Tetrabromosilane, or silicon tetrabromide, is an inorganic molecule with one Si atom and four Br atoms bound together by covalent bonds.
At high temperatures (T>600°C), it is made from silicon and bromine vapours.
SiBr4 is a colourless liquid with a strong stench. As indicated in the equation, its capacity to hydrolyze and emit HBr gas (an disagreeable odour) is responsible for its distinctive odour.
SiBr4 + 2H2O → SiO2 + 4 HBr
The density is 2.79 g cm-3, and the molar mass is 347.701 grammes per mole. 15°C and 153°C are the melting and boiling points, respectively.
It’s a reactive substance. It’s utilised to make Si and its compounds, such as SnO2 whiskers and Si3N4. Si3N4 is also used to make coatings, ceramics, and cutting tools, among other things.
Because it is caustic and irritating, caution should be exercised when using it.
In this post, we’ll take a closer look at some of SiBr4’s features using a variety of concepts.
First, we’ll learn about bonding using the Lewis concept, then we’ll look at molecular geometry using the VSEPR theory, and finally we’ll talk about hybridization and polarity in SiBr4.
Lewis Structure of SiBr4
The octet rule states that main group elements prefer to have eight electrons in their outermost shell. The element is deemed unstable if the number of electrons is greater than or equal to eight.
These unstable atoms create bonds to obtain stability by either transferring or sharing electrons.
In main group elements, the completion of the octet rule is the driving factor for bond formation.
Although the Lewis structure may not totally describe the molecule, it does provide useful information about its creation and properties.
It follows the octet rule and provides a 2-D diagrammatic representation of bonding through the arrangement of valence electrons.
The Lewis structure is drawn using electron-dot notation. Electrons are shown as dots surrounding the element’s symbol in this diagram.
Drawing the Lewis structure of SiBr4 in steps
Step 1: To calculate the total valence electron count, count the number of valence shell electrons on each atom of the molecule.
SiBr4 is made up of two elements: Si and Br.
Si has the atomic number 14 and is a member of group 14.
The valence electron for group 14 is 4. In addition, Si’s electrical configuration is 1s22s22p63s23p2. The valence shell has four electrons, two in the 3s subshell and two in the 3p subshell.
This diagram depicts the Lewis dot structure for Si.
Br is a member of group 17 and has an atomic number of 35. The valence electron for group 17 is 7. In addition, Br’s electronic configuration is
1s22s22p63s23p63d104s24p5. The valence shell has seven electrons, two in the 4s subshell and five in the 4p subshell.
Bromine’s Lewis dot structure is depicted.
Total valence electrons = 1(valence shell electron in Si) + 4(valence shell electron in Br) = (14) + (47) = 4+28= 32
Step 2: Pick the most central atom.
The centre atom, Si, is chosen to provide stability to the molecule and to allow for a better distribution of electron density.
The electron density of the core atom is meant to be shared with the surrounding atoms. As a result, a more electronegative atom (in this example, Br) is unsuitable for usage as a central atom since it tends to attract electrons to itself.
Step 3: Sketch down a skeleton diagram.
Make a skeletal diagram with the core atom and the atoms around it in mind.
Step 4: Using electron-dot notation, place the valence electrons around each atom.
The electrons in the entire valence shell are ordered according to bond formation.
Step 5: Form bonds to complete the octet of atoms. Two electrons make up a single bond.
There are seven valence electrons in each Br atom. To complete the octet, it shares one electron with Si.
The valence electrons in Si are four. To complete the octet, it shares one electron with all four Br atoms.
Step 6: Charge Formally
A net charge is present on the entire molecule for polyatomic ions.
The net charge of SiBr4 is zero.
If a molecule is neutral as a whole, that does not imply that all of its atoms are neutral. It is preferable to give each constituent a formal charge.
On Si, the formal charge is 4-0- (0.5*8) = 0.
Br’s formal charge is 7-6-(0.5*2) =0.
All atoms have the same formal charge, which is zero. As a result, the lewis structure created in step 5 is correct.
Check out this movie for a visual representation of SiBr4’s Lewis structure.
Geometry of SiBr4
The arrangement of all the atoms and bonds in a molecule is described by molecular geometry.
The Lewis Structure has a number of flaws, one of which is that it cannot account for the geometry of all molecules.
This flaw is overcome by the VSEPR hypothesis (Valence Shell electron pair repulsion theory). The VSEPR theory states that
• Because the valence electron pairs resist each other, the system becomes unstable.
