Boron Trifluoride or trifluoroborane (BF3) is an inorganic compound with toxic properties. It is a colorless gas. It has a pungent smell and forms white fumes when exposed to moist air. Boron trifluoride is highly toxic when inhaled.
It reacts with heat when exposed for a prolonged period of time and causes a rupture with a blast. It is soluble in water. Boron trifluoride is hydrolyzed and gives off a highly corrosive material namely hydrofluoric acid when reacted with cold water. Boron trifluoride is considered a versatile chemical compound as it acts as a building block for other boron compounds.
Boron Trifluoride is a very important Lewis acid and is used in the production of different adhesives, lubricants, and other additives. It is also used as a catalyst in alkylation, esterification, and condensation reactions.
Properties of BF3
- It is a colorless, pungent gas with toxic properties.
- It is a volatile liquid at room temperature.
- It reacts with moisture and gives off white fumes.
- It reacts with cold water and produces corrosive hydrofluoric acid.
- It has a molar mass of 67.81 g/mol and a density of 2.76 kg/m3.
- The melting and boiling point of Boron trifluoride is -126.8°C and -100.3°C respectively.
- It has a vapor pressure of 50 atm at 20°C.
| Compound Name | Boron Trifluoride |
| Chemical Notation | BF3 |
| Valence electrons of BF3 | 24 |
| Formal charges | 0 (zero) |
| Molecular geometry of BF3 | Trigonal planar |
| Electron geometry of BF3 | Trigonal planar |
| BF3 Hybridization | sp2 type |
| BF3 Bond Angle | 120 |
| Dipole moment of BF3 | 0 (zero) |
| BF3 Polar or Nonpolar? | Nonpolar |
BF3 Lewis Structure
Gilbert N. Lewis gave a method to represent a molecule along with its chemical bonds and lone pair of electrons diagrammatically called Lewis or electron dot structure. The total number of lone and bound pairs of electrons in the atom is also shown in the Lewis structure. The Lewis structure only considers the electrons in a molecule’s valence shell, ignoring the interior shells. A dot represents a lone pair of electrons in a Lewis structure, while a straight line represents a bonded pair of electrons.
Boron trifluoride or trifluoroborane consists of two kinds of elements, i.e. boron and fluorine. There is one boron atom and three fluorine atoms in a boron trifluoride molecule as the name suggests. In this molecule, boron acts as the central atom. It is because Boron is less electronegative (2.04) than Fluorine (3.98) and we know the lesser the electronegativity value of an element the more likely it is to behave as a central atom. Thus, three fluorine atoms are bonded to a central boron atom.
Valence Electrons
Valence electrons are the total number of electrons present on an atom’s outermost shell. Only the valence electrons participate in the creation of chemical bonds. For bond formation, they are either transmitted or shared (totally or partially). Fluorine has 7 electrons on the outermost shell so the number of valence electrons of Fluorine is 7. Similarly, Boron has 3 valence electrons on the outermost shell and hence the number of valence electrons of Boron is 3.
Octet Rule
The urgency or tendency of each atom to have 8 electrons on its valence shell by losing, acquiring, or sharing electrons is known as the octet rule. The atoms acquire the electronic configuration of the nearest noble gas by gaining, losing, or sharing electrons. Except for hydrogen and helium, all elements follow the octet rule. (The duplet rule applies to hydrogen and helium.)
Note: There are some elements like Boron, Beryllium, Aluminum, Hydrogen, Helium, and Lithium that do not complete octet in their valence shell and become stable without eight electrons.
E.g.: Boron and Aluminum become stable with only six electrons on their valence shell.
Formal Charge
The difference between the number of valence electrons in the neutral atom and the number of valence electrons around the atom in the molecule is known as a formal charge. The Lewis diagram is more stable when the formal charge of an atom is lower. A formula can be used to compute formal charge:
Formal Charge (FC) = V- N – B/2 where,
V= Number of Valence Electrons
N = Number of nonbonded electrons
B = Number of bonded electrons
Steps for drawing Lewis dot structure of BF3
- Count the total number of valence electrons present on each atom of BF3
The total number of valence electrons of the BF3 molecule is 24.
