Silane, also known as monosilane is the simplest of all the chemical compounds belonging to ‘silane’ groups which refer to binary silicon-hydrogen and organosilicon compounds having terminal hydrides.
SiH4 is the structural composition of silane/silicane. It is a flammable and colorless gaseous compound bearing a strong repulsive odor.
Silicane has its application in the semiconductor industry: it is used as a source of hyperpure silicon.
Other than this, SiH4 is used to manufacture several silicon-containing compounds and also as doping agents.
Due to its flammable nature, it is highly explosive and dangerous. It can cause fatal accidents via ignition and combustion due to leakage.
It has a molar mass of 32.117 g/mol and a density of 1.313 g/l.
It is even lighter than air and can be a cause of skin and eye irritation. SiH4 is pyrophoric in nature.
The below-mentioned reactions show the manufacturing of Silane:
Si + 3HCl ——-> HSiCl3 + H2
4HSiCl3 ——> SiH4 + 3SiCl4
Mg2Si + 4HCl ——> 2MgCl2 + SiH4
SiH4 Lewis Structure
Lewis Structure is a two-dimensional diagrammatic approach towards finding the nature of chemical bonding present inside any given molecule.
Here, we use dot notations to represent the electrons, and hence this is also known as the electron-dot structure.
In this article, we will find out the most appropriate and possible Lewis Structure of Silicon hydride or SIH4.
At the very beginning, we will focus on the valence electron concept.
Valence electrons refer to the electrons present in the outermost or valence shell of an atom of any element. In bonding, the valence electrons take part.
Here’s a look at the Periodic table.
As we can all see, elements of the same group (vertical) have the same number of valence electrons in their atoms.
Hydrogen belongs to group 1 and so has only 1 electron in its outermost shell.
Silicon, on the other hand, belongs to group 14 and has a valency of 4.
Now, we will calculate the total number of valence electrons in a SiH4 molecule.
Total number of valence electrons = 4 + 1*4 = 8.
Now, we will have a look at the Pauling electronegativity chart to find out their corresponding electronegativity values.
H value is 2.20 whereas that of Si is 1.90.
The general rule states that the more electropositive element must form the central atom. Hence, Si will act as the central atom surrounded by the four hydrogen atoms at its sides.
Now, here comes the Octet rule.
According to this rule, the elements in the main group of the periodic table tend to achieve the octet or the outer shell electronic configuration of the noble gas elements.
Exception: Hydrogen only needs two electrons since it attains the Helium configuration.
Now, as we have put the electron dot notations according to the probable bond formation (one electron pair sharing between two constituent atoms form a single bond), we will check the octet fulfillment.
As we can see very clearly, both Si and H have attained their respective noble gas valence shell configurations ( Ar and He respectively).
So, we can now go to our last step towards sketching the Lewis Structure of SiH4.
The formal charge is the charge which is assigned to constituent atoms inside a molecule with the assumption that electrons are shared equally among the atoms participating in bond formation.
We calculate formal charge with the help of the following formula:
For Si, formal charge = 4 – 0.5*8 – 0 = 0.
For each H atom, formal charge = 1 – 0.5*2 – 0 = 0.
All the five atomic elements are present in their least possible formal charge values. We have got our most suitable Lewis Structure sketch for SiH4.
SiH4 Molecular Geometry
In the above section, we discussed in detail the step-by-step procedure to put forward the pictorial representation of the Lewis Structure of SiH4 which gives us a vivid idea of the type of bond formation and 2-dimensional approach.
However, if we can decipher the 3D structural geometry of silane, it will be a lot easier for us to understand the chemical bonding occurring inside the molecule.
For that, we need the help of the VSEPR model, short for the Valence Shell Electron Pair Repulsion theoretical model.
VSEPR theory is therefore used to predict the 3D molecular shape of a given molecule from its Lewis Structure diagram.
While drawing the electron-dot sketch, we found out that the valence electrons take part in bond formation as electron-pairs. Also, the unbonded valence electrons act as lone pairs.
