Molecular Geometry, Hybridization, and Polarity of SiH4 Lewis Structure

The simplest of all the chemical compounds belonging to the’silane’ groups, which correspond to binary silicon-hydrogen and organosilicon compounds with terminal hydrides, is silane, also known as monosilane.

SiH4 is the chemical structure of silane/silicane. It’s a combustible, colourless gaseous chemical with a strong noxious stench.

Silicane has a usage in the semiconductor sector, where it is employed as a source of ultra-pure silicon.

Aside from that, SiH4 is used to make a variety of silicon-based chemicals as well as doping agents.

It is highly explosive and deadly due to its flammable nature. Due to leaking, it can cause fatal mishaps by igniting and combusting.

It has a density of 1.313 g/l and a molar mass of 32.117 g/mol.

It’s even lighter than air and might irritate your skin and eyes. In nature, SiH4 is pyrophoric.

The reactions listed below demonstrate how Silane is made:

Si    +     3HCl    ——->     HSiCl3    +    H2

4HSiCl3    ——>   SiH4    +    3SiCl4

Also,

Mg2Si     +    4HCl     ——>     2MgCl2 + SiH4

Lewis Structure of SiH4

Lewis Structure is a two-dimensional diagrammatic way to determining the chemical bonding type of any given molecule.

The electrons are represented using dot notations, hence this structure is also known as the electron-dot structure.

We shall discover the most acceptable and probable Lewis Structure of Silicon hydride, or SIH4, in this article.

At first, we’ll concentrate on the valence electron notion.

The electrons in the outermost or valence shell of an atom of any element are referred to as valence electrons. Valence electrons are involved in bonding.

The Periodic Table is shown below.

Elements of the same group (vertical) have the same number of valence electrons in their atoms, as can be shown.

Hydrogen is in group 1, which means it only has one electron in its outermost shell.

Silicon, on the other hand, is a member of group 14 with a valency of 4.

The total number of valence electrons in a SiH4 molecule will now be calculated.

The total number of valence electrons is equal to 4 + 14 = 8.

Now we’ll look at the Pauling electronegativity chart to see what electronegativity values they relate to.

The H value is 2.20, whereas the Si value is 1.90.

The more electropositive element must form the centre atom, according to the general rule. As a result, Si will serve as the centre atom, surrounded on all sides by four hydrogen atoms.

The Octet rule is now in effect.

The elements in the main group of the periodic table tend to acquire the octet or outer shell electronic configuration of noble gas elements, according to this criterion.

Hydrogen, on the other hand, just need two electrons to achieve the Helium structure.

We’ll examine the octet fulfilment now that we’ve placed the electron dot notations according to the likely bond creation (one electron pair sharing between two constituent atoms forms a single bond).

Both Si and H have acquired their respective noble gas valence shell configurations, as can be seen plainly ( Ar and He respectively).

We can now go to the final step in sketching the Lewis Structure of SiH4.

The formal charge is the charge ascribed to constituent atoms inside a molecule based on the premise that electrons are shared evenly among the atoms involved in bond formation.

The following formula is used to determine formal charge:

Formal charge = 4 – 0.5*8 – 0 = 0 for Si.

Formal charge = 1 – 0.5*2 – 0 = 0 for each H atom.

All five atomic elements are present in the lowest formal charge values allowed. We’ve come up with the best Lewis Structure sketch for SiH4.

Molecular Geometry of SiH4

We went over the step-by-step technique for presenting a visual representation of the Lewis Structure of SiH4 in the previous part, which provides us a good notion of the type of bond creation and 2-dimensional approach.

However, understanding the chemical bonding occuring inside the molecule will be much easier if we can unravel the 3D structural geometry of silane.

The VSEPR model, or Valence Shell Electron Pair Repulsion theoretical model, comes in handy for this.

The VSEPR theory is thus utilised to estimate a molecule’s 3D molecular form from its Lewis Structure diagram.

We discovered that valence electrons participate in bond formation as electron-pairs when sketching the electron-dot sketch. The valence electrons that are not bound also act as lone pairs.

Around the nucleus, all of these negatively charged subatomic particles form a cloud-like environment. These similar charges are attracted to one other by repulsive forces.

