An Orbital View of Bonding

3 Standing Waves in Three Dimensions Atomic Orbitals 

4 Mixing Atomic Orbitais into Moiecuiar Orbitais 

Since the antibonding molecular orbital is raised more than the bonding is lowered, if we try to fill both molecular orbitals (with 4 electrons), the overall energy is increased compared to the isolated atoms. This is why helium does not form a diatomic molecule. With both the bonding and antibonding orbitals filled with two electrons, there is no net bond and the helium atoms drift apart. With three electrons, the antibonding orbital has just one electron in it, so the bond order of He2 is 1/2, resulting in a very weak bond. 

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AN ORBITAL VIEW OF BONDING (A Supplementary, More Advanced Explanation)... 

An understanding of interaction diagrams is not absolutely necessary for using the principle of electron flow to predict organic reaction products. However, it is useful for understanding reactivity trends and the stability of reactive intermediates. This section relies on the principles discussed in Section 1.6, An Orbital View of Bonding. 

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