Students often get confused regarding the polarity or non-polarity of BF3 (Boron Trifluoride) due to the presence of three Fluorine atoms which have a very high electronegativity value when compared to the Boron atom. So, is BF3 polar or non-polar? BF3 (Boron Trifluoride) is Non-Polar because of its highly symmetric shape. It has a Trigonal Planar geometry which cancels out the dipole moments of the three BF bonds making the resultant Dipole Moment of the compound equal to 0 (Zero). Let’s try to understand this in some detail. To answer our question, we will analyze various factors, responsible for polarity, with respect to BF3.
Molecular Structure of BF3 (Boron Trifluoride)
The BF3 (Boron Trifluoride) molecule has 1 atom of B (Boron, atomic no: 5) and 3 atoms of F (Fluorine, atomic no: 9). The valency of B (Boron) is 3 and of F (Fluorine) is 7, thus the Lewis structure of BF3 can be drawn as shown in the figure:
We can see that each F (Fluorine) atom has 3 lone pairs of electrons thus the molecular structure is balanced and symmetric. According to the VSEPR (Valence Shell Electron Pair Repulsion) Theory, the BF3 (Boron Trifluoride) molecule has a Trigonal planar geometry with each F-B-F bond angle equal to 120 degrees which again adds to the balance of the molecule making it highly symmetric, as shown:
Electronegativity & Bond Polarity of BF3 (Boron Trifluoride)
In BF3 (Boron Trifluoride) molecule there are three bonds of B-F, as shown clearly in Lewis diagram above. The electronegativity of B (Boron) is 2.04 and of F (Fluorine) is 3.98 (maximum) according to the Pauling scale which means that F (Fluorine) will pull the shared electrons towards itself and will thus acquire a partial negative charge (δ-) and B (Boron) will have a partial positive charge (δ+). To determine if a bond is polar or not we must find out the difference between the electronegativity values of the two atoms sharing a bond. The difference between the electronegativity values of B and F is equal to 1.94 (3.98 – 2.04 = 1.94) which is greater than 0.5. Thus, each B-F bond in BF3 (Boron Trifluoride) molecule is polar. It means that the electrons are not shared equally by the two atoms (B and F) instead they are pulled towards F (Fluorine). However, we must keep in mind that bond polarity is different from overall molecule polarity. The bond polarity doesn’t always result in overall molecular polarity.
Dipole Moment of BF3 (Boron Trifluoride)
The three bonds of B-F in BF3 (Boron Trifluoride) are polar (as discussed above) and thus they have a bond dipole moment as shown below:
But the highly symmetrical structure and geometry of BF3 (Boron Trifluoride), as discussed earlier, allows the bond dipole moments to be canceled such that the resultant Molecular Dipole Moment is equal to 0 (Zero). Thus, making the molecule BF3 (Boron Trifluoride) as Non-Polar. Now, we know that the Non-Polarity of BF3 (Boron Trifluoride) is due to its highly symmetrical Trigonal Planar geometry, as predicted by the VSEPR (Valence Shell Electron Pair Repulsion) Theory, which other molecules, like H2O, doesn’t have. To find out whether a given molecule is polar or non-polar, we must also understand the factors which are responsible for the polarity of a molecule. These factors are explained below in detail
Molecular Structure & Geometry
The polarity of a molecule is very much dependent on its structure and geometry. As the bond angle between the bonds determines if the resultant dipole moment of the overall molecule is equal to 0 (Zero) or not. VSEPR (Valence Shell Electron Pair Repulsion) Theory helps to identify the geometrical shape of the molecule which gives us the bond angle. The Lewis structure of the molecule gives us an overview of all the lone pairs of electrons that are present in the molecule. If these lone pairs of electrons remain unbalanced i.e. unsymmetrical then the molecule acquires a polarity (like in NH3 where the Nitrogen atom has the lone pair). The molecules which are highly symmetrical are Non-Polar (like BF3). For more detailed information, you must also check out the article written on BF3 Lewis Structure, Molecular Geometry, and Hybridization.
