Sulfur tetrafluoride (SF4) Lewis dot structure, molecular geometry or shape, electron geometry, bond angle, formal charge
SF4 is the chemical formula for sulfur tetrafluoride, a colorless gas with a distinct rotten-egg-like odor. Its molar mass is 108.4 g/mol thus it is heavier than air.
Sulfur tetrafluoride is a highly toxic gas that can have adverse effects on human skin and eyes if handled inappropriately in the chemistry laboratory.
In this article, we have discussed some important chemical properties of SF4 such as its Lewis structure, molecular geometry or shape, electron geometry, bond angle, hybridization, formal charge, etc.
The Lewis structure of sulfur tetrafluoride (SF4) consists of one sulfur (S) and four fluorine (F) atoms. There are a total of five electron pairs present on the central S atom. 4 out of these 5 electron pairs are bond pairs while there is 1 lone pair in lewis’s structure of SF4.
Sulfur (S) occupies a central position in the Lewis structure of SF4, it is surrounded by 4 fluorine (F) atoms at the sides.
You can easily draw the Lewis structure of SF4 by following the simple guidelines given below.
Steps for drawing the Lewis dot structure of SF4
1. Count the total valence electrons in SF4
The Lewis structure of a chemical molecule is a simplified representation of all the valence electrons present in it. So, the first step in drawing the Lewis structure of SF4 is to determine the total valence electrons present in it.
An easy way to do that is by making use of the Periodic Table of elements. Find from the Periodic Table, in which groups are your concerned elements (in this case S and F) placed.
Sulfur (S) is situated in group 6 A of the Periodic Table, so it has a total of 6 valence electrons. Similarly, Fluorine (F) is placed in group 7 A of the Periodic Table, so it has 7 valence electrons.
SF4 has one sulfur and four fluorine atoms. So, the total valence electrons available for the SF4 Lewis structure = 6+4(7)= 34 valence electrons.
2. Find the least electronegative atom and place it at the center
Electronegativity is the ability of an atom to attract a shared pair of electrons from a chemical bond.
So, the atom which is least electronegative or most electropositive is placed at the center of a molecule’s Lewis structure.
This is because it is most likely to share its electrons with the more electronegative atoms surrounding it.
Thus, we will place an S atom at the center of the Lewis diagram while all the four F atoms will be placed in their surroundings, as shown below.
3. Connect outer atoms with the central atom
Now, join all outer atoms with the central atom using single bonds.
In the SF4 molecule, the fluorine (F) atoms act as the outer atoms while sulfur (S) is the central atom. So, all the four F atoms will be joined to an S in the center, as shown in the figure below.
If we count the number of valence electrons used in the structure above, there are four single bonds and every single bond represents an electron pair i.e., 2 electrons. So, in the figure above, (4 x 2)= 8 valence electrons are used for drawing the Lewis structure.
Total valence electrons available – electrons used in bonding = 34-8 = 26 valence electrons.
So, we still have 26 valence electrons to be placed in the Lewis structure of SF4.
4. Complete the octet of outer atoms
Fluorine atoms are the outer atoms in SF4. Each F atom needs a total of 8 electrons in the valence shell to complete its octet electronic configuration.
Each F already contains 2 electrons, if we place 3 electron pairs (6 more electrons) in its surroundings, F will achieve a complete octet, as shown in the figure below.
5. Complete the octet of the central atom
Total electrons used till step 4 = Four single bonds + 4 (electrons placed around each F atom as dots) = 8 + 4(6)= 32 valence electrons.
We are still left with 34-32 = 2 valence electrons, so we will place these 2 electrons as a lone pair on the central S atom.
In this way, sulfur (S) has a total of 10 electrons in its valence shell.
This situation falls under the expanded octet concept. Atoms such as sulfur, phosphorus, chlorine, and silicon can accommodate more than 8 electrons during chemical bonding.
