As we move across a period of the periodic table, why do the sizes of the transition elements change more gradually than those of the representative elements?

Atomic size refers to the distance between the nucleus and the outermost electrons of an element. It depends on various factors such as atomic number, screening or shielding effect, effective nuclear charge, and the number of energy levels.

In general, atomic size decreases as we move from left to right across a period of the periodic table. This is mainly due to the increase in atomic number (number of protons in the nucleus) which causes a greater effective nuclear charge attracting the electrons, pulling them closer to the nucleus.

Representative elements or main group elements are those elements that belong to groups 1, 2, and 13-18 of the periodic table. They exhibit a more predictable trend in their properties, including atomic size, compared to transition elements. Their outermost electrons are present in the same energy level or shell, which directly affects the atomic size.

Transition elements are found in the d-block of the periodic table. As we move from left to right across a period of transition elements, the additional electrons go into the inner d-orbital, which is closer to the nucleus and have a slightly poor shielding effect. Due to this poor shielding effect, the electrons in the outermost shell experience a higher effective nuclear charge that gradually pulls the outer electrons closer to the nucleus.

Transition elements change more gradually in size compared to representative elements because the additional electrons in the inner d-orbital of transition elements provide poor shielding. For representative elements, the outermost electrons are in the same energy level, and the shielding effect remains constant across the period. In summary, the sizes of the transition elements change more gradually than those of the representative elements due to the effect of inner d-orbital electrons providing poorer shielding compared to the representative elements. This leads to increased effective nuclear charge on outer electrons and the atomic size decreases less abruptly as we move from left to right across a period in the d-block of the periodic table.