Electronegativity
Electronegativity is denoted by symbol χ (Greek letter Chi). It can be defined as the capability or the force with which an atom of an element pulls the shared electrons of a bond towards itself making the sharing as unequal. The higher the electronegativity of an atom, the more tendency it has to attract the shared electron pairs of a bond towards itself. It is decided by two factors, firstly, the atomic number of the atom and secondly, the distance which is present between the valence electrons of an atom and its positively charged nucleus. A periodic table of electronegativity of elements has been given by Linus Pauling. According, to which Fluorine element has the highest electronegativity equal to 3.98.
Bond Polarity
To know whether a compound is polar or non-polar, we must first have an idea about what do we mean by Polarity. When atoms of different elements come together to form a compound, they do so by developing a bond between them. In this bond sharing of electrons take place but this sharing is always not equal between the elements. The element with a higher electronegativity value attracts the shared electrons towards itself thus acquiring a partial negative charge (δ-) and the element with less electronegativity value acquires a partial positive charge (δ+). As a result, a separation of charge within the bond takes place and bond polarity develops. For a bond to be polar, the electronegativity difference between the two atoms should be greater than or equal to 0.5. However, bond polarity alone does not make a molecule polar.
Dipole Moment
When a bond acquires polarity i.e. separation of charge occurs then a bond dipole moment is developed. It is a measure of the polarity of the bond made by two atoms. Chemists represent the dipole moment with the slight variation in arrow symbol with the arrowhead pointing towards the negative center, as depicted: δ+ ———————> δ- Since dipole moment is a vector quantity i.e. it has both magnitude and direction, therefore, it can also be 0 (Zero) when the two opposite bond dipoles cancel each other due to the symmetrical geometry of the molecule, like in BF3. It is measured in Debye Unit, denoted by ‘D’. 1 D = 3.33564 × 10-30 C.m, where C is Coulomb and m stands for a meter. To calculate the dipole moment we must know the shape and structure of the molecule. Mathematically, the dipole moment of a bond can be calculated by using the formula given below: Dipole Moment (µ) = Charge (Q) * distance of separation (r) A molecule may have all its bonds polar and yet it might be non-polar (like BF3), it happens due to the canceling of dipole moments of all the bonds making the resultant dipole moment equal to 0 (Zero). Now, with the concepts clear in our minds we can clearly understand how the BF3 molecule is non-polar despite the presence of F (Fluorine), a highly electronegative element
Properties of BF3
Boron Trifluoride is toxic when present in its gaseous state but dissolves easily in cold water due to its high solubility and gives Hydrofluoric Acid which is highly corrosive in its nature. It can corrode metals including stainless steel. Its solubility in cold H2O is 106%. Vapors of BF3 are heavier than air and the prolonged exposure of BF3 containers to heat or fire may have a rupturing or rocketing effect. Even though it is colorless but forms dense white fumes in moist air due to the presence of H2O molecules, however, it is stable in a dry atmosphere. BF3 is also dangerous upon heating for decomposition as it produces toxic and corrosive fumes of Hydrogen Fluoride. BF3 (Boron Trifluoride) can also be used to polymerize unsaturated compounds.
Uses of BF3 (Boron Trifluoride)
BF3 (Boron Trifluoride) has many uses in various industries. It is used as adhesives and sealant chemicals, adsorbents and absorbents, fuels and fuel additives, oxidizing/reducing agents, binding agent for industrial manufacturing, plastics industry, drug production, etc. (Source: https://www.epa.gov/chemical-data-reporting) The paper manufacturing industry also uses BF3 (Boron Trifluoride) for the manufacturing of pulp. It is also used as a lubricant for engines, brake fuels, oils, etc. Apart from this, it is useful in manufacturing oils, Crude oil, crude petroleum, refined oil products, fuel oils, drilling oils, etc. The most important use of BF3 (Boron Trifluoride) happens to be in organic synthesis as a catalyst for many reactions that are useful in industrial productions. Some of them are mentioned below:
Friedel-Crafts alkylation reactions Cleavage of ethers to alcohols Esterification reactions Nitration and Sulfonation of aromatic compounds (Source: Brotherton RJ et al; Ullmann’s Encyclopedia of Industrial Chemistry 7th ed. (1999-2012))
By now, you must have clearly understood the concept behind the polarity and non-polarity of a molecule including the reasons which are specific to BF3 (Boron Trifluoride) to make it non-polar. If you have any doubts or confusion regarding the same, please feel free to leave your queries in the comment section below. We will get back to you, as soon as possible.
