Is BH3 polar or nonpolar? - Polarity of Boron trihydride
BH3 is the chemical formula for boron trihydride, aka borane or borine. It is an unstable, spontaneously flammable, and highly reactive molecule with a molar mass of 13.83 g/mol.
An interesting fact is that this extremely low molecular weight chemical compound, borane BH3 is used as one of the high-energy fuel sources for rockets and jet aircraft.
So are you excited to know whether this highly reactive molecule is polar or non-polar? Continue reading to find out the answer to this question.
Is BH3 polar or non-polar?
Borane (BH3) is a non-polar molecule. It consists of one boron (B) and three hydrogens (H) atoms. The B-atom is kept at the central position, while all three H-atoms are at the surrounding positions, making a perfectly symmetrical, trigonal planar molecule.
An electronegativity difference of 0.16 units exists between the bonded atoms of boron and hydrogen in the B-H bonds of BH3.
Thus, each B-H bond is individually slightly polar in BH3 and possesses a small dipole moment value (symbol µ).
However, in BH3, these small B-H dipole moments get canceled uniformly in the symmetrical trigonal planar shape of the molecule.
Hence, the charged electron cloud stays uniformly distributed in the molecule overall. As result, the BH3 molecule is overall non-polar (net µ = 0).
A molecule is polar if there is a non-uniform charge distribution present in it.
If the charge distribution gets equally balanced in different parts, then that molecule or molecular ion is considered non-polar.
The following three factors mainly influence the polarity of a molecule:
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now, let us discuss the effect of the above three factors one by one to prove that the borane (BH3) molecule is overall non-polar.
Factors affecting the polarity of BH3
Electronegativity
It is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity decreases down the group in the Periodic Table of elements while it increases across a period.
Greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
Boron (B) is present in Group III A (or 13) of the Periodic Table. The electronic configuration of a boron atom is 1s2 2s2 2p1. As per this electronic configuration, a B-atom has a total of 3 valence electrons. It is, thus, short of 5 valence electrons that are required so that the boron atom can achieve a complete octet electronic configuration.
Conversely, hydrogen (H) belongs to Group I A (or 1) of the Periodic Table. The electronic configuration of a hydrogen atom is 1s1. Hence, each H-atom in the BH3 molecule has a deficiency of 1 more valence electron to obtain a full outer shell.
The Lewis dot structure of BH3 displays a total of 3 B-H single covalent bonds. All three valence electrons of boron consumed in covalent bonding imply there is no lone pair of electrons on the central B-atom in the BH3 molecule.
Hence there is no distortion witnessed in the shape and geometry of the molecule.
An interesting fact here is that each terminal H-atom has a complete duplet in BH3; however, the central B-atom only has a total of 6 valence electrons around it and thus an incomplete octet.
This is because elements such as boron and beryllium are exceptions that do not holistically follow the octet rule.
Atom
Electronic configuration
Valence electrons
Boron (5B)
1s22s22p1
3
Hydrogen (1H)
1s1
1
The electronegativity difference in BH3 between the B-atom (E.N = 2.04) and H-atom (E.N = 2.20) in each of the three B-H bonds is 0.16 units. It is much less than that required for a covalent bond to be polar, as per Pauling’s electronegativity scale.
However, we know that a purely non-polar covalent bond is only formed between identical atoms such as H-H.
In B-H, there are two dissimilar atoms involved; thus, the H-atom being slightly more electronegative than the B-atom attracts the B-H shared electron cloud to a slightly greater extent.
Hence, the hydrogen atom with relatively higher electronegativity gains a partial negative (Hδ-) charge while the central boron atom obtains a partial positive (Bδ+) charge, as shown below.
In this way, each B-H bond is considered weakly polar in the BH3 molecule.
Dipole Moment
Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.
It is mathematically calculated as a product of the magnitude of charge (Q) and charges separation (r). The dipole moment is expressed in a unit called Debye (D).
The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.
So in BH3, the dipole moment of each B-H bond points from Bδ+ to Hδ-(as shown below).
Molecular geometry
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, BH3 is an AX3E0 or simply AX3-type molecule. To one B-atom at the center (A), three bonded atoms (X) are attached (i.e., three H-atoms), and the central B-atom contains no lone pair of electrons (E).
Thus, the molecular geometry or shape of BH3 is trigonal planar, which is identical to its ideal electron pair geometry.
There are no lone pair-lone pair or lone pair-bond pair electronic repulsions present in the molecule; hence the bonded atoms form a mutual bond angle of (∠H-B-H) 120°.
Due to the absence of any lone pair of electrons on the central B-atom, the BH3 molecule has an identical molecular and electron pair geometry i.e., symmetrical trigonal planar.
Relatively highly electronegative terminal H-atoms attract the shared electron cloud of each B-H bond to a greater extent than the central B-atom. Still, because each B-H bond is directed at an angle of 120° to each other in a symmetrical plane, hence, the dipole moment generated in the B-H bonds gets canceled out uniformly.
The charged electron cloud in the trigonal planar shape of BH3 stays uniformly distributed overall. Consequently, borane (BH3) is overall a non-polar molecule (net µ = 0).
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), nitric oxide (NO), nitrous oxide (N2O), etc.
BH3 has very weakly polar covalent bonds due to the small electronegativity difference of 0.16 units between the bonded B-atom (E.N = 2.04) and H-atoms (E.N = 2.20).
BH3 is a symmetrical trigonal planar, So, the charged electron cloud stays uniformly distributed and the dipole moment of individually polar B-H bonds gets canceled.
Thus, BH3 is overall a non-polar molecule with a net dipole moment of µ = 0.
This uneven dispersion of the electron cloud is the reason that the individual dipole moments in the molecule do not get canceled, which results in an overall polar molecule (net µ = 0).
Contrarily, BH3 has a symmetrical trigonal planar molecular shape. The charged electron cloud in the molecule stays uniformly distributed, and the small dipole moment of each B-H bond in the BH3 molecule gets canceled uniformly, ensuring the non-polarity of the molecule.
Hence, NH3 is a polar molecule with a net dipole moment of µ = 1.40 D. Contrarily, BH3 is overall a non-polar molecule with a net dipole moment of µ = 0.
What are the formal charges present on the bonded atoms in BH3?
Formal charge on an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For the central B-atom
Valence electrons = 3
Bonding electrons = 6
Non-bonding electrons = 0
∴ Formal charge on the central B-atom = 3 – 0 – 6/2 = 3 – 0 – 3 = 3 – 3 = 0
All atoms in the BH3 molecule including boron and hydrogen obtain a formal charge equal to zero.
Hence, the overall charge present on the borane BH3 molecule is also zero.
Summary
Borane (BH3) is a non-polar molecule.
It consists of three B-H bonds.
B-H bonds are slightly polar, having an electronegativity difference of 0.16 units between the bonded B-atom (E.N = 2.04) and H-atom (E.N = 2.20).
Borane BH3 has a trigonal planar molecular and electron pair geometry with a bond angle ∠ H-B-H = 120°.
Due to the absence of any lone pair of electrons on the central B-atom, the BH3 molecule has a symmetrical trigonal planar shape.
In BH3, the individual dipole moments of slightly polar B-H bonds get canceled equally in opposite directions.The net dipole moment in BH3 is thus net µ= 0, so borane is overall a non-polar molecule.
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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