Sulfur has its valence electrons in the 3rd energy level, so the incoming electrons also have access to the 3d subshell. So, after completely filling 3p orbitals, the excess electrons are placed in 3d.
Now we have used all the 34 valence electrons available for drawing the Lewis structure of SF4.
So, finally, we just need to check the stability of this Lewis structure. We can do that by checking whether there is a formal charge present on SF4 or not.
6. Check the stability of Lewis’s structure with the help of the formal charge concept
The less the formal charge on the atoms of a molecule, the better the stability of its Lewis structure.
The formal charge can be calculated using the formula given below.
Hence in the above Lewis structure of SF4, all the atoms have a zero formal charge. This makes sure that this Lewis structure is stable, and that we have drawn it correctly.
What are the electron and molecular geometry of SF4?
The sulfur tetrafluoride (SF4) molecule has an asymmetric seesaw shape or molecular geometry. The ideal electronic geometry of SF4 is trigonal bipyramidal but it adopts a different molecular geometry due to the presence of a lone pair on the central sulfur (S) atom.
Molecular geometry of SF4
The SF4 molecule has an irregular molecular geometry or shape known as a seesaw. In the seesaw shape, S lies at the center while lone-pair bond pair repulsions push the F atoms away to occupy the axial and equatorial positions.
Lone pair-bond pair repulsions are significantly greater than bond pair-bond pair repulsions. As a result, it occupies a seesaw shape. The word ‘seesaw’ comes from the observation that this shape resembles the playground seesaw swing.
It should be noted that the molecular geometry or shape is determined based on the distinction of bond pairs and lone electron pairs at different positions on the molecule. On the other hand, the ideal electronic geometry depends on the total electron pairs present around the central atom.
Thus, while determining the molecular geometry or shape of SF4 we seriously take into account the repulsive effect of the lone pair present on the central S atom, but this repulsive effect can be ignored while considering the ideal electronic geometry of SF4.
Electron geometry of SF4
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electronic geometry of a molecule that has a total of 5 electron pairs (i.e., 5 regions of electron density) around the central atom is trigonal bipyramidal. The trigonal bipyramidal molecule has a triangular base and two pyramids, one on each side.
A region of electron density refers to the group of bonding and non-bonding electrons present around the central atom in a molecule.As there are 5 electron density regions around S in the SF4 molecule so its electronic geometry is trigonal bipyramidal.
A simpler way of finding the electron and the molecular geometry of SF4 is the AXN method.
AXN is a simple formula that represents the number of bonded atoms and lone pairs present on the central atom to predict the shape or geometry of the molecule using the VSEPR concept.
AXN notation for SF4 molecule
A in the AXN formula represents the central atom. In the SF4 molecule, sulfur is present at the center so A=S.
X denotes the atoms bonded to the central atom. In SF4, four Fluorine (F) atoms are bonded to the central S so X=4.
N stands for the lone pairs present on the central atom. As per the Lewis structure of the SF4 molecule, there is 1 lone pair on S so N=1.
So, the AXN generic formula for the SF4 molecule is AX4N1.
Now, you may have a look at the VSEPR chart below.
According to this chart, a molecule with an AX4N1 generic formula has a seesaw shape while its electronic geometry is trigonal bipyramidal.
Hybridization of SF4
The central sulfur in the SF4 molecule is sp3d hybridized.
The electronic configuration of sulfur (S) is 1s22s22p63s23p4.
One 3s electron of sulfur shifts to the empty 3d orbital. The 3s orbital then hybridizes with three 3p orbitals and one 3d orbital to produce five sp3d hybrid orbitals. These hybrid orbitals are not all equivalent.
There is one set of 2 equivalent orbitals and another second set of 3 equivalent hybrid orbitals. One sp3d has two paired electrons which are situated as a lone pair on S while the other four sp3d hybrid orbitals have 1 electron each to form single covalent bonds with F atoms.