Boron lies on group 13 in the periodic table and contains a valency of 3.
Fluorine lies on group 17 in the periodic table and contains a valency of 7. There is total of three fluorine atoms bonded to the central boron atom.
Valency of BF3 molecule = Valency of Boron atom + Valency of three Fluorine atoms
= 3 + 7*3
= 3 + 21
= 24
- Determine the total number of Valence electron pairs for BF3 molecule
A molecule’s valence electron pair can be computed by summing the molecule’s bonds and lone pair of electrons. There are two sorts of bonds: sigma and pi. By adding sigma bonds, pi bonds, and the lone pair of electrons in a molecule, we may determine the valence electron pair of the molecule.
The total number of valence electron pairs in a molecule can also be calculated by dividing the total number of valence electrons in the molecule by two.
So, the total number of valence electron pairs of BF3 molecule is 24/2, i.e. 12.
- Identify the central atom and draw a simple skeleton diagram
The central atom is the atom that all other atoms are bound around. The identification of a central atom is critical for drawing the Lewis structure of a molecule. A central atom is an atom in a molecule that is the least electronegative of all the atoms.
In the BF3 molecule, Boron (2.04) is less electronegative than fluorine (3.98). Therefore, Boron is the central atom in BF3 and Fluorine is the bound atom.
- Put lone pair of electrons on each atom
All the atoms require eight electrons in their valence shell to fulfill their octet (Exceptions: Boron, Beryllium, Aluminum, Hydrogen, Helium, and Lithium). To complete their octet, the atoms form bonds with other atoms in the molecule. Bonding may not involve all of the electrons in the valence shell. Bonding electrons are those that participate in chemical bonding, while non-bonding or lone pair electrons are those that do not participate in chemical bonding.
In the BF3 molecule, Boron contains 0 lone pair electrons. And Fluorine contains 3 lone pairs of electrons (6 lone electrons).
- Complete the octet of all atoms and make covalent bond if necessary
The atoms have a behavior of losing, gaining, or sharing electrons to make a net number of electrons to eight in their valence shell. Boron is an exception to this behavior as it only requires 6 electrons in its valence shell to become stable. This nature makes Boron stand out from all other elements (Aluminum is also an element that requires 6 electrons in its valence shell to become stable.)
The Boron Trifluoride molecule has two elements: boron and fluorine. Boron has 3 valence electrons and requires 3 more electrons to become stable (as it only requires 6 electrons to become stable). Fluorine contains 7 valence electrons and requires 1 more electron to complete its octet.
Here central atom Boron shares 3 electrons with 3 fluorine atoms and makes 3 covalent bonds. Both the Boron and three Fluorine atoms become stable by the formation of three covalent bonds.
- Calculate the formal charge distribution on all atoms and check the stability
The formal charge of all the atoms is calculated to check the stability. The formula to calculate formal charge distribution is:
Formal charge (FC) = [Valence electrons (V) – Lone pair electrons (L) – Bonded pair electrons (B)/2]
In Boron Trifluoride molecule,
Formal charge of Boron = 3 – 0 – 6/2 = 3 – 0 – 3 = 0
Formal charge of Fluorine = 7 – 6 – 2/2 = 7 – 6 – 1 = 0
All the atoms in the boron trifluoride molecule are stable by forming covalent bonds and they do not possess any formal charge on them. We know that the lesser the value of formal charge more stable the Lewis structure of the given molecule is. Since our molecule boron trifluoride has no formal charge on any of the atoms, the above-mentioned structure is the best Lewis dot structure of BF3 molecule (with 3 covalent bonds between boron and three fluorine atoms.)
BF3 Molecular Geometry and Shape
The molecular geometry of a molecule means the three-dimensional arrangement of the atoms that constitute the molecule. The study of the molecular geometry of any molecule is important as it presents information on many physical and chemical properties of the compound like polarity, reactivity, phase of matter, color, magnetism, biological activities, and so on.
