The molar mass and melting point of beryllium chloride are 79.91 g/mol and 399 °C, respectively. The chemical bonding in Beryllium Chloride is studied by writing down its Lewis structure by following the Lewis approach. After lewis structure, there is a need of understanding its molecular geometry and hybridization of the central atom, Beryllium. The molecular orbital (MO) theory will be used to understand the MO diagram of beryllium chloride.
BeCl2 Lewis Structure
The electrons present in the outermost shell of an atom are shown in the Lewis structure of any molecule. These electrons will be both bonding as well as non-bonding electrons. The electronic configuration of beryllium is [He] 2s2and chlorine is [Ne] 3s23p5. The number of electrons on the valence shell of Be and Cl is 2 and 7 electrons, respectively. As there are two chlorine atoms in beryllium chloride, the total number of valence electrons would be 2 + (7 X 2) =16 electrons. Lewis structure is also known as electron dot structure or Lewis dot structure because the valence electrons are represented as dots in the Lewis structure of the molecule. It is the two-dimensional structure in which every atom in the molecule tends to complete its octet either by sharing or gaining or losing electrons. However, there are some exceptions as well. For example, Hydrogen, Helium, Lithium, and Beryllium cannot complete their octet, and hence, they prefer to have duplets (two electrons). Now, we have 16 valence electrons and we need to arrange these electrons in the Lewis structure of BeCl2. The beryllium will be surrounded by the two electrons, as mentioned earlier. The chlorine atom would like to have eight electrons around it to complete its octet. Beryllium will act as the central atom and chlorine atoms will surround it. Therefore, the possible Lewis structure of beryllium chloride would be:
It can also be represented in the bond form as two shared electrons will form a single bond. Hence, the Lewis structure of beryllium chloride can be drawn as:
Both representations are correct and hence, they can be represented by either of two. Each chlorine atom completes its octet by sharing its one electron with the beryllium atom. Therefore, there is a single bond between Beryllium and Chlorine atom. The octet of beryllium is incomplete and hence, Beryllium chloride is electron-deficient and acts as Lewis acid. Now, what would be the molecular shape of BeCl2? We cannot say anything based on the lewis structure. For that, we have to study the Valence shell electron pair repulsion (VSEPR) theory to predict the shape of beryllium chloride. Let us move towards the VSEPR theory.
BeCl2 Molecular Geometry
VSEPR theory determines the molecular shape of the molecule on the basis of Bond pair – Bond pair, Bond pair – Lone pair, and Lone pair- Lone pair repulsions. It also includes only valence shell electron, which may be bonded or non-bonded. According to the lewis structure of Beryllium chloride, Beryllium is a central atom and it has only two bond pairs. Its shape can easily be predicted by the following table. As the Beryllium atom forms two bond pairs with two chlorine atoms, its general formula will be AX2. Hence, Beryllium Chloride will have a linear shape or we can say its molecular geometry is Linear. The linear geometry of Beryllium Chloride leads to the bond angle (Cl-Be-Cl) of 180° to minimize bond pair-bond pair repulsions. If the bond angle is either greater than or lower than 180°, then bond pair-bond pair repulsion will not be minimum. Now, let us move towards the valence bond theory (VBT) to know the hybridization of Beryllium in Beryllium Chloride.
BeCl2 Hybridization
According to valence bond theory (VBT), atomic orbitals of the central atom fuse together and form hybrid orbitals of equivalent energy. These hybrid orbitals overlap with the atomic orbitals of surrounding atoms and hence, bond formation takes place. The ground state electronic configuration of Beryllium is [He] 2s2. As there is only one pair of the electron, which is paired in 2s orbital. The paired orbitals cannot participate in bond formation as per VBT. Hence, one of the electrons from the 2s orbital will excite to the 2p orbital of Beryllium. The amount of energy required for the excitation of the electron is known as promotion energy, which comes from bond formation between Beryllium and Chlorine atom. Now, the excited-state electronic configuration of Beryllium will be [He] 2s12p1. One 2s orbital and one 2p orbital of Beryllium atom will fuse and form two sp hybrid orbitals of the equivalent energy. These sp hybrid orbitals of Beryllium atom will overlap with 3p orbitals of chlorine atoms and hence, sigma bond formation takes place between Beryllium and chlorine. The same can be represented by its orbital diagram. The orbital diagram of Beryllium chloride would be: Therefore, the hybridization of the Beryllium atom in the Beryllium chloride is sp. It can also be calculated from a steric number. Steric number = Number of atoms bonded to central atom + Number of lone pair of electrons at the central atom Now, there are two atoms, which are bonded to the Beryllium atom and there are no lone pair of electrons at the Beryllium atom. Hence, the steric number is 2 and hybridization will be sp.
