The correct answer is option 4, Atomic radius.

The atomic radius refers to the size of an atom. It can be defined as the distance between the nucleus and the outermost electron orbital.

As we move from left to right across the periodic table, the atomic radius generally decreases. This is because the number of protons and electrons increases, resulting in a stronger attractive force between the nucleus and the electrons. As a result, the electrons are pulled closer to the nucleus, making the atomic radius smaller.

On the other hand, as we move from top to bottom in a group or column of the periodic table, the atomic radius generally increases. This is because each subsequent element in a group has an additional electron shell, further away from the nucleus. The additional electron shells shield the outer electrons from the attractive force of the nucleus, causing the atomic radius to increase.

It is important to note that while other properties mentioned in the options, such as ionization energy, electron affinity, and electronegativity, do exhibit trends across the periodic table, they do not follow the same pattern of decreasing from left to right and increasing from top to bottom.