Explain the trend in reactivity along period 3 of the periodic table?

As you move from left to right along period 3 (from K to Kr) the amount of protons in the nucleus increases therefore proton density increases. Shielding stays the same as it remains in the same electron orbital (shielding increases when more orbitals are occupied between the protons in the nucleus and electrons in the outer orbital). Therefore the atomic radius will decrease as there is a stronger attraction between the protons in the nucleus and electrons in the outer orbital. This means that reactivity will decrease.