Answer:
Atoms tend to get smaller as you move across the periodic table due to an increase in the effective nuclear charge. The effective nuclear charge is the positive charge experienced by the outermost electrons of an atom.
As you move across a period from left to right, the number of protons in the nucleus increases, leading to a greater positive charge. While the number of electrons also increases, the additional electrons are added to the same energy level or shell. Therefore, the increase in positive charge exerts a stronger pull on the electrons, resulting in a stronger electrostatic attraction between the nucleus and the outermost electrons.
As a result, the outermost electrons are drawn closer to the nucleus, causing a decrease in the atomic radius and making the atom smaller. This phenomenon is known as the trend of decreasing atomic size or increasing electronegativity across a period in the periodic table.