How do valence electrons affect reactivity?

Hint: The outermost shell of the atom is known as the valence shell. The electrons which are present in the outermost shell of the atom are known as the valence electrons. They tend to determine the reactivity of the atom and also determine its tendency to form the chemical bonds.

Complete answer:
So in metals or the most reactive metallic elements are present from the group 1 such as sodium, potassium. So in the group 1 elements there are only single valence shell electrons. So this single valence electron can be easily lost so that a positive ion can be formed with the electronic configuration of ${s^2}{p^6}$ . So as it has only one electron to lose so it can be easily bonded and so it will be more reactive.
Now the metals present in group 2 have two valence electrons in its valence shell. So to form the positive metal ion they need to lose these two valence electrons. But to lose two electrons will be more tough than to lose one electron. So these metals will be less reactive as they have more electrons to lose and to form bonds with these metals will be more tough than the group 1 elements.
Within each group of the metals the reactivity tends to increase down the group. As the valence electrons become less bound to the nucleus so they will be easily removed and the atomic size also increases as the number of shells increases by 1 down the group.
In the non metals, they need to attract electrons towards itself to form their bond. It could share electrons with the neighbouring atom so that the covalent bond could be formed, else they could remove one electron to form the ionic bond.
So the most reactive nonmetals are the halogens as they need only one electron to form bonds. So they tend to remove an electron from another atom or they tend to share an electron from the other storm to form a covalent bond.
In groups the reactivity of the non metal decreases because the valence electrons are at the progressively higher energies and the atoms could not gain stability by gaining the electrons.
So the metal such as potassium present in group 1 and the non metal such as fluorine present in group17 would be reacting violently as they both will fulfil their needs and get benefits from the reaction. potassium will lose one electron and the fluorine would gain an electron and form an ionic bond.

Note: If the valence shell of the atom is fulfilled that is either it form octet or duplet then it will be stable. The stable compounds usually have 8 electrons or 2 electrons in its valence shell. Such stable compounds are the noble gases which have full outer energy level.