BF3 is the chemical formula for a colorless toxic gas that produces a pungent odor, called boron trifluoride. It is an inorganic compound that is widely used as a catalyst for a series of chemical reactions.
As its name suggests, boron trifluoride consists of a boron (B) atom in the center, surrounded by three atoms of fluorine (F). Each B-F bond is polar individually but the overall BF3 molecule is non-polar. Why is that? Let’s discover together by discussing the basic concepts associated with the polarity of BF3.
Is BF3 polar or non-polar?
Boron trifluoride (BF3) is a non-polar molecule. This non-polarity is attributed to the molecule’s highly symmetric geometrical structure. BF3 has trigonal planar geometry. Each B-F bond is polar due to a difference in electronegativity between the central boron and each halogen i.e., fluorine.
The polarity of each B-F bond gets canceled overall lending the BF3 molecule a net dipole moment of zero.
BF3 is a flat molecule with all four atoms lying in one plane. Three fluorine atoms surround the central boron in a triangular (3 vertices) arrangement therefore the geometrical shape of boron trifluoride is known as trigonal planar.
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory of chemistry, BF3 is an AB3-type molecule with three bond pairs and no lone pair around the central boron.
This makes BF3 a highly symmetrical molecule. All three B-F bonds are equivalent thus the overall BF3 molecule is non-polar.
Three factors dominate the polarity of all molecules in general:
Electronegativity
Dipole moment
Molecular geometry
Let us discuss each of the three concepts to understand the non-polar nature of BF3 better:
Factors affecting the polarity of BF3
Electronegativity:
Electronegativity is the ability of an atom to attract a shared pair of electrons from a covalent bond.
Each F forms a single covalent bond with the central B in BF3 so that each F has a complete octet configuration.
Atom
Group number
Electron configuration
Valence electrons
Boron (B)
III-A (or 13)
1s22s22p1
3
Fluorine (F)
VII-A (or 17)
1s22s22p5
7
The electronegativity of F (E.N= 3.98) is much higher than the electronegativity of B (E.N= 2.04). Rather, fluorine is the most electronegative element in the Periodic Table.
So, each F strongly attracts the shared pair of electrons from each B-F bond. This makes the central boron partially positively charged (Bδ+) while each fluorine gains a partial negative charge (Fδ-) in the BF3 molecule.
For a bond to be polar, the electronegativity difference between the two atoms involved should be equal to or great than 0.5.
Atom
Electronegativity
Electronegativity difference
Boron (B)
3.98
3.98 – 2.04 = 1.94
Fluorine (F)
2.04
Thus, each B-F bond in the BF3 molecule is polar.
However, the bond polarity does not always determine a molecule’s overall polarity. We must have realized the fact by now that a molecule’s bond polarity may or may not be the same as its overall polarity because there are two other factors involved.
Dipole moment
The dipole moment (µ) measures the separation between opposite electrical charges.
It is defined as the product of the magnitude of charge (Q) with the charge separation i.e., bond length (r)
It has a symbol called Debye (D). 1 D= 3.33564 x 10-30 C.m. The symbol C represents Coulomb (unit of charge) while m is for the meter (unit of distance).
A polar bond’s dipole moment is directed from the partial positive to the partial negative center.
Considering the electronegativity difference between a B and an F atom i.e., 3.98-2.04= 1.94, the dipole moment of each B-F bond is 1.94 D. The direction of this dipole moment is from boron (Bδ+) to fluorine (Fδ-).
But the net dipole moment of BF3 is zero because of its highly symmetrical geometry which leads us to discuss the third and final factor controlling BF3 polarity i.e., molecular geometry.
Molecular geometry
BF3 molecule has a trigonal planar geometry. Each boron in the molecule is sp2 hybridized, also referred to as trigonal hybridization.
One s and two p atomic orbitals combine to form three new equivalent sp2 hybrid orbitals.
The three sp2 hybrid orbitals of boron lie in the same plane and are directed towards the three corners of an equilateral triangle to form a bond with the p orbital of the F atom.
Therefore, the name trigonal planar is given to the geometrical shape of BF3. Each B-F bond length is equal i.e., 130 pm, forming a mutual bond angle of 120°.