• To make the electron configuration stable, the repulsions between them must be reduced.
• As a result, electrons align themselves with the least amount of repulsion and the greatest distance between them.
• The molecule geometry is determined by the stable arrangement of atoms’ valence electron pairs.
Electrons are subatomic particles with a negative charge.
Bonding pair of electrons (bp) are valence shell electrons that are involved in bonding, while lone pairs of electrons are valence shell electrons that are not involved in bonding (lp).
The order of repulsion between lone pairs and bond pairs of electrons is lp – lp > lp – bp > bp –bp.
The secret of using VSEPR to predict geometry
• Count the number of valence shell electrons (N) in a molecule (as determined in step 1 of Lewis’ structure drawing).
The total number of valence shell electrons in SiBr4 is 32.
• Divide total valence shell electrons by 8 if N is greater than 8, otherwise divide by 2.
We split 32 by 8 for SiBr4 since 32>8. The result is a quotient of four.
• If it is divisible by 8 exactly and the quotient equals the number of surrounding atoms, it means the molecule has no lone pairs.
The form of the molecule is the same as the molecular geometry in this example.
This means there are no lone pairs on the core atom of SiBr4.
• If the number of surrounding atoms is not divisible by 8 or the quotient is greater than the number of central atoms, lone pairs are present on the central atom, and the geometry is not the same as the shape.
• If N is not exactly divisible by 8, divide the remainder of (N/8) by 2 or subtract the number of surrounding atoms from the quotient if quotient> surrounding atoms to find the number of lone pairs.
• Using the table below, we may anticipate the form by calculating the total number of lone pairs and bond pairs.
The total domain is equal to the sum of the lone pair and the bond pair. For additional assurance, the generic formula can be matched.
The total number of electron pairs in SiBr4 is four.
As a result, SiBr4 has a tetrahedral geometry and form.
Hybridization is a term that describes the geometrical shape and bonding of polyatomic molecules.
An orbital is a three-dimensional region around the nucleus where the chances of finding an electron are greatest.
The mixing of pure atomic orbitals to generate hybrid atomic orbitals is known as hybridization. If the pure atomic orbitals are of similar form and energy, this mixing is possible.
The number of hybrid orbitals equals the number of pure orbitals combined for hybridization.
2s and one 2p, for example, can generate sp hybrid orbitals, while 2s and 5d can’t. In addition, one 2s and three 2p orbitals are combined in C (carbon atom) to generate four comparable sp3 hybrid orbitals.
Because the orbitals arrange themselves in such a way that repulsion between electrons in them is minimal, the type of hybridization also indicates the molecule’s geometry.
Hybridization of SiBr4
The core atom of the SiBr4 molecule is Si, which is the least electronegative atom in the molecule.
Si has the electrical arrangement 1s22s22p63s23p2. In hybridization, only valence orbitals are utilised.
One electron from the 3s orbital is promoted to the 3p orbital in the excited state. Electron promotion, on the other hand, does not always occur.
These four orbitals (one 3s and three 3p) are now hybridised to produce four sp3 orbitals, which will make bonds with the atoms in the vicinity.
There are four surrounding atoms in SiBr4. With one sp3 hybrid orbital, each atom creates a bond.
With sp3 hybrid orbitals, all Br atoms form a sigma bond. As a result, SiBr4 is sp3 hybridised.
The secret to determining the type of hybridization
In the last step of VSEPR theory, we estimated the total electron pairs. It turns out to be 4 for SiBr4.
We can anticipate hybridization using total electron pairs or steric numbers from the table below. The SiBr4 molecule has a bond angle of roughly 109.5°, which can also be observed in one of the photos above.
Polarity of SiBr4
The net dipole moment of a substance determines whether it is polar or non-polar.
The difference in electronegativity of elements, charge separation, and molecule shape all influence polarity.
Si has an electronegativity of 1.9, while Br has a value of 2.96. Because the difference is 1.06, the bonds are polar and covalent.
The shape of the molecule determines its net polarity.
Tetrahedral is a symmetrical shape, and SiBr4 is tetrahedral. The dipole moment vectors cancel one other out, resulting in a non-polar molecule as a whole.
If the form had not been symmetrical, the compound would have been polar.
Each Br and Si make a covalent connection. All atoms have a formal charge of zero. SiBr4 has a tetrahedral molecular geometry and form.
SiBr4 has an sp3 hybridization. It’s a molecule that isn’t polar.
I hope you enjoyed the SiBr4 chemistry.