(i) Bond formation in Beryllium chloride takes place by sp hybridisation.
(ii) The valence shell of Beryllium has the electronic configuration as follows:

(iii) In BeCl₂ both the Be-Cl bonds are equivalent and it was observed that the molecule is linear. VB theory explains this observed behaviour by sp hybridisation. One of the paired electrons in the 2s orbital gets excited to 2p orbital.
(iv) Now the 2s and 2p orbitals hybridise and produce two equivalent sp hybridised orbitals which have 50% s-character and 50% p-character. These sp hybridised orbitals are oriented in opposite direction.
(v) Each of the sp hybridised orbitals linearly overlap with Pz orbital of the chlorine to form a covalent bond between Be and CI atoms as follow.

(i) When half filled orbitals of two atoms overlap, a covalent bond will be formed between them.
(ii) The resultant overlapping orbitals are occupied by the two electrons with opposite spins. For example when H2 is formed, the two Is electron of two hydrogen atoms get paired up and occupy the overlapped orbitals.
(iii) The strength of a covalent bond depends upon the extent of overlap of atomic orbitals. Greater the overlap, larger is the energy released and stronger will be the bond formed.
(iv) Each atomic orbital has a specific direction (except s-orbital which is spherical) and hence orbital overlap takes place in the direction that maximises overlap.
(v) Depending upon the nature of overlap, the bonds are classified as c covalent bond and n covalent bond.
(vi) When two atomic orbitals overlap linearly along the axis, the resultant bond is called a sigma (c) bond. This overlap is also called "head-on-overlap" or "axial overlap".
(vii) When two atomic orbitals overlap sideways the resultant covalent bond is called a pi (n) bond.

(i) Two hydrogen atoms Ha and Hb are separated by infinite distance. At this stage, there is no interaction between these two atoms and the potential energy of this system is arbitrarly taken as zero.
(ii) As these two atoms approach each other, in addition to electrostatic attractive forces between the nucleus and its own electrons, the following new forces begins to operate.

(iii) The new attractive forces (. arrows) arise between:
(a) nucleus of H_a,and valence electron of H_b (b) nucleus of H_b and the valence electron of Ha.
(iv) The new repulsive forces (arrows) arise between:
(a) the nucleus of H_a and H_b
(b) the valence electrons of H_a and H_b.
(v) The attractive forces tend to bring H_a and H_b together whereas the repulsive forces tends to push them apart.
(vi) At the initial stage, as the two hydrogen atoms approach each other, the attractive forces are stronger than repulsive forces and the potential energy decreases.
(vii) A stage is reached where the net attractive forces are exactly balanced by repulsive forces and the potential energy of the system acquires a minimum energy.
(viii) At this stage, there is a maximum overlap between the atomic orbitals of H_a, and H_b and atoms H_a and H_b are now said to be bonded together by a covalent bond.