Variation in Atomic and Ionic Sizes — Core Principles

Core Principles

The size of transition elements, represented by atomic or ionic radii, is influenced by effective nuclear charge ($Z_{eff}$), shielding effect, and electron-electron repulsion. Across a transition series, atomic radii initially decrease, then remain relatively constant in the middle, and may slightly increase towards the end.

This non-uniform trend is due to the interplay between increasing $Z_{eff}$ and the poor shielding by d-electrons, which is eventually balanced or overcome by electron-electron repulsion. Down a group, atomic radii increase from the 3d to 4d series.

However, a crucial anomaly is observed between the 4d and 5d series: elements in the 5d series have atomic and ionic radii almost identical to their 4d counterparts. This phenomenon is known as lanthanoid contraction, caused by the very poor shielding ability of the 4f electrons that are filled before the 5d orbitals.

Lanthanoid contraction leads to higher effective nuclear charge for 5d elements, resulting in smaller-than-expected sizes. Its consequences include similar sizes and chemical properties for 4d and 5d elements, and higher densities for 5d elements.

Important Differences

vs Atomic Radius vs. Ionic Radius

Aspect	This Topic	Atomic Radius vs. Ionic Radius
Definition	Atomic Radius: Half the internuclear distance between two identical atoms (metallic or covalent).	Ionic Radius: The effective distance from the nucleus to the outermost electron shell of an ion in an ionic crystal.
Measurement	Measured from metallic or covalent bonds in elemental form.	Estimated from internuclear distances in ionic compounds, considering charge and coordination number.
Relation to Parent Atom	Represents the size of a neutral atom.	Cations are