SIF4 is a covalent compound, which consists of silicon and fluorine atoms. It is named tetrafluorosilane or silicon tetrafluoride.
The melting and boiling point of silicon tetrafluoride is -95.0 °C and -90.3 °C and hence, it exists as a gas at room temperature.
Silicon tetrafluoride is a colorless, toxic, corrosive, and non-flammable gas with a pungent smell. Silicon tetrafluoride is prepared in the laboratory by decomposition of BaSiF6 at a temperature greater than 300 °C.
BaSiF6 —–> SiF4 (volatile) + BaF2 (residue)
The volatile nature of silicon tetrafluoride limits their use and hence, it is restrained to only organic synthesis and microelectronic.
The density of SiF4 is 1.66 g/cm3, which makes it heavier than air. Its molar mass is 104 g/mol.
In this article, we are going to discuss the chemical bonding of silicon tetrafluoride by understanding its Lewis structure, molecular geometry, and the hybridization of the central atom. Then, we will study the polarity of SiF4 i.e., whether SiF4 is a polar or nonpolar molecule.
SiF4 Lewis Structure
First of all, there is a need to understand the Lewis structure of silicon tetrafluoride for studying its chemical bonding.
The Lewis structure refers to the two-dimensional representation of the molecule with valence electrons surrounding the atoms in the molecule. These valence electrons or outermost shell electrons of the atoms are represented as dots and hence, it is also known as electron dot structure.
Let us draw the Lewis structure of silicon tetrafluoride.
Step 1: Find out the total number of valence electrons in silicon tetrafluoride.
Silicon tetrafluoride contains one silicon atom and four fluorine atoms. The silicon and fluorine atoms belong to group 14, and group 17, respectively. Hence, the silicon atom has four valence electrons whereas the fluorine atom has seven valence electrons.
Therefore, silicon tetrafluoride consists of 4 + (7*4) = 32 valence electrons in total.
Step 2: Find out the central atom in silicon tetrafluoride.
Out of silicon and fluorine atoms, silicon is less electronegative than fluorine, which makes it a central atom in silicon tetrafluoride. The four fluorine atoms will surround the silicon atom.
Step 3: Organization of all valence electrons in silicon tetrafluoride by following the octet rule.
Octet Rule: Every element in the periodic table (except Hydrogen and Helium atom) tend to have a noble gas configuration during the formation of a chemical bond with another atom.
Here, we have arranged the 32 valence electrons in silicon tetrafluoride, as shown in the following diagram:
We can observe that both silicon and fluorine have completed their octet i.e., they have eight electrons in their outermost shell.
Now, let us calculate the formal charge on the silicon and fluorine atoms in SiF4, which is defined as:
Formal charge = Number of valence electrons − 1/2(bonding electrons) – nonbonding electrons
The formal charge on the Silicon atom, Si = 4 −12(8) − 0 = 0
The formal charge on the Fluorine atom, F = 7 −12(2) − 6 = 0
Therefore, the silicon and fluorine atoms do not carry any charge in the Lewis structure of silicon tetrafluoride. Two electrons shared between the silicon and fluorine atom form a single bond in silicon tetrafluoride. Hence, the most suitable Lewis structure of silicon fluoride in bond notation form would be:
The Lewis structure of Silicon tetrafluoride gives us information that the silicon atom forms four single bonds with four fluorine atoms.
The Lewis structure is the foundation of the chemical bonding of any molecule. With the help of Lewis structure and Valence shell electron pair repulsion (VSEPR) theory, we can anticipate the molecular geometry of the silicon tetrafluoride.
SiF4 Molecular Geometry
For shedding more light on the chemical bonding of nitryl fluoride, there is a need to understand the molecular geometry and hybridization of the silicon atom in silicon tetrafluoride.
The molecular geometry or shape is a three-dimensional representation of the molecule. It is determined by VSEPR theory with the help of the Lewis structure.
According to VSEPR theory, the shape of the molecule depends on repulsion between the valence shell electrons, which may be bonding or non-bonding electrons.
