SiH4 is the chemical formula of silicon tetrahydride, most commonly known as silane. It is a colorless, pyrophoric, toxic gas with a sharp and repulsive smell. SiH4 has a molar mass of 32.12 g/mol.
It is used as a bio-inert layer on titanium implants and in some fiberglass and composites to improve mechanical strength and electrical properties.
This article will discuss a very important property of the silane (SiH4) molecule, i.e., its polarity. So is silane (SiH4) polar or non-polar? Continue reading to find out the answer to this question.
Is SiH4 polar or non-polar?
Silane (SiH4) is a non-polar molecule. It consists of one silicon (Si) atom and four hydrogens (H) atoms. The silicon is kept at the central position, and all hydrogen atoms are at the terminal positions, making a perfectly symmetrical tetrahedral molecular shape and geometry.
An electronegativity difference of 0.30 units exists between the bonded atoms of silicon and hydrogen in Si-H in SiH4.
Thus, each Si-H bond is slightly polar individually in SiH4 and possesses a specific dipole moment value (symbol µ).
However, it is due to the symmetrical shape of SiH4 that the dipole moments of individually polar Si-H bonds get canceled uniformly on each side of the molecule.
No lone pair of electrons is present on the central silicon atom. Hence, the molecule possesses an identical tetrahedral electron and molecular geometry or shape in which the charged electron cloud stays uniformly distributed. As a result, the SiH4 molecule is overall a non-polar molecule (net µ = 0).
A molecule is polar if there is a non-uniform charge distribution present in it. If the charge distribution gets equally balanced in different parts, then that molecule or molecular ion is considered non-polar.
The following three factors mainly influence the polarity of a molecule:
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now, let us discuss the effect of the above three factors one by one to prove that silane (SiH4) is overall a non-polar molecule.
Factors affecting the polarity of SiH4
Electronegativity
It is defined as the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity increases across a period in the Periodic Table while it decreases down the group.
Greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
Silicon (Si) is present in the Group 14 of the Periodic Table. The electronic configuration of silicon is 1s2 2s2 2p6 3s2 3p2. As per the electronic configuration, a Si-atom has a total of 4 valence electrons; it is thus short of 4 more electrons that are required so that the silicon atom can achieve a complete octet electronic configuration.
Conversely, hydrogen (H) belongs to Group 1 of the Periodic Table. The electronic configuration of a hydrogen atom is 1s1. Hence, each H-atom in SiH4 molecule has a deficiency of 1 more valence electron to obtain a full outer shell.
The SiH4 Lewis dot structure represents four Si-H bonds. All four valence electrons of silicon consumed in covalent bonding denote there is no lone pair on the central Si-atom in SiH4.
Via chemical bonding in SiH4, the central Si-atom has a complete octet, while each H-atom obtains a complete duplet electronic configuration.
Due to the absence of any lone pair of electrons on the central Si-atom in SiH4, there is no distortion in the symmetrical tetrahedral shape or geometry of the molecule.
Atom
Electronic configuration
Valence electrons
Silicon (14Si)
1s22s22p63s23p2
4
Hydrogen (1H)
1s1
1
The electronegativity difference in SiH4 between a Si-atom (E. N= 1.90) and an H-atom (E. N= 2.20) in each of the four Si-H bonds is 0.30 units.
Although it is a very small electronegativity difference, still the hydrogen atom with higher electronegativity attracts Si-H bonded electrons to a greater extent as compared to the Si-atom.
All the terminal H-atoms thus gain partial negative (δ–) charges while the central silicon atom obtains a partial positive (δ+) charge.
In this way, in SiH4, all Si-H bonds are slightly polar, with an electronegativity difference of 0.30 units.
Dipole Moment
Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.
It is mathematically calculated as a product of the magnitude of charge (Q) and charges separation (r). The dipole moment is expressed in a unit called Debye (D).
The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.
So in polar bonds of SiH4, the dipole moment of each Si-H bond points fromSiδ+ to Hδ-(as shown below).
Molecular geometry
According to the valence shell electron pair (VSEPR) theory of chemical bonding, SiH4 is an AX4E0 or simply AX4-type molecule. To one Si-atom at the center (A), four bonded atoms are attached (four H-atoms), and the central Si-atom contains no lone pair (E).
The molecular geometry or shape of SiH4 is thus tetrahedral, which is identical to its electron pair geometry. To minimize the repulsion and to attain stability in this tetrahedral structure, the bonded atoms of the SiH4 molecule possess a mutual bond angle of (∠H-Si-H) 109.28°.
Due to the absence of any lone pair of electrons on the central Si-atom, the SiH4 molecule has an identical molecular and electron geometry that is tetrahedral. There is no distortion in the shape and symmetry of the molecule.
It is due to the symmetrical tetrahedral shape that the charged electron cloud stays uniformly distributed, and the Si-H dipole moments get canceled equally on each side of the molecule.
Consequently, silane (SiH4) is overall a non-polar molecule (net µ = 0).
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), phosphorus trifluoride (PF3), nitrogen trifluoride (NF3), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), ethane (C2H6), propane (C3H8), silane (SiH4), etc.
SiH4 has four Si-H bonds. The electronegativity difference between the bonded Si-atom (E. N= 1.90) and H-atom (E. N= 2.20) is 0.30 units.
Hence, each hydrogen atom with relatively high electronegativity gains a partial negative (Hδ-) charge while the central silicon atom obtains a partial positive (Siδ+) charge.
Each Si-H bond thus possesses a small dipole moment.
However, it is due to the symmetrical tetrahedral shape of SiH4 that the individual Si-H dipole moments get canceled. The electron cloud stays uniformly distributed in the molecule overall.
Thus, SiH4 is overall a non-polar molecule with net µ = 0.
How the tetrahedral shape of SiH4 affects its molecular polarity?
Bond polarity depends on the electronegativity difference between bonded atoms, while molecular polarity is governed by the overall shape of the molecule.
In SiH4 there is no lone pair of electrons on the central Si-atom. Hence, no distortion is witnessed in the shape and geometry of the molecule.
Consequently, it adopts a symmetrical tetrahedral geometry, which leads to Si-H dipole moment cancellation and, thus, an equal charge distribution overall, leading to a non-polar SiH4
The net dipole moment of three downwards-pointing Si-H bonds gets canceled with the dipole moment of an upwards-pointing Si-H bond. So, net µ = 0.
Which one is more polar, SiH4 or CH4?
The dipole moment of any molecule depends on the difference in electronegativity between the bonded atoms.
The greater the electronegativity difference, the higher the bond polarity, resulting in a high net dipole moment value.
All atoms in theSiH4 molecule including silicon and hydrogen obtain a formal charge equal to zero, hence overall charge present on the silicon tetrahydride molecule is Zero.
Summary
Silane (SiH4) is a non-polar molecule.
It consists of four Si-H bonds.
Si-H bonds are slightly polar with an electronegativity difference of 0.30 units between the bonded Si-atom (E. N= 1.90) and H-atom (E. N= 2.20).
Silane (SiH4) possesses an identical tetrahedral molecular and electron geometry with a bond angle of (∠H-Si-H) 109.28°.
Due to the absence of any lone pair of electrons on the central Si-atom, there is no distortion witnessed in the shape and geometry of the SiH4
The dipole moments of individually polar Si-H bonds get canceled uniformly in the symmetrical shape of the molecule; thus, SiH4 is overall non-polar (net µ = 0).
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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