The electronic configuration of Fluorine (F) is 1s22s22p5. Fluorine uses its unpaired p orbital electron to form an S-F sigma bond with sulfur by sp3d-p overlap.
A shortcut to finding the hybridization present in a molecule is by using its steric number against the table given below. The steric number of central S in SF4 is 5 so it has sp3d hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The bond angles of SF4
Different bond angles are present in the SF4 molecule. SF4 has an asymmetric shape. Lone pair-bond pair and bond pair-bond pair repulsions lead to different bond angles. Lone pair-lone pair repulsions are stronger than bond pair-bond pair repulsions.
So, the axial fluorine atoms are more pushed away from the central S atom as compared to the equatorial fluorine atoms. So, the axial F-S-F bond angle is 187° while the equatorial F-S-F bond angle is 101.6°.
SF4 has polar bonds present because of an electronegativity difference between bonded S (E.N =2.58) and F (E.N = 3.98) atoms.
An electronegativity difference of 3.98-2.58 = 1.4 units > 0.5 units exist so each S-F bond is polar according to Pauling’s scale and it has a specific dipole moment value.
It is due to the asymmetric seesaw shape of SF4 that the bond polarities do not get canceled in the molecule overall.
Rather, the electronic cloud stays non-uniformly distributed in the molecule. Thus, SF4 is a polar molecule overall with a net dipole moment > 0.
How many lone pairs are present in the Lewis structure of SF4?
A total of 13 lone pairs are present in the Lewis structure of SF4.
The lone pairs represent unbonded electrons present in a molecule’s Lewis structure. They are shown as dots in the Lewis structure of SF4. Each dot represents a single electron.
There are 26 dots in total. 2 dots together are considered a lone pair. So, 26 dots = 26 unbonded electrons = 26/2 = 13 lone pairs.
How many bond pairs and lone pairs are present around the central atom in the Lewis structure of SF4?
Sulfur (S) acts as the central atom in the Lewis structure of SF4. Every single bond around S represents a bond pair i.e., 2 electrons while the dots denote a lone pair.
There are four single bonds and 2 dots in the Lewis structure of SF4.So, there are 4 bond pairs and 1 lone pair around the central atom in SF4.
How do you memorize the electron and molecular geometry of SF4?
The Lewis structure of SF4 can be used to determine its electron and molecular geometries via the AXN formula. There are four single bonds around the central S atom in SF4 so X=4. There is one lone pair (shown as 2 dots) so N=1.
Thus, the AXN formula for SF4 is AX4N1. Now you can use the VSEPR chart (given in this article) to find the electron and molecular geometries of SF4 i.e., trigonal bipyramidal and seesaw respectively.
Why is the shape or molecular geometry of SF4 different from its electron geometry?
There are five electron density regions around the central S atom in SF4, so its electron geometry is trigonal bipyramidal. But all the electron pairs are not bond pairs. There are four bond pairs and also a lone pair around S.
The presence of this lone pair distorts the ideal shape of the SF4 molecule. It leads to lone pair-bond pair repulsions in addition to the bond-pair bond pair repulsions already present. As a result, the molecule adopts a seesaw or irregular tetrahedron shape and molecular geometry.
The total valence electrons available for drawing the Lewis structure of sulfur tetrafluoride (SF4) is 34.
SF4 has a seesaw shape and molecular geometry.
The electron geometry of SF4 is trigonal bipyramidal.
In the Lewis dot structure of SF4, there are a total of 4 bond pairs and 1 lone pair around the central sulfur atom.
The central S atom in SF4 is sp3d hybridized.
The F-S-F bond angles in the SF4 molecule are 101.6° and 187° respectively.
SF4 is a polar molecule (net μ= 0.632 D). The dipole moments of individual S-F bonds do not get canceled in the overall structure of SF4 due to the molecule’s asymmetric shape.
The overall formal charge on SF4 is zero so it is a stable 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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5. Complete the octet of the central atom
