Carbon tetrabromide is a colorless, monoclinic crystalline solid represented by the chemical formula CBr4. The vapors of this non-flammable compound are highly narcotic in excess concentration.
The IUPAC name of CBr4 is tetrabromomethane. CBr4 is extensively used as a laboratory solvent and has a potential role as a sedative.
So what is the polarity of this highly narcotic compound (CBr4)? Is it a polar or a non-polar molecule? Let’s find out in this article.
Is CBr4 polar or non-polar?
Carbon tetrabromide (CBr4) is a non-polar molecule. The central carbon (C) atom in the CBr4molecule is surrounded by four bromine (Br) atoms via single covalent bonds, forming a symmetric tetrahedral molecule.
The electronegativity of the bromine (Br) atom is slightly more than the carbon (C) atom. The higher electronegative Br atom attracts the shared electron clouds with more influence in the CBr4 molecule.
Thus, all the bonds are individually polar and possess a specific dipole moment value.
However, the individual C-Br dipole moments get canceled in the symmetric tetrahedral shape of the CBr4 molecule. Thus, CBr4 is a non-polar molecule with a net dipole moment equal to 0.
A molecule with an equal charge distribution between different centres of bonded atoms is a non-polar molecule.
It is formed by the covalent bond between two same atoms leading to a symmetric electron density in the molecule overall.
In the case of symmetric molecules, If the dipole moments of individually polar bonds are cancelled, the molecule will be non-polar such as CBr4.
Hence a non-polar molecule has an equal distribution of the electronic charge. Contrarily, if the electronic charge is not evenly distributed over the molecule, in that case, it will be a polar molecule.
The following three factors influence the polarity of any covalent molecule:
Electronegativity.
Dipole moment.
Molecular geometry or shape.
In the next section, we will discuss how these factors lead to the non-polar nature of the CBr4 molecule.
Factors affecting the polarity of CBr4
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 the bonded atoms in a molecule, the higher the bond polarity.
Carbon belongs to group IV-A (or 14) of the Periodic Table. The electronic configuration of carbon is 1s2 2s2 2p2, so it has 4 valence electrons available for bonding.
On the other hand, bromine belongs to group VII-A (or 17) of the Periodic Table. The electronic configuration of bromine is 1s22s22p63s23d104s24p5, indicating the presence of 7 valence electrons.
The four Br-atoms thus form a single covalent bond with the central C-atom on each side of the CBr4 molecule.
Atom
Electronic configuration
Valence electrons
Carbon (6C)
1s22s22p2
4
Bromine (35Br)
1s22s22p63s23d104s24p5
7
Bromine (E.N = 2.96) is slightly more electronegative than carbon (E.N = 2.55). An electronegativity difference of only 0.41 units is present between these two bonded atoms.
Due to this electronegativity difference, bromine attracts the shared electron cloud from the C-Br bond with more influence in the CBr4 molecule. The bonded electrons are held slightly close to the bromine.
The Br-atom thus gains a partial negative (Brδ-) charge, while the carbon atom, less electronegative, obtains a partial positive (Cδ+) charge. In this way, oppositely charged poles develop in the CBr4 molecule.
Consequently, each C-Br bond in the CBr4 molecule is polar.
Dipole Moment
The dipole moment is the product of electrical charge (Q) and bond length (r) between two bonded atoms. It is a vector quantity expressed in Debye (D) units.
It is represented by a Greek symbol µ and measures the polarity of a bond.
The dipole moment of any molecule depends on the difference in electronegativity between the bonded atoms. The greater the electronegativity difference, the higher the bond polarity, resulting in a high dipole moment value.
It points from the partial positive (δ+) center to the partial negative (δ–) center of a bond or molecule.
The difference in electronegativity between the bonded atoms in the CBr4 molecule leads to dipoles from Cδ+ to Brδ-.
Thus, each bond in the CBr4 molecule is polar and has a dipole moment value.
Molecular geometry
As discussed earlier, a carbon tetrabromide (CBr4) molecule consists of four single C-Br covalent bonds. There are a total of 32 valence electrons in the overall molecule. Each bromine atom has three lone pairs of electrons.
According to the Valence Shell Electron Pair Repulsion Theory (VSEPR) theory of chemical bonding, CBr4 is an AX4-type molecule. Around the central carbon atom (A) are four bond pairs of electrons (X).
To minimize the electronic repulsions between the atoms, the carbon tetrabromide (CBr4) molecule adopts a symmetric tetrahedral geometry with a Br-C-Br bond angle of 109.5°.
As a result of the symmetric arrangement of atoms around the central carbon, the individual dipole moments of the bonds get canceled in the carbon tetrabromide (CBr4) molecule. There is an equal distribution of charge over the molecule.
In conclusion, CBr4is a non-polar molecule with a net dipole moment value equal to zero.
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), bromine pentafluoride (BrF5), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), carbon tetrabromide (CBr4), etc.
Although the individual C-Br bonds in the carbon tetrabromide (CBr4) molecule are polar, the overall molecule is non-polar.
Why is carbon tetrabromide (CBr4) non-polar?
The individual C-Br bonds in the CBr4 molecule are polar due to an electronegativity difference between Br and C atoms.
But the molecule (CBr4) overall is non-polar because it has a symmetric tetrahedral geometry.
The individual dipole moments get canceled in opposite directions. So, the net dipole moment µ becomes zero in the molecule.
Thus, CBr4 is a non-polar molecule with a zero net dipole moment.
Compare the polarity of CBr4 and CHBr3 molecules.
CBr4 is a non-polar molecule because it has a symmetric tetrahedral geometry, and the individual dipole moments get canceled in opposite directions. So, net µ becomes zero in the molecule.
Due to the unsymmetrical arrangement of atoms in the tetrahedral CHBr3 molecule, the individual dipole moments of the bonds are not canceled in the molecule overall. Thus, CHBr3 is a polar molecule
Thus, CHBr3 is polar while CBr4 is a non-polar molecule.
Is there a formal charge on the bonded atoms in the CBr4 molecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For carbon atom
Valence electrons = 4
Bonding electrons = 8
Non-bonding electrons = 0
∴ The formal charge on the central carbon atom = 4-0-8/2 = 4-4 = 0
For bromine atoms
Valence electrons = 7
Bonding electrons = 2
Non-bonding electrons = 6
∴ The formal charge on the bromine atoms = 7-6-2/2 = 1-1 = 0
Thus, no formal charge is present on the carbon tetrabromide (CBr4) molecule.
Summary
Carbon tetrabromide/tetrabromomethane (CBr4) is a non-polar molecule.
The individual C-Br bonds are polar due to an electronegativity difference of 0.41 units between the bonded atoms.
The difference in electronegativity between the bonded atoms leads to the formation of dipoles pointing from Cδ+ to Brδ-.
Due to the symmetric tetrahedral molecular geometry of CBr4, the individual C-Br dipole moments get canceled in opposite directions.
In conclusion, CBr4 is a non-polar molecule with net µ =0.
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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