All these negatively charged subatomic particles form an expansive cloud-like atmosphere around the nuclei. These like charges experience repulsive forces amongst themselves.
VSEPR theory states that the electrostatic repulsive forces can be minimized for molecular stability if the electrons stay farther away from each other (in linear geometry, the bond angle is 180 degrees).
Let us see what the molecular geometry for Silane is:
In VSEPR theory, we have AXnEx notation where
A: central atom of a molecule
X: surrounding atoms of a molecule
E: lone pairs on the central atom
Here in SiH4,
A stands for Silicon (Si)
X stands for the four Hydrogen atoms, ∴ n = 4
E stands for no lone pairs on Si, ∴ x = 0.
Our VSEPR notation for silane is AX4E0.
Now, we will have a look into the VSEPR chart that contains all the molecular geometries with respect to their AXE notations:
As we can clearly see, Silicon hydride or Silane has a tetrahedral molecular geometry. The approximate bond angle for a general tetrahedral 3D molecule is 109 degrees.
The Si-H bond length is around 1.4798 Å.
Hybridization, or better known as orbital hybridization, is an important concept of chemistry. We already know about covalent bond formation, in SiH4 we have four covalent Si-H bonds.
Hybridization is a model which is used to explain the phenomenon of covalent bond formation.
In this model, we will talk first about AOs or atomic orbitals.
Orbitals like s,p,d,f are mathematical probability functions giving us an idea of electron presence in a given regional space.
In hybridization, we talk about atomic orbitals of the same atom inside a molecule to come together and fuse to form hybrid orbitals.
When a direct head-on overlap occurs, a sigma (𝛔) bond is formed, and parallel side-to-side overlap occurs to give rise to a pi (𝝅) bond.
Si, the central atom of silane has the following electronic configuration:
Si: 1s2 2s2 2p6 3s2 3p2
Si: [Ar] 3s2 3p2
If we look at the diagram, we can see that the s and the three p orbitals come together and combine to form 4 hybridized sp3 orbitals.
These hybridized orbitals form sigma bonds with each hydrogen atom. (Single bond refers to sigma bond formation).
Also, if we calculate the steric number, the formula is as below.
Steric number = Number of atoms bonded to central atom inside a molecule + Number of lone pair of electrons attached to the central atom
Steric number in silane = 4 + 0 = 4.
There are four electron-rich regions around central Si.
Therefore, the hybridization for Si is sp3 in SiH4.
We will now talk about polarity, another crucial topic to discuss with respect to our molecule, Silane.
Polarity deals with the separation of electric charges inside a chemical compound.
Electronegativity, which is defined as the degree to which an element can take in negatively charged electrons, is used to explain polarity and find out whether a molecule is polar or non-polar.
To check whether SiH4 is polar or non-polar, first, we have to understand the nature of the chemical covalent bonds formed inside the molecule.
We have only one type of bond formation in silane: Si-H bond.
Now, we will have a look at the electronegativity values of both elements. H has an electronegativity value of 2.20 whereas that of Silicon is 1.90.
The difference is quite low but it is said that Si-H covalent bonds are slightly polarized. Si has a δ+ partial charge making it vulnerable to nucleophiles whereas H has a δ- partial charge.
If we focus on the whole molecule, we can say that electrons are slightly in more abundance near the terminal hydrogen regions due to H’s higher electronegativity value.
Since we have four Si-H bonds, the net dipole of the silane molecule is the vector sum of all the dipole moments which results in a zero value.
BTW, I have specifically written an article on it. You can check it out on the polarity of SiH4.
The resultant molecule is therefore non-polar in nature.
In this article on SiH4 or Silicane, we have covered the chemical bonding nature of the molecule. We have done an in-depth discussion on the initial sketch of the Lewis Structure, then proceeded to talk about the 3D molecular geometry.
We have also included the concepts of hybridization and polarity.
Note: SiH4 or Silane, 4 Si-H single covalent bonds, tetrahedral molecular geometry, sp3 hybridization, non-polar in nature.