Molecular Orbital Diagram of H2, He2, Li2 and Be2 Molecules.

0:15 Molecular Orbital Diagram of Hydrogen Molecule
1:39 Molecular Orbital Diagram of Helium Molecule
2:54 Molecular Orbital Diagram of Lithium Molecule
4:00 Molecular Orbital Diagram of Beryllium Molecule

As we have discussed in our previous video how formation of molecular orbitals takes place in diatomic molecules. Lets take first example of hydrogen molecule and create its molecular orbital diagram,

So generally we can write simple reaction as two hydrogen atoms combine to make one hydrogen molecule as,
As we know the number of electrons in Hydrogen atom is one its electronic configuration will be 1s1
When two hydrogen atom contribute one electron each, Hydrogen molecule will have two electrons and its electronic configuration will be sigma 1s2.

Filling of these two electrons takes place from the bonding molecular orbital and then pairing takes place.So both the electrons are in sigma 1s orbitals.and anti bonding molecular orbitals are empty.

So as all the electrons in hydrogen molecule are paired up, it is diamagnetic. And bond order can be calculated as bond order = number of electrons in bonding mo - number of electrons in anti bonding mo divide by 2. Which is 2-0/2 comes to be one . Therefore single covalent bond is formed.
Similarly we can draw the molecular orbital diagram of He and find the number of covalent bond formed between two helium atoms. He has two electrons with electronic configuration 1s2. Thus He molecule will have total 4 electrons and its electronic configuration will be sigma 1s2 , sigma star 1s2.
So 4 elections can be filled as per the afbaus principle, first electrons will be filled in bonding Mo and later in anti bonding MO.
In this case also all electrons are paired up so molecule should be diamagnetic and bond order can be calculated from the same formula where number of electrons in bonding mo is 2 and number of electrons in anti bonding mo is also 2. So 2-2/2 is zero. This means there is no bond formation between two helium atoms. And thus helium molecule does not exist.
Now lets discuss formation of lithium molecule, as lithium contains 3 electrons its electronic configuration is 12 2s1. So two lithium atoms will combine to form a molecule of Li. And now lithium molecule will have total 6 electron so its electronic configuration will be sigma 1s2 sigma star 1s2 and sigma 2s2 which can also be written as KK sigma 1s2
In this case also all the electrons are paired so it should be diamagnetic. And bond red calculated from the formula is again 1 so single bond between two Li atoms is formed.
Similarly we can draw the mo diagram for beryllium molecule. So no of electron = 4 and electronic configuration 1s2 2s2


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