Sulfur difluoride, represented by the formula SF2, is a highly volatile inorganic chemical compound. Having a molar mass of 70.062 g mol-1, it comprises two fluorine atoms linked with the central sulfur atom. It is a synthetic fiber and is used as an electrophilic warhead.
The biggest dilemma that chemists have about this compound is its chemical nature. Is it polar or nonpolar? To find that out, continue reading this article.
Is SF2 polar or nonpolar?
Sulfur difluoride (SF2) is a polar molecule.The central sulfur (S) atom in SF2 is surrounded by two fluorine (F) atoms forming a bent or V-shaped molecule.
A fluorine (F) atom is more electronegative than a sulfur (S) atom. Thus both S-F bonds are individually polar in the SF2 molecule and possess a specific dipole moment value.
The asymmetric bent shape of SF2 further enhances the polarity effect as the S-F dipole moments are not canceled in the molecule overall. Thus, SF2 is a polar molecule with a net dipole moment µ =0.52 D.
A molecule with unequal charge distribution between different centers of bonded atoms is a polar molecule.
It is formed by the covalent bond between two unlike atoms leading to an asymmetric electron density.
In this case, the atoms acquire partial negative (δ–) and partial positive (δ+) charges.
Three factors influence the polarity of any covalently bonded molecule, namely:
The electronegativity difference between two or more bonded atoms
Dipole moment
Molecular geometry or shape
How do oppositely charged poles develop in an SF2 molecule, and how do the three factors given above contribute to its polar nature? Let’s find out in a detailed discussion.
Factors affecting the polarity of SF2
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.
Fluorine (F) present in SF2 belongs to group VII-A of the Periodic Table. The electronic configuration of fluorine is 1s22s22p5. Thus each F-atom needs only 1 more electron in its valence shell in order to gain a stable octet electronic configuration.
On the other hand, sulfur (S) belongs to group VI-A of the Periodic Table. The electronic configuration of sulfur is 1s22s22p63s²3p⁴, so it has a total of 6 valence electrons.
The 2 F-atoms thus form a single covalent bond with the central S-atom on each side of the SF2 molecule. In this way, each F and S atom attains a stable octet configuration.
Two valence electrons consumed in bonding out of the six initially available leaves behind 4 valence electrons, i.e., 2 lone pairs on the central S-atom in SF2.
Atom
Electronic configuration
Valence electrons
Fluorine (9F)
1s2 2s2 2p5
7
Sulfur (16S)
1s2 2s2 2p6 3s² 3p⁴
6
According to the Pauling scale, a covalent chemical bond is polar if the electronegativity difference of bonded atoms ranges from 0.5 to 1.6 units.
Fluorine (E.N = 3.98) is a highly electronegative element. Each F-atom is more electronegative than a sulfur atom (E.N = 2.58). There is an electronegativity difference of 1.40 units between these two atoms.
So, the F-atoms strongly attract the shared electron cloud from each S-F bond in the SF2 molecule. The bonded electrons are held significantly close to the fluorine atoms in each S-F bond.
Each terminal F-atom thus gains a partial negative (Fδ-) charge, while the central S-atom gains a partial positive (Sδ+) charge. In this way, oppositely charged poles develop in the SF2 molecule. As a result, each S-F bond is strongly polar in SF2.
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.
By convention, the direction of a dipole moment is from the center of the positively charged pole to the negatively charged center. The greater the electronegativity difference between two bonded atoms, the higher the bond polarity, leading to a high dipole moment value.
Considering the electronegativity difference between S and F atoms, the dipole moment of individual S-F bond points from Sδ+ to Fδ-.
Molecular geometry
As discussed earlier, SF2 consists of two S-F single covalent bonds, and 2 lone pairs are present on the central S-atom (as shown below).
According to the Valence Electron Pair Repulsion (VSEPR) theory of chemical bonding, SF2 is an AX2E2-type molecule. Around the central sulfur atom (A), there are two bond pairs (X) and two lone pairs of electrons (E).
The presence of two lone pairs on the central S-atom leads to strong lone pair-lone pair and lone pair-bond pair electronic repulsions in the SF2 molecule. This strong repulsive effect distorts the regular symmetry of the molecule.
The sulfur difluoride molecule thus adopts an “asymmetric bent” shape in which the bond angle is also reduced to 98° from an ideal F-S-F bond angle of 109.5 ° in an AX4-type molecule.
The individual S-F dipole moments are not canceled in the asymmetric bent geometry of sulfur difluoride (SF2). There is a non-uniform distribution of the electron cloud over the molecule. Thus SF2 is polar (net µ = 0.52 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), Sulfur difluoride (SF2), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), sulfur trioxide (SO3), etc.
SF2 has polar bonds because of a definite electronegativity difference between bonded S and F atoms in each S-F bond.
Due to the asymmetric bent shape of SF2, the bond polarities do not get canceled in the molecule overall.
Thus, SF2 is a polar molecule with a net dipole moment value of 0.52 D.
Is there any formal charge on the bonded atoms in the SF2 molecule?
Formal charge on an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For sulfur atom
Valence electrons = 6
Bonding electrons = 4
Non-bonding electrons = 4
∴ Formal charge on the central sulfur atom = 6- 4- 4/2 = 6-4-2 = 6-6 = 0.
For fluorine atoms
Valence electrons = 7
Bonding electrons = 2
Non-bonding electrons = 6
∴ Formal charge on each F-atom = 7- 6- 2/2 = 7-6-1 = 7-7 = 0.
Thus, no formal charge is present on the sulfur difluoride molecule overall.
Why does SF2 have a smaller bond angle than H2O, although both have a bent shape?
The lower the electronegativity of the central atom, the smaller the bond angle will be. Sulfur (E.N = 2.58) is less electronegative than oxygen (E.N = 3.44).
Thus bond pairs in SF2 are more away from the central atom than in H2O, and thus the repulsive forces between bond pairs are smaller, producing a smaller bond angle (98° vs 104.5° ).
Why is SF6 non-polar while SF2 is a polar molecule?
The asymmetric bent shape of the SF2 molecule leads to an unequal charge distribution, and the dipole moments are not canceled. Thus, SF2 is a polar molecule.
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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