In silicon tetrafluoride, the silicon atom is the central atom and it forms four sigma bonds with four fluorine atoms, as observed from its Lewis structure. The silicon atom has no lone pair of electrons and thus, its molecular geometry or shape can be predicted from the following table:
|Number of bond pairs
In the above-mentioned table, A is the central atom and X is the surrounding atom, which is bonded to the central atom. Here, silicon tetrafluoride molecule well matches with AX4 species and it will have ideal tetrahedral geometry as all X atoms are the same i.e., the fluorine atom.
In the tetrahedral geometry of silicon tetrafluoride, two Si-F bonds are in a plane, one Si-F bond is above the plane and another Si-F bond is present below the plane.
The tetrahedral geometry of silicon tetrafluoride leads to the F-Si-F bond angle of 109.5 °. The bond length of the Si-F bond is 154 pm.
Let us move towards the learning of the hybridization of the silicon atom in silicon tetrafluoride.
For understanding the hybridization of the silicon atom in silicon tetrafluoride, we should be aware of the Valence Band Theory (VBT).
This theory gives information regarding the hybridization of the central atom in the molecule. According to VBT, the hybrid orbitals of the central atom, formed by combining atomic orbitals of similar energy, overlap with the atomic orbitals of another atom. The overlapping of the orbitals corresponds to the formation of the covalent bond.
Let us determine the hybridization of the silicon atom, Si, in the SiF4 molecule by VBT.
The ground state electronic configuration of the silicon atom will be [Ne] 3s23p2. There are two unpaired electrons in the ground state. But, the silicon atom forms four sigma bonds as per the Lewis structure of silicon tetrafluoride and VSEPR theory.
Hence, one of the electrons from the 3s orbital excite to the 3p orbital of the silicon atom and provide four unpaired electrons for the formation of four sigma bonds with four fluorine atoms. It leads to the excited state configuration of silicon atom as [Ne] 3s13p3.
Now, one 3s orbital fuses with three 3p orbitals of the silicon atom and provide four sp3 hybrid orbitals of equivalent energy, as shown in the following figure.
These four sp3 hybrid orbitals arrange themselves in the tetrahedral geometry with a bond angle of 109.5°. The four sp3 hybrid orbitals will overlap with 2p orbitals of four fluorine atoms and form four Si F covalent bonds.
Hence, the hybridization of the silicon atom in silicon tetrafluoride is sp3 hybridization with tetrahedral geometry.
The hybridization of the silicon atom and the shape of the silicon tetrafluoride can also be determined from the steric number. The steric number is defined as:
steric number = number of atoms bonded to the central atom + number of lone pairs attached to the central atom
In SiF4, four fluorine atoms are bonded to the silicon atom and no lone pair is attached to the silicon atom.
Therefore, the steric number of silicon tetrafluoride is four, indicating the tetrahedral geometry of SiF4 and sp3 hybridization of the silicon atom in it.
Now, let us understand the concept of polarity in silicon tetrafluoride.
In silicon tetrafluoride, four fluorine atoms are bonded to the central silicon atom. According to the Pauling scale, the electronegativity values of the silicon atom and the fluorine atoms are 1.90 and 3.98, respectively. The electronegativity difference for the Si-F bond is 3.98 – 1.90 = 2.08.
The high electronegativity difference indicates that the electron pair is strongly attracted towards the fluorine atom. Hence, the Si-F bond is highly polar and acts as a dipole with a partial negative charge and a partial positive charge on the fluorine and silicon atom, respectively,
According to VBT and VSEPR theory, the silicon tetrafluoride has tetrahedral molecular geometry. The tetrahedral geometry is symmetrical and hence, polarities of the Si-F bond cancel each other. It leads to zero net dipole moment of the silicon tetrafluoride.
Therefore, silicon tetrafluoride is a nonpolar molecule in spite of the highly polar nature of the Si F bond.
Here, we have learned the chemical bonding of the silicon tetrafluoride or tetrafluorosilane.
In brief, the silicon atom is the central atom in silicon tetrafluoride and forms four sigma bonds with four fluorine atoms. The silicon tetrafluoride has tetrahedral molecular geometry and it shows sp3 hybridization at the central silicon atom. The symmetrical nature of molecular geometry makes the silicon tetrafluoride the nonpolar molecule.