Across period four on the periodic table, the general trend in reactivity is to decrease, but why is this the case?

An atom is reactive if it is easy to remove the electrons for a chemical reaction. When atoms are smaller in size, their outer electrons are closer to their positive nucleus meaning more energy is needed to remove them. The larger the atom, the larger the separation between the nucleus and the electron, meaning it is easier to remove, and thus, the atom is more reactive.

We also see the general trend in atomic size across the period is to decrease, which gives the cause for this correlation.

The Reactivity series of period 4 metals, copper, iron and potassium ordered to show increasing reactivity

Figure 1 - The Reactivity series of period 4 metals. As the atomic radius increases, the reactivity increases.