This type of radiation is smallest and highest in energy

As your question is vague, I am assuming that you are talking Alpha, Beta and Gamma radiation. Out of these three radiation, Gamma radiation is the smallest in size compared to Alpha or Beta, but it has the highest energy levels. Gamma radiation is also known as photons. In other words, photons are light particles.

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.