Molecular Geometry, Hybridization, and Polarity of BeH2 Lewis Structure

BeH2, also known as beryllium hydride or beryllium dihydride, is a beryllium hydride or beryllium dihydride. It’s an inorganic chemical that belongs to the alkaline earth hydride family. At ordinary temperature and pressure, it appears as an amorphous white solid. It’s also known as (BeH2) n in polymeric form.

The reaction of dimethyl beryllium (Be (CH3)2) with lithium aluminium hydride produces beryllium hydride (LiAlH4).

Be (CH3)2    +    LiAlH4    —–>    BeH2    +    LiAl (CH3)2H2

Beryllium hydride decomposes into beryllium hydroxide and hydrogen gas when exposed to water. Toluene and diethyl ether are insoluble in it. Beryllium hydride has a molar mass of 11.03 g/mol. It decomposes at 250°C, which is its melting point.

Because beryllium hydride is a Lewis acid, it forms dimeric or trimeric adducts with Lewis bases like dimethylamine and trimethylamine.

Let’s have a look at how the chemical bonds in the beryllium hydride molecule work.

Following the octet rule, we shall first draw its two-dimensional structure, i.e. the Lewis structure. Then, using VSEPR theory and VBT, we’ll figure out its three-dimensional structure, or the molecule’s true shape. Finally, we’ll find out whether the molecule of beryllium hydride is polar or nonpolar.

Lewis Structure of BeH2

The simplest depiction of every molecule is the Lewis structure, which comprises the molecule’s atoms as well as the atoms’ valence electrons.

Why are valence electrons important while core electrons aren’t?

Because only the valence electrons, or electrons in the atom’s outermost shell, are available for forming a chemical bond between two atoms. Because core electrons are securely bonded to the atom’s nucleus, they do not contribute to chemical bonding.

The bonding process does not involve all of an atom’s valence electrons. Some of them may choose to remain non-bonding and act as a lone pair (s).

In the Lewis structure of the atom, the valence electrons are shown as dots surrounding the atom. As a result, it’s also called the electron dot structure.

Let’s draw the beryllium hydride molecule’s Lewis structure.

The hydrogen atom belongs to group 1 (alkali metal) of the current periodic table, while the beryllium atom belongs to group 2 (alkaline earth metal).

As a result, the valence electrons of the Be and H atoms are 2 and 1, respectively.

One beryllium atom and two hydrogen atoms make up the beryllium hydride molecule. As a result, in beryllium hydride, the total number of valence electrons is 2 + (1 x 2) = 4 electrons.

The molecule’s Lewis structure is based on the concept of the octet rule.

The Octet Rule states that during bond formation, every atom in the periodic table strives to complete its octet, or eight valence electrons around it, whether with the same atom or with another element in the molecule. However, the octet rule does not apply in a number of situations.

a. Formation of the central atom’s expanded octet: In this situation, the centre atom is surrounded by more than eight electrons, all of which are stable. Consider the following scenario: The phosphorous atom, which is the core atom in PCL5, is surrounded by ten electrons instead of eight, although it nevertheless exists in nature.

b. Incomplete octet of the central atom formation: In this case, the centre atom has fewer than eight electrons surrounding it, and these molecules are likewise stable. When the centre atom has fewer than four valence electrons, an incomplete octet is formed.

One example of an incomplete octet is beryllium hydride. Because the beryllium atom in the BeH2 molecule only has two valence electrons, it cannot complete its octet by forming a connection with the hydrogen atom.

We have a total of four valence electrons to arrange in the Lewis structure of the beryllium hydride molecule. By forming a connection with another atom, the hydrogen atom always forms a duplet (one of the exceptions to the octet rule), and so there will be two electrons between hydrogen and beryllium atom.

Between hydrogen and the beryllium atom, these two electrons will create a single connection. As a result, the Lewis structure of the beryllium hydride molecule that is most appropriate is:

Each hydrogen atom has completed its duplet by creating a single bond with the beryllium atom, but the beryllium atom’s octet is still unfinished. As a result, beryllium hydride is an electron-poor molecule that functions as a Lewis acid.

The next question that will cross your mind is what the form or geometry of the BeH2 molecule is.

We obviously can’t answer this issue based on its two-dimensional structure, the Lewis structure.

