Ammonia (NH3) lewis dot structure, molecular geometry or shape, electron geometry, bond angle, formal charge
Ammonia is a colorless gas that has a strong pungent odor. It has a chemical formula of NH3. Ammonia gas is lighter than air.
In this article, we will discuss Ammonia (NH3) lewis structure, molecular geometry or shape, electron geometry, bond angle, hybridization, formal charge, etc.
Ammonia is soluble in chloroform, ether, ethanol, etc. Its conjugate acid is Ammonium and its conjugate base is Amide.
Ammonia (NH3) lewis structure is made up of one nitrogen (N) atom and three hydrogens (H) atoms. In, lewis’s structure of NH3, three bond pairs, and one lone pair are present. The nitrogen (N) atom is situated at a central position and the hydrogen (H) atoms are at the outside position in the lewis diagram.
The drawing of the NH3 Lewis structure is very easy and simple. Let’s see how to do it.
Steps for drawing the Lewis dot structure for NH3
1. Count total valence electrons in NH3
First of all, determine the valence electron that is available for drawing the lewis structure of NH3 because the lewis diagram is all about the representation of valence electrons around atoms.
So, an easy way to find the valence electron of atoms in the NH3 molecule is, just to look at the periodic group of nitrogen and hydrogen atoms.
As the nitrogen atom belongs to the 5A group in the periodic table and hydrogen is situated in the 1A group, hence, the valence electron for the nitrogen is 5, and for the hydrogen atom, it is only 1.
⇒ Total number of the valence electrons in hydrogen = 1
∴ Total number of valence electrons available for the NH3 Lewis structure = 5 + 1×3 = 8 valence electrons [∴ NH3 molecule has one nitrogen and three hydrogen atoms]
2. Find the least electronegative atom and place it at center
An atom with a less electronegative value is preferable for the central position in the lewis diagram because they are more prone to share the electrons with surrounding atoms.
It should be noted that “Hydrogen always go outside in lewis diagram” Because, Hydrogen atom can form only one single bond.
Hence, put the nitrogen atom at the central position of the lewis diagram and all three hydrogen atoms outside it.
3. Connect outer atoms to the central atom with a single bond
In this step, join all outer atoms to the central atom with the help of a single bond.
In, the NH3 molecule, hydrogen is the outer atom, and nitrogen is the central atom. Hence, joined them as shown in the figure given below.
Count the number of valence electrons used in the above structure. There are 3 single bonds used in the above structure, and one single bond means 2 electrons.
Hence, in the above structure, (3 × 2) = 6 valence electrons are used from a total of 8 valence electrons available for drawing the NH3 Lewis structure.
∴ (8 – 6) = 2 valence electrons
So, we are left with only 2 valence electrons.
4. Complete the octet of all atoms
In the 3rd step structure, the hydrogen atoms completed their octet since they have 2 electrons(one single bond means 2 electrons) in their outer shell.
Hydrogen atom only need 2 electrons to fulfill the outer shell.
Now the Nitrogen central atom, in the NH3 molecule, requires a total of 8 electrons to have a full outer shell.
If you look at the 3rd step structure, the nitrogen atom is attached to three single bonds that means it have 6 electrons, so, it just short of 2 electrons.
We already have the remaining 2 valence electrons, hence, put these two electrons on the nitrogen atom to complete its octet as well.
In the above structure, we see, that each atom completed its octet comfortably, now, Let’s check the formal charge for the above NH3 lewis structure to verify whether it’s stable or not.
5. Check the stability of the NH3 lewis structure with the help of a formal charge concept
The lesser the formal charge on atoms, the better the stability of the lewis diagram.
To calculate the formal charge on an atom. Use the formula given below-
What are the electron and molecular geometry of NH3?
The molecular geometry or shape of NH3 is a Trigonal pyramid.
The nitrogen (N) central atom is at the apex position and the three hydrogens (H) atoms are at the corners of the trigonal base.
The electron geometry of NH3 is Tetrahedral, which is different from the Pyramid geometry.
The molecular geometry of NH3
The molecular geometry or shape of NH3 is a Trigonal pyramid, because, the lone pair present on the central Nitrogen (N) atom exerts repulsion on the bonded pairs, therefore, the lone pair push itself away from the bonded pairs, it results, in the three bonds(N-H) are pushed down even further away from their respective position, and the final shape of NH3 appears like Trigonal pyramid.
It should be noted that, Molecular geometry only consider bonded atom while determining the shape of molecule, it doesn’t count lone pair, but the influence of lone pair(repelling effect) will be counted on overall shape of molecule.
So, while determining the molecular geometry of NH3, we will consider the repelling effect of the lone pair present on the nitrogen central atom, but the lone pair will be invisible when we look at the actual molecular geometry of NH3.
What is the electron geometry of NH3?
The electron geometry consider bond pair as well lone pair while determining the geometry of any molecule.
The electron geometry of NH3 is Tetrahedral, because, the nitrogen central atom has one lone pair and it is attached to three bonded pairs as well. So, there are 4 regions of electron density(3 bond pairs + 1 lone pair) around the central atom.
According to the VSEPR theory, the central atom with four regions of electron density adopts a tetrahedral electron geometry. Because repulsion is minimum in electron pairs at this position.
“A region of electron density means the group of bonding or nonbonding electrons that present around the atom. “
According to the VSEPR theory, “the maximum distance four regions of electron density can get away from affords a geometry called Tetrahedral.”
