone pair repulsions > lone pair-bond pair repulsions > bond pair-bond pair repulsions. This makes the O-H bonds bend slightly towards each other and away from the lone pairs.
Consequently, the molecule occupies an asymmetric bent shape. O lies at the top of an inverted V-shape while the H-atoms lie at the terminals, as you can see in the figure below.
An important point to remember is that the molecular geometry or shape of a molecule depends on the difference in the number of lone pairs and bond pairs around the central atom in the molecule.
Contrarily, the ideal electron geometry depends on the total regions of electron density around the central atom in the molecule. Each electron pair is considered a separate region of electron density, regardless of the fact whether it’s a bond pair or a lone pair.
Let’s figure out how this concept works with reference to the H2O molecule.
Electron geometry of H2O
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electronic geometry of a molecule having a total of 4 electron density regions around the central atom is tetrahedral.
In line with that, the ideal electron geometry of H2O is tetrahedral as there are 2 bond pairs + 2 lone pairs = 4 electron pairs = 4 regions of electron density around the central oxygen (O) atom.
A much more straightforward way of finding the electron geometry of H2O or any other molecule in that case is using the AXN method.
AXN is a simple formula to represent the number of atoms bonded to the central atom in a molecule and the number of lone pairs present on the central atom.
It is used to predict the geometry and shape of a molecule against the VSEPR chart given below.
AXN notation for H2O molecule
A in the AXN formula represents the central atom present in the molecule. As oxygen (O) is the central atom in the H2O molecule so A=O.
X denotes the number of atoms bonded to the central atom. As 2 hydrogen (H) atoms are bonded to the central O atom in the Lewis structure of H2O so X=2.
N stands for the number of lone pairs present on the central atom. As per the H2O Lewis structure, there are 2 lone pairs on the central O atom so N=2.
Thus, the AXN generic formula for the H2O molecule is AX2N2.
Now have a glance at the VSEPR chart given below and identify where you find AX2N2.
The VSEPR chart tells us that the AX2N2 generic formula is representative of the molecules having a tetrahedral electron geometry while their molecular geometry or shape is bent, as we already noted down for the H2O molecule.
Hybridization of H2O
The central oxygen (O) atom in the H2O molecule is sp3 hybridized.
The electronic configuration of oxygen is 1s2 2s2 2p4.
During chemical bonding, the 2s orbital mixes with the three 2p orbitals to yield four sp3 hybrid orbitals. Each sp3 hybrid orbital represents a 75% p character and a 25% s character.
Two of the four sp3 hybrid orbitals contain paired electrons which are situated as lone pairs on the central O atom. The other two sp3 hybrid orbitals containing a single electron each overlap with the s orbitals of H atoms to form a sigma (σ) O-H bond on each side of the H2O molecule.
A short trick for finding the hybridization present in a molecule is memorizing the table given below. The steric number of a molecule can be used against this table to determine the hybridization of the central atom in the molecule.
There are 4 electron pairs so four regions of electron density around the central O atom in the H2O molecule hence its steric number is 4. Consequently, the O atom has sp3 hybridization in H2O.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The H2O bond angles
The H-O-H bond angle in the H2O molecule is 104.5°.
The ideal bond angle in a symmetrical tetrahedral molecule is 109.5° but it is due to the lone pair repulsive effect that the symmetry of the molecule gets disturbed. The molecule adopts an asymmetric bent shape, and the H-O-H bond angle decreases from the ideal 109.5° to 104.5°. The O-H bond length in the water molecule is 95.8 pm, as shown in the figure below.
Pauling’s electronegativity scale states that any bond having an electronegativity difference greater than 0.5 units between its bonded atoms is considered polar.
Oxygen (O) is more electronegative than the hydrogen (H) atoms to which it is bonded in the H2O molecule. An electronegative difference of 1.24 units exists between the bonded O (E.N = 3.44) and H (E.N = 2.20) atoms in each O-H bond. Thus, each O-H bond present in the H2O molecule is polar and possesses a specific dipole moment value (symbol μ).
The asymmetric bent shape of the Water (H2O) molecule ensures that the electron cloud stays non-uniformly distributed in the molecule overall. The dipole moment effect of each O-H bond adds up. In conclusion, water is a polar molecule with net μ=1.85 D.
How many lone pairs are present in the Lewis structure of H2O?
There are 2 lone pairs around the central O atom in the Lewis structure of H2O.
Why there is no lone pair around the outer H atoms in H2O Lewis’s structure?
The electronic configuration of hydrogen (H) is 1s1. Naturally, it is short of a single electron only to achieve its most stable duplet configuration. Therefore, H atoms only form a single bond with the central O atom in the Lewis structure of H2O.
Once that is done, both the H-atoms complete their duplet, and no electrons are required as lone pairs around the outer H-atoms.
On the other hand, the water (H2O) molecule has a tetrahedral electron geometry while its molecular geometry or shape is bent. There are a total of 4 electron density regions (2 bond pairs + 2 lone pairs) around the central O atom in the molecule.
What are some examples of a similar Lewis structure to H2O?
The Lewis structures of hydrogen disulfide (H2S) and oxygen difluoride (F2O) are similar to that of water and can be drawn using all the steps used for drawing the H2O Lewis structure.
What is the shape of water (H2O) according to the VSEPR theory?
According to the VSEPR theory, water is an AX2N2 type molecule. It’s ideal electron geometry is tetrahedral while the molecular geometry or shape of water is bent. The bent shape is also called angular or V-shape.
The presence of lone pairs on the central atom i.e., O in H2O makes the molecule adopt a different shape from its ideal electronic geometry.
Why H2O has a bent shape and not linear?
There are 2 lone pairs present on the central O atom in the H2O molecule. Lone pair-lone pair repulsions and lone pair-bond pair repulsions exist in the molecule. Therefore, the molecule cannot attain a linear shape.
Rather, it adopts an asymmetric bent shape to minimize the repulsive effect. The O-H bonds tilt inwards and the H-O-H bond angle decreases.
Why do CH4, NH3, and H2O differ in shape when they have the same geometry?
All the three molecules i.e., methane (CH4), ammonia (NH3), and water (H2O) have a total of 4 electron pairs around the central C, N, and O atoms respectively.
Four electron pairs around the central atom mean 4 regions of electron density thus each molecule has a tetrahedral electron geometry as per the VSEPR concept.
However, the molecular geometry or shape of a molecule depends on the distinction between the bond pairs and lone pairs around the central atom.
The three molecules have a different lone pair-bond pair combination which accounts for their different shapes.
In CH4: 4 electron pairs = 4 bond pairs so its shape is identical to its electron geometry i.e., tetrahedral.
In NH3: 4 electron pairs = 3 bond pairs + 1 lone pair so it has a trigonal pyramidal shape.
In H2O: 4 electron pairs = 2 bond pairs + 2 lone pairs therefore it has a bent 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/
Topblogtenz is a website dedicated to providing informative and engaging content related to the field of chemistry and science. We aim to make complex subjects, like chemistry, approachable and enjoyable for everyone.