The dipole moment of the downwards pointing B-F bond (red arrow) gets canceled with the combined dipole moment of the remaining two B-F bonds (blue arrow). Thus, the BF3 molecule overall has a net dipole moment of 0 and BF3 is a non-polar molecule.
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than 0
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3) and sulfur dioxide (SO2), etc.,
Examples include oxygen (O2), nitrogen (N2), methane (CH4), and Boron trifluoride (BF3).
Uses of Boron trifluoride
As a catalyst in organic synthesis
As an initiator in polymerization reactions
In pest control via fumigation
As adhesive, adsorbent, and/or a fuel additive in automotive and industrial manufacturing
For the pharmaceutical drug production
As a binding agent in the paper pulp manufacturing industry
FAQ
Is BF3 polar or non-polar?
The bonds present in BF3 are polar, but, the overall (BF3) molecule is nonpolar.
Because of its symmetric structure that results in the cancellation of the dipole moment.
The net dipole moment for BF3 is zero and this show, its nonpolar nature.
How do you know if a bond is polar?
The difference in electronegativity between the atoms involved determines the polarity of a bond.
The atoms should have a difference in electronegativity greater than 0 for a bond to be polar.
What three factors determine the polarity of a molecule?
Electronegativity, dipole moment, and molecular geometry are the three factors that control the polarity of a molecule.
Why is BF3 not polar?
BF3 has a symmetric geometrical structure. The polarities of B-F bonds get canceled in opposite directions giving a net dipole moment of zero to the BF3 molecule. Therefore, it is not polar.
Does BF3 have polar bonds in it?
All three bonds in BF3 are polar because of the existence of the electronegativity difference between them.
What is the molecular geometry of BF3?
BF3 has trigonal planar geometry. The central boron is sp2 hybridized. It forms an sp2 -p bond with each fluorine, lying at the three corners of the equilateral triangle.
The bond angles in the BF3 molecule are 120°.
Which one is polar between NH3 and BF3?
BF3 is non-polar while NH3 is polar. The polarity of NH3 is due to its asymmetric shape. It has a trigonal pyramidal geometry with three bond pairs and one lone pair on central nitrogen.
The polarities of N-H bonds do not get canceled so it has a net dipole moment greater than zero, unlike BF3 which has a zero net dipole moment.
Boron trifluoride (BF3) is a gas at room temperature and pressure.
Its melting point is -126.8°C while the boiling point of BF3 is -100.3°C.
Molar mass of BF3 = atomic mass (B) + 3 [atomic mass (F) ] = 10.8+ 3(19)= 67.8 g/mol.
It has a high solubility in cold water i.e., about 106%. 332 g of BF3 dissolves in 100 grams of water.
It is toxic in nature and is corrosive to metals.
It exhibits a pungent smell.
It is colorless but white fumes of BF3 can be seen in moist air due to its interaction with H2O.
Boron trifluoride is a Lewis acid, it can donate a pair of electrons and participate in many different chemical reactions.
Chemical reactions of BF3
Acid-base reaction:
Acting as a Lewis acid, BF3 can react with a Lewis Basis (that tends to accept electrons) such as ether and form an addition product called an adduct.
Hydrolysis reaction
Boron trifluoride can undergo a chemical decomposition reaction with the addition of water. Boric acid and fluoroboric acid are formed.
⇒ 4BF3 + 3H2O → 3HBF4 + B(OH)4
Halide exchange reaction
BF3 can undergo a halide exchange reaction with other boron halides such as boron trifluoride (BCl3).
⇒ BF3 + BCl3 → BF2Cl + BCl2F
Other reactions of BF3 may include its involvement as a catalyst in the Friedel craft alkylation, nitration, sulfonation, and esterification of aromatic organic compounds.
Summary
Boron trifluoride (BF3) is a non-polar molecule.
The non-polarity of BF3 is because of its highly symmetric trigonal planar geometry with equal bond lengths and a mutual bond angle of 120°.
Each B-F bond is polar because of a difference in electronegativity between a boron and fluorine atom.
The dipole moments of the polar bonds get canceled because of the symmetric geometry.
The net dipole moment in the BF3 molecule is zero which leads to BF3 non-polarity.
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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