Now, a very simple way to determine the electron and molecular geometry of NH3 is the AXN method.
AXN is a simple formula that represents the number of the bonded atom to central atom and number of lone pair on the central atom to predict the shape or geometry of the molecule using the VSEPR chart.
AXN notation for NH3 molecule:
A denotes the central atom, so, Nitrogen (N) is the central atom in NH3 molecule A = Nitrogen
X denotes the number of bonded atoms to the central atom, Nitrogen (N) is bonded with three hydrogens (H) atoms. Therefore, X = 3
N represents the number of lone pairs on the central atom, as per NH3 Lewis structure, the Nitrogen central atom has one lone pair. Hence, N = 1
So, the AXN generic formula for the NH3 molecule becomes AX3N1.
As per the VSEPR chart, if a molecule gets AX3N1 generic formula then its molecular geometry or shape will be a trigonal pyramid, and its electron geometry will be tetrahedral.
Therefore, according to the VSEPR chart or AXE notation, the electron geometry for NH3 is a Tetrahedral and its molecular geometry is Trigonal pyramidal.
Hybridization of NH3
“Hybridization is a theory that helps us understand the shape of molecular orbitals upon bonding to compounds”
A steric number is used to determine the hybridization of an atom.
Steric number = (Number of bonded atoms attached to central atom + Lone pair on central atom)
When the Steric number is equal to 2, then the hybridization of that atom is sp, and if it is equal to 3 then Sp²…..so on.
Steric number
Hybridization
2
Sp
3
Sp²
4
Sp³
5
Sp³d
6
Sp³d²
According to the Lewis structure of NH3, nitrogen is the central atom that is attached to the three bonded atoms(hydrogens) and it has one lone pair as well.
Hence, (3 + 1) = 4 is the steric number of central atom nitrogen (N) in the NH3 molecule that gives Sp3 hybridization.
The bond angle of NH3
We know, the molecular geometry of NH3 is a Trigonal pyramid or distorted tetrahedral. The ideal bond angle for regular tetrahedral geometry is 109.5º.
But, the bond angle, in the NH3 molecules is 107º. This is because the presence of lone pair on the central Nitrogen (N) atom exerts a repulsive force on the bonding pairs, this will contract the bond angle in NH3 by approx 2.5º.
Therefore, the bond angle decrease from its ideal value(109.5º to 107º).
∴ So, the H-N-H bond angle in NH3 is 107º.
It should be 109.5° for Perfect tetrahedron but the lone pair electrons occupy more space, so, it will contract the angle between bonding orbitals.
“The bond angle decreases due to the presence of lone pairs, which cause more repulsion on the bond pairs and as a result the bond pairs tend to come closer.”
Therefore, due to the lone pair effect, the bond angle in NH3 has a value of 107º irrespective of 109.5º.
Well, we know the polar molecule has some dipole moment because of unequal distribution of charges whereas the non-polar molecule has an equal distribution of charges that cause zero dipole moment because they cancel out each other due to the symmetrical shape of the molecule.
So, Is NH3 polar or nonpolar? NH3 is a polar molecule. Because its molecular geometry is Trigonal pyramidal which is not symmetrical since there is one lone pair present on the central atom Nitrogen (N) that causes distortion in a molecule, so, it results in unequal distribution of charges over the atoms, which results in some net dipole moment, and, makes, NH3 is a polar molecule.
How many lone pairs are present in the lewis structure of NH3?
Lone pairs are those represented as dots in the lewis diagram that do not take part in the formation of bonds and are also called nonbonding electrons.
By looking at the NH3 Lewis structure, we see, that there is only 1 lone pair present(2 dot electrons on the central Nitrogen (N) atom).
The total number of bond pairs present in the lewis structure of NH3?
Bonding pairs are the pair of electrons that are in a bond. A single bond has one bond pair means 2 bonding electrons.
Two bonding electron between the atoms forms a single covalent bond.
Now, as per the NH3 Lewis structure, the central atom nitrogen is attached with three single covalent bonds, and one single covalent bond means 2 bonding electrons.
Hence, the total bonding electrons is (3 × 2) = 6 bonding electrons that make 3 bond pairs.
∴ In the NH3 Lewis dot structure, a total of 3 bond pairs are present.
Why the molecular geometry of NH3 is Trigonal pyramid and the electron geometry is Tetrahedral?
Two types of geometry can be predicted with the help of VSEPR theory- (a). Electron geometry (b). Molecular geometry
So, the molecular geometry or shape of NH3 is a Trigonal pyramid but its electron geometry is Tetrahedral. Let’s find out why.
Look at the basic concept-
Electron geometry considers all electrons(Bonding and Lone pair electrons) whereas molecular geometry considers only Bonding atoms to determine the geometry of any molecule.
Now, look at the figure given below-
It’s very simple when we evaluate the electron geometry for NH3, then we will consider lone pair as well as bond pair around the central atom.
So, In NH3, the nitrogen central atom has one lone pair and has three bond pairs as well, which makes up, 4 regions of electron density around the central atom, according to the VSEPR theory, “the maximum distance four regions of electron density can get away from affords a geometry called Tetrahedral.”
Now for the molecular geometry or shape of Ammonia (NH3), we will only look at the bonded pair, excluding the lone pair. But, the effect of lone pairs on bonded atoms will be considered.
Based on this concept, the molecular geometry of Ammonia (NH3) will be a Trigonal pyramidal, as a lone pair electron is invisible, although still exerting its influence.
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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