Question

Why does atomic radius decrease as you move from left to right across a period or row of elements, and why does reactivity increase as you move down a group or column of elements?

Answer 1

Atomic radius decreases because you are adding protons to the nucleus as you move from the left to the right of the period. These additional protons pull with more force on the electrons outside the nucleus, drawing them all in closer, and making the radius of the atom smaller.

Further, let's consider what makes a metal reactive? its outer electrons. the alkali group (group 1) are the most reactive because they only have one outer electron in their outermost energy level. speaking of outer energy levels, the period names on the periodic table signal the outermost level for elements in that period. so as you go down the group, you add one more energy level than the previous element. the addition of energy levels makes the elements increase in size going down a column. okay, back to the reactivity. as the atomic size gets bigger, the outer electron gets farther from the nucleus, giving it less of a pull. shielding effect also takes place (each energy level blocks the electrons further out). therefore, even though the outer electron has the same amount of charge, it does not feel the same amount of attraction when the atomic size increases. this makes it easier for another atom (probably a halogen) to take the outer electron.
in short: the reactivity increases down a column, even Group I, because the atomic size gets bigger, which pushes the outer electron away from the nucleus, making it easier for another element to take.