Question

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?

Short answer

As we move across a period on the periodic table, the sizes of transition elements change more gradually than those of the representative elements because the electron shielding provided by d electrons in transition elements is less efficient. This causes a smaller contrast in effective nuclear charge experienced by the valence electrons, leading to a more gradual decrease in atomic size.

Step-by-step solution

Define Atomic Size

Atomic size refers to the distance between the nucleus of an atom and its outermost valence electrons. This distance is generally represented by the atomic radius. As we move across a period on the periodic table, atomic size generally decreases, except for certain cases in transition elements.

Understand the Concept of Effective Nuclear Charge

Effective nuclear charge is the net positive charge experienced by an electron in an atom. An increase in the effective nuclear charge means that the outermost electrons are more strongly attracted to the nucleus, causing a decrease in atomic size. As we move across a period, the number of protons in the nucleus increases, leading to an increase in effective nuclear charge.

Understand Electron Shielding

Electron shielding is the phenomenon where the outermost electrons in an atom are partially shielded from the positive charge of the nucleus by the inner electrons. The more shielding an outer electron experiences, the less it will be attracted to the nucleus, causing an increase in atomic size. The electron shielding effect remains relatively constant as we move across a period because the electrons are being added to the same energy level.

Compare Representative and Transition Elements in Terms of Effective Nuclear Charge and Electron Shielding

Representative elements have their valence electrons in the s and p orbitals, while transition elements have their valence electrons in the d orbitals. As we move across a period, both groups experience an increase in effective nuclear charge due to the addition of protons to the nucleus. However, the electron shielding effect in representative elements remains relatively constant, whereas in transition elements, the electron shielding provided by d electrons is less efficient due to their larger size and shape.

Conclude with the Answer to the Question

As we move across a period on the periodic table, the sizes of transition elements change more gradually than those of the representative elements because the electron shielding provided by d electrons in transition elements is less efficient. This causes a smaller contrast in effective nuclear charge experienced by the valence electrons, leading to a more gradual decrease in atomic size.