BeCl2 Polarity
BeCl2 molecule is considered a nonpolar molecule. The shape of this molecule is symmetric ie; linear due to which the net dipole of the entire molecule becomes zero leaving behind no partial charge. The electronegativity of Both Chlorine atoms is the same and thus has equal influence on the shared electrons. For detailed information, check out the polarity of BeCl2.
BeCl2 MO diagram
The Molecular orbital theory, given by Mulliken and Hund, further explains the chemical bonding in the molecule. It provides the molecular orbital diagram or energy level diagram, which is based on the linear combination of atomic orbitals. According to this theory, atomic orbitals of similar energy and symmetry around the molecular axis combine to form molecular orbitals. The number of atomic orbitals combining will be equal to the number of molecular orbitals. For example, two atomic orbitals combine to form two molecular orbitals, one is bonding and the other is antibonding. Bonding molecular orbitals are lower in energy and hence, more stable than individual atomic orbitals whereas antibonding molecular orbitals will be higher in energy than individual atomic orbitals and hence, less stable. The molecular orbital diagram of the BeCl2 molecule is drawn by the combination of Beryllium atomic orbitals and chlorine group orbitals. As there are two chlorine atoms and hence, first they combine to form group orbitals. The electronic configuration of Cl is [Ne] 3s23p5. Now, the 3s atomic orbital of one chlorine atom will combine with the 3s atomic orbital of other chlorine atoms and provide two 3s group orbitals of the same energy. Similarly, three 3p orbitals of one chlorine atom will combine with three 3p orbitals of other chlorine atoms and yield six 3p group orbitals of equivalent energy. We have two 3s and six 3p group orbitals with the following symmetry. These are also obtained by the linear combination of atomic orbitals. We named bonding group orbitals as 3s, 3px, 3py, and 3pz (left hand side) whereas antibonding group orbitals as 3s*, 3px*, 3py*, and 3pz* (right hand side). As chlorine is more electronegative than beryllium and hence, its energy will be relatively lowered than beryllium. The molecular orbital diagram of BeCl2 will be drawn by combining atomic orbitals of beryllium atom and group orbitals of chlorine atom having similar energy and symmetry around a molecular axis. The 3s group orbitals of chlorine atom will remain non-bonding because their energy is very low as compared to the 2s and 2p atomic orbitals of beryllium atom. Similarly, 3px* and 3py* group orbitals also remain non-bonding because their symmetry does not match with 2s and 2p atomic orbitals of the beryllium atom. Now, the 2s atomic orbital of the beryllium atom will combine with the 3pz* group orbital of a chlorine atom and gives bonding and antibonding molecular orbitals. These two orbitals are similar in symmetry and energies are also nearby. Similarly, the 2pz atomic orbital of the beryllium atom will combine with the 3pz group orbital of the chlorine atom and gives bonding and antibonding molecular orbitals. The 2px and 2py atomic orbitals of the beryllium atom will combine with the 3px and 3py group orbital of a chlorine atom and gives two bonding and two antibonding molecular orbitals. Hence, the molecular orbital diagram of beryllium chloride would be:
The sixteen valence electrons are filled in molecular orbitals according to the Aufbau principle, which follows Pauli’s exclusion principle and Hund’s rule.
Conclusion
Beryllium chloride is an inorganic compound, which is soluble in various polar solvents. It is a Lewis acid and hence, used as a catalyst in Friedel-craft reaction. Here, we have studied every aspect of chemical bonding in beryllium chloride. We have started from its two-dimensional representation i.e., Lewis structure. The molecular shape of beryllium chloride is linear and it shows sp hybridization. The molecular orbital diagram of beryllium chloride is also studied. It is based on the linear combination of atomic orbitals of the beryllium atom and group orbitals of the chlorine atom. In brief, we have covered all the basic properties of beryllium chloride. More suggestions are welcome for further improvement. Thank you for reading.
