When a sample of solid lithium is placed in a flask of hydrogen gas then following reaction happened 

This statement is

The given reaction is a nuclear reaction, which can take place only if a proton (a hydrogen nucleus) comes into contact with a lithium nucleus. If the hydrogen is in the atomic form, the interaction between its electron cloud and the electron cloud of a lithium atom keeps the two nuclei from getting close to each other. Even if isolated protons are used, they must be fired at the Li atom with enough kinetic energy to over come the electric repulsion between the proton and Li atom