After one half life ___% of the atoms will change to a stable

the answer is 30% if the atoms will change to a stable

beta decay is the answer

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.