Selenium dioxide is a colorless solid represented by the chemical formula SeO2. It is an important selenium compound, mainly used as a colorant in the printing and glass industry. SeO2 is also used as an oxidizing agent in organic synthesis. The IUPAC name of SeO2 is selenium (IV) oxide.
SeO2 can be wrongly perceived as a non-polar molecule, but that is not the case. It is actually polar in nature. This article brings you an explanation of this very important question about the polarity of SeO2.
So, let’s start reading.
Is SeO2 polar or nonpolar?
Selenium dioxide (SeO2) is a polar molecule. The central selenium (Se) atom in the SeO2 molecule is surrounded by two oxygen (O) atoms via double bonds, forming a bent or V-shaped molecule.
The electronegativity of the oxygen (O) atom is greater than the selenium (Se) atom. Thus each Se=O bond in the SeO2 molecule is individually polar and thus possesses a specific dipole moment value.
The asymmetric bent shape of SeO2 further enhances the polarity effect as the Se=O dipole moments do not get canceled in the molecule overall. Thus, SeO2 is a polar molecule with a net dipole moment value > 0.
A molecule with unequal charge distribution between different centers of bonded atoms is a polar molecule.
It is usually formed via covalent bonding between two different atoms leading to an asymmetric electron density. In this case, the atoms acquire partial negative (δ–) and partial positive (δ+) charges.
If the dipole moments of individually polar bonds are not canceled due to the asymmetrical shape of the molecule, the molecule will be polar, such as SeO2.
Hence a polar molecule has a non-uniformly distributed electron cloud. Contrarily, if the electronic charge is evenly spread over the molecule, in that case, it will be a non-polar molecule overall.
Three factors that control the polarity of any covalently bonded molecule are:
Electronegativity.
Dipole moment.
Molecular geometry or shape.
In the next section, we will discuss how these factors control the polarity of the SeO2 molecule.
Factors affecting the polarity of SeO2
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.
Both oxygen (O) and selenium (Se) belong to group VI-A (or 16) of the Periodic Table. The electronic configuration of selenium is [Ar] 3d104s24p4, so it has a total of six valence electrons available for bonding.
On the other hand, the electronic configuration of an oxygen atom is 1s22s22p4. Thus, each O-atom also has six valence electrons and needs two more electrons in its valence shell to complete its octet. The two O-atoms thus form a double covalent bond with the central Se-atom on each side of the SeO2 molecule.
Four valence electrons consumed in bonding out of the six initially available leaves behind two valence electrons, i.e., one lone pair on the central Se-atom in the SeO2 molecule.
Atom
Electronic configuration
Valence electrons
Oxygen (8O)
1s2 2s2 2p4
6
Selenium (34Se)
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4
6
Oxygen is a highly electronegative element (E.N= 3.44). It is more electronegative than selenium (E.N = 2.55). There is an electronegativity difference of 0.89 units between these two atoms.
Due to this electronegativity difference, oxygen strongly attracts the shared electron cloud from both the Se=O bonds in the SeO2 molecule. The bonded electrons are held significantly close to the oxygen atoms in each Se=O bond.
The central Se-atom thus gains a partial positive (Seδ+) charge, while the oxygen atoms having greater electronegativity obtain partial negative (Oδ-) charges. Thus, oppositely charged poles develop in the SeO2 molecule. That is why bonds present in SeO2 are 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.
It is noted that a molecule’s dipole moment depends on the electronegativity difference 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.
Thus the dipole moment of each Se=O bond in the SeO2 molecule points from Seδ+ to Oδ- due to the electronegativity difference between the Se and O atoms.
Thus, each Se=O bond in the SeO2 molecule is polar, with a net dipole moment of 2.62 D.
Molecular geometry
As discussed earlier, a selenium dioxide (SeO2) molecule consists of two double covalent Se=O bonds. There are a total of 18 valence electrons in the SeO2 molecule. One lone pair of electrons is present on the central selenium (Se) atom.
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory of chemical bonding, SeO2 is an AX2E1-type molecule. Around the central selenium atom (A), there are two bond pairs (X) and only one lone pair of electrons (E).
The presence of a lone pair of electrons on the central Se-atom leads to strong lone pair-bond pair electronic repulsions in the SeO2 molecule. These strong repulsions contribute to the distortion in the regular symmetry of the molecule.
The selenium dioxide (SeO2) molecule thus adopts an “asymmetrical bent” shape.
As a result of the bent shape, the individual Se=O dipole moments do not get canceled in the selenium dioxide (SeO2) molecule. Therefore is an unequal distribution of the electronic charge over the molecule.
Thus, SeO2 is an overall polar molecule with a net dipole moment µ = 2.62 D.
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), Selenium dioxide (SeO2), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), sulfur trioxide (SO3), etc.
SeO2 has polar bonds because of a definite electronegativity difference between bonded Se and O atoms in each Se=O bond.
Due to its asymmetric bent or V shape, the dipole moments do not get canceled in the molecule overall.
Thus, SeO2 is a polar molecule with a net dipole moment µ = 2.62 D.
Why is SeO2 polar while CO2 is a non-polar molecule?
The asymmetric bent shape of the SeO2 molecule leads to an unequal charge distribution, and the dipole moments do not get canceled. Thus, SeO2 is a polar molecule.
Due to the symmetric linear shape of CO2, the individual C=O dipole moments get canceled in opposite directions. Thus, CO2 is a non-polar molecule.
How do we compare the polarity of SeO2 with SO2?
Both SeO2 and SO2 are polar molecules.
They have comparable symmetry, shape, and, thus, polarity. In both cases, the central atom (Se and S) belongs to the same group, and it is bonded to two O-atoms via double covalent bonds, one on each side.
A lone pair of electrons on the central atom leads to an asymmetric bent shape of the molecule.
Thus, the dipole moments of individually polar Se=O or S=O bonds do not get canceled, which leads to an overall molecular polarity.
SeO2, however, has a higher dipole moment (net µ = 2.62 D) as compared to that of SO2 (net µ = 1.62 D).
Is there a formal charge on the bonded atoms in the SeO2 molecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For oxygen atoms
Valence electrons = 6
Bonding electrons = 4
Non-bonding electrons = 4
∴ The formal charge on each O-atom = 6-4-4/2 = 6-4-2= 6-6 = 0
For selenium atom
Valence electrons = 6
Bonding electrons = 8
Non-bonding electrons = 2
∴ The formal charge on the central selenium atom = 6-2-8/2 = 6-2-4 = 6-6 = 0
Thus, no formal charge is present on the selenium dioxide (SeO2) molecule overall.
Summary
Selenium dioxide (SeO2) is a polar molecule.
It consists of two polar Se=O bonds due to an electronegativity difference of 0.89 units between the bonded Se and O atoms.
Oxygen attracts the shared electron cloud strongly from each Se=O bond.
The asymmetric bent shape of the SeO2 molecule leads to a non-uniform charge distribution in the molecule.
Due to the unsymmetrical arrangement of atoms, the individual Se=O dipole moments are not canceled in the molecule overall. Thus, SeO2 is polar (net μ= 2.62 D).
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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