According to VSEPR theory, electrostatic repulsive forces can be decreased for molecule stability by keeping electrons farther apart (in linear geometry, the bond angle is 180 degrees).

Let’s have a look at Silane’s molecular geometry:

We have AXnEx notation in VSEPR theory, where

A: a molecule’s core atom

X: the area around a molecule’s atoms

E: the core atom’s lone pairs

In SiH4, we call it SiH4 for short.

Silicon is represented by the letter A. (Si)

X denotes the four hydrogen atoms, while n = 4 denotes the number of hydrogen atoms.

On Si, E denotes no lone pairs, and x = 0.

AX4E0 is the VSEPR nomenclature for silane.

Now we’ll take a look at the VSEPR chart, which lists all of the molecular geometries and their AXE notations:

Silicon hydride, often known as Silane, has a tetrahedral chemical shape. A general tetrahedral 3D molecule has an estimated bond angle of 109 degrees.

The length of the Si-H bond is around 1.4798.

Hybridization of SiH4

Hybridization, often known as orbital hybridization, is a fundamental chemical topic. Covalent bond formation is well-known; SiH4 contains four covalent Si-H bonds.

Hybridization is a theory that is used to explain the development of covalent bonds.

We’ll start with AOs, or atomic orbitals, in this model.

Orbitals such as s,p,d,f are mathematical probability functions that indicate the presence of electrons in a certain location.

Hybridization refers to the coming together and fusing of atomic orbitals of the same atom within a molecule to generate hybrid orbitals.

A sigma () bond is created when a direct head-on overlap occurs, while a pi () bond is formed when parallel side-to-side overlap occurs.

The electronic configuration of Si, the core atom of silane, is as follows:

1s2 2s2 2p6 3s2 3p2 Si: 1s2 2s2 2p6 3s2 3p2

3s2 3p2 Si: [Ar] 3s2 3p2

The s and three p orbitals come together and unite to generate four hybridised sp3 orbitals, as seen in the diagram.

Each hydrogen atom forms a sigma bond with these hybridised orbitals. (The term “single bond” refers to the development of sigma bonds.)

Also, the formula for calculating the steric number is as follows.

Number of atoms bound to the core atom inside a molecule + Number of lone pairs of electrons attached to the central atom = Steric number

In silane, the steric number is 4 + 0 = 4.

Around centre Si, there are four electron-rich areas.

As a result, in SiH4, the hybridization for Si is sp3.

Polarity of SiH4

Now we’ll talk about polarity, which is another important topic to explore in relation to our molecule, Silane.

The separation of electric charges within a chemical substance is referred to as polarity.

Electronegativity is used to explain polarity and determine whether a molecule is polar or non-polar. It is defined as the degree to which an element can accept negatively charged electrons.

To determine whether SiH4 is polar or non-polar, we must first comprehend the chemical covalent connections produced within the molecule.

In silane, there is only one type of bond: the Si-H bond.

We’ll now examine the electronegativity ratings of both elements. H has an electronegativity of 2.20, whereas Silicon has an electronegativity of 1.90.

Although the difference is small, Si-H covalent bonds are considered to be somewhat polarised. Because Si has a positive partial charge, it is susceptible to nucleophiles, whereas H has a negative partial charge.

If we look at the entire molecule, we can see that due to H’s greater electronegativity value, electrons are slightly more abundant near the terminal hydrogen regions.

Because there are four Si-H bonds, the net dipole of the silane molecule is equal to the vector sum of all dipole moments, which is zero.

BTW, I’ve published a whole article about it. On the polarity of SiH4, you can check it out.

As a result, the resulting molecule is non-polar.

Conclusion

The chemical bonding nature of SiH4 or Silicane has been discussed in this article. We started with a detailed discussion of the Lewis Structure’s first sketch, and then moved on to 3D molecular geometry.

The notions of hybridization and polarity have also been included.

Read more: Is CaO an ionic or a covalent compound?

Misha Khatri
Misha Khatri is an emeritus professor in the University of Notre Dame's Department of Chemistry and Biochemistry. He graduated from Northern Illinois University with a BSc in Chemistry and Mathematics and a PhD in Physical Analytical Chemistry from the University of Utah.

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