How does atomic size affect reactivity?

Hint: The reactivity of the atom of any element can be found by a couple factors and these help us determine if they will and how will they react. ionization energy is a factor which is the measure of how easily an electron can be removed from an element’s stable atom. Electronegativity of the element is one of the factors which determines how badly an element can take other electrons from other elements.

Complete step-by-step answer:It is known that larger an atom is, the more reactive it will be. The attraction between the protons and electrons which are present in the nucleus and the shells surrounding the nucleus help to stabilize the atom. Larger the atom, the farther away the valence electron shell will be. This causes the loss of electrons to be easier.
This makes it easier for the valence electrons to leave the atom and form an ionic bond or to interact with another atom to form a covalent bond.
If electrons can leave more easily, then the atom is more likely to react.
When we talk about the metals which are present in the periodic table, they follow a trend. They are present towards the left hand side of the periodic table. When we go from left to right of the periodic table, chemical reactivity decreases. As we go from top to bottom in a group, the chemical reactivity increases.

Note:It is known that as we go down a group, it is easier for electrons to be lost. This results in high chemical reactivity. Also, when we go from left to right, there are more electrons in their valence shells. To lose these electrons, there is a need of high energies and thus this causes the chemical reactivity to lower down.