BF3 molecule has a trigonal planar shape. Boron trifluoride molecule has three B – F bonds on central atom Boron. This creates three regions of electron density around the central Boron atom. The charge distribution around the central atom is symmetrical and the three fluorine atoms lie in positions that make an equilateral triangle. This makes the shape of the boron trifluoride molecule trigonal planar.
We can also determine the geometry and shape of the BF3 molecule using the VSEPR theory. For this, we can use the AXN method.
AXN Notation for BF3 molecule:
- A represents the central atom, In BF3 molecule the central atom is Boron. So, A = Boron
- X represents the bonded atoms to the central atom. In BF3 molecule, 3 fluorine atoms are bonded to the central atom Boron. So, X = 3
- N represents lone pair of electrons on the central atom. The central atom Boron has 0 lone pair of electrons. So, N = 0
Now the AXN generic formula for BF3 becomes AX3N0 or AX3.
According to VSEPR theory, if the molecule has an AX3 formula then the molecule has trigonal planar molecular geometry and electron geometry.
Also Read:
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- I3- Lewis structure, Molecular geometry, Hybridization, Polarity
BF3 Hybridization
Boron Trifluoride (BF3) has sp2 hybridization as it has three sigma bonds (B–F). The steric number is 3 which indicates sp2 hybridization.
We can also calculate hybridization with the use of the following method:
- If the total number of valence electrons of the molecule is less than 8, divide it by 2.
- If the total number of valence electrons of the molecule is greater than 8 and less than 56, divide it by 8. Then divide the remainder by 2. Add the two quotients to get the number of orbitals.
- If the total number of valence electrons of the molecule is greater than 56, divide it by 18. Then divide the remainder by 2. Add the two quotients to get the number of orbitals.
| Total No. of Orbitals | Hybridization |
| 2
3 4 5 6 7 |
sp
sp2 sp3 sp3d sp3d2 sp3d3 |
Our molecule boron trifluoride (BF3) has a total of 24 valence electrons. So we will use the second step of the above-mentioned method.
Dividing 24 by 8 we get quotient 3 and reminder 0. Since we got reminder 0 we need not further divide by 2 and we can take quotient 3 as our required value of orbitals.
The number of orbitals is equal to 3. Hence, the hybridization in the boron trifluoride (BF3) molecule is sp2.
Bond Angle of BF3
Boron trifluoride has a trigonal planar molecular geometry with three electron-rich regions placed symmetrically around the central atom making an equilateral triangle. Due to the symmetrical placement of the Fluorine atoms around the central atom Boron, the bond angle of the BF3 molecule is 120°.
Is BF3 Polar or Nonpolar?
The Boron Trifluoride (BF3) molecule is nonpolar.
The polarity of a molecule is determined by its dipole moment. A dipole moment is given by the product of the magnitude of the charges and the distance between the center of positive and negative charges. It is denoted by µ. A dipole moment is generated when the molecule has an asymmetrical arrangement of the atoms surrounding the central atom. The Boron Trifluoride molecule has three electron-rich regions placed in a perfectly symmetrical arrangement with the central atom. This perfect placement of the surrounding atoms results in the happening of zero net dipoles.
Zero net dipole means a molecule will have an equal distribution of positive and negative charges. A molecule with zero net dipole moment is non–polar.
Electronegativity of BF3
The degree to which an atom attracts electrons in a chemical bond is described by electronegativity. The chemical bond is determined by the difference in electronegativity between two atoms.
- The bond will have an ionic character if the electronegativity difference is bigger than 1.7.
- The bond will have a polar covalent character if the electronegativity difference is between 0.4 and 1.7.
- The bond will have a non–polar covalent character if the electronegativity difference is smaller than 0.4.
In our molecule Boron Trifluoride, Boron has an electronegativity of 2.04 and Fluorine has an electronegativity of 3.98. The electronegativity difference between the two types of atoms is 2. This value implies that the bond will have an ionic character in the Boron Trifluoride molecule.
Summary