To do so, we must first comprehend the BeH2 molecule’s three-dimensional structure, which can be predicted using the Valence shell electron pair repulsion (VSEPR) theory.

What exactly is the VSEPR theory?

Molecular Geometry of BeH2

Sidgwick and Powell’s VSEPR theory provides a straightforward method for determining the shape of a covalent molecule. The repulsive interactions between the valance electrons of the atoms provide the basis for this hypothesis.

The bond pairs find a place in space to minimise repulsive interactions and maximise the distance between them due to electron repulsion. Lone pairs are non-bonded valence electrons that have stronger repulsions than bond pairs.

The beryllium atom is the core atom in the Lewis structure of the beryllium hydride molecule, with two hydrogen atoms surrounding it. As a result, the beryllium atom has two bond pairs and no lone electrons.

The form of the molecules is depicted in the table below, where the centre atom has only bond pairs. The number of bond pairs on the core atom can thus be used to predict the shape of the molecule.

General formulaNumber of bond pairsMolecular shape/geometry
AX1Linear
AX22Linear
AX33Trigonal planar
AX44Tetrahedral
AX55Trigonal bipyramidal
AX66Octahedral

Beryllium hydride’s general formula is AX2, and it has a linear shape and geometry as a result.

The Beryllium Hydride molecule’s linear geometry results in a bond angle (H-Be-H) of 180°, which minimises repulsions between two B-H bonds in space.

Hybridization of BeH2

The Valence Band Theory (VBT) and the steric number can both be used to estimate the hybridization of the central atom in the molecule as well as the shape of the molecule.

Method of Steric Number: Steric number is defined as

Number of Bond pairs + Number of Lone pairings = Steric number

Two hydrogen atoms are bound to the beryllium atom in this structure, while the centre element, beryllium, has no lone pair. As a result, the BeH2 molecule has a steric number of 2.

The sp hybridization of the beryllium atom with the linear geometry of the BeH2 molecule is caused by the two steric number.

The hybridization of the beryllium atom in the BeH2 molecule is computed using the VBT method:

The ground state electronic configuration of the beryllium atom is [He] 2s2. In the excited state, the electronic configuration of the beryllium atom is [He] 2s12p1.

The beryllium atom’s 2s orbital now fuses with its 2p orbital, resulting in two sp hybrid orbitals of equivalent energy that align in a linear geometry.

The beryllium atom’s sp hybrid orbital overlaps with the hydrogen atom’s 1s atomic orbital, as illustrated in the orbital diagram of the beryllium hydride molecule:

As a result of the VBT approach, the Beryllium atom in Beryllium hydride with linear geometry undergoes sp hybridization.

Polarity of BeH2

The electronegativity differential between the bond’s atoms determines the polarity of the covalent connection. The covalent bond can be polar or nonpolar depending on the electronegativity difference.

Let’s look at how the Be-H bond in the BeH2 molecule works.

On the Pauling scale, the electronegativity of the beryllium atom and the hydrogen atom are 1.57 and 2.20, respectively. Because the electronegativity difference between the B-H bond and the electronegativity difference between the B-H bond is 0.63, the B-H bond is a polar covalent bond.

On the beryllium and hydrogen atoms, it will function as a dipole with a partial positive and partial negative charge, respectively. As a result, the dipole moment of the B-H bond points towards the hydrogen atom.

The net dipole moment of the BeH2 molecule determines its polarity, which is further determined by its shape or geometry.

Because the BeH2 molecule has a linear form, two B-H dipoles of equal magnitude but opposite directions cancel each other out.

The net dipole moment of the BeH2 molecule is zero as a result.

As a result, the BeH2 molecule is a nonpolar molecule.

Conclusion

We looked at the chemical bonding of the BeH2 molecule in this article.

In summary, the BeH2 molecule is a Lewis acid due to the beryllium atom’s incomplete octet, which can be seen in its Lewis structure. BeH2 has a form that is linear with the beryllium atom’s sp hybridization. The Be-H bond is a polar covalent bond, however due to its linear shape and zero net dipole moment, the BeH2 molecule is nonpolar.

Suggestions and questions are appreciated.

Thank you, Yo.

Read more: Is PF5 a polar or nonpolar 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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