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Li and Na have 1 electron in their outermost shells or valence shells.
These elements can easily lose one electron and gain the stable electronic configuration of the nearest noble gas,Helium in case on Lithium and Neon in case of Sodium.
As reactivity of an element is based on how quickly the element can reach its stability.
So both are highly reactive.
However Francium,the last element of the first period is the most reactive,as the electrons are added in different shells,the distance between nucleus and electrons increases and francium can lose electrons easily than lithium or sodium,because Li and Na are more attracted to nucleus as they are closer to the nucleus than Francium which is far away from the nucleus as it has a bigger atom than Li and Na

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Because they can easily lose their valence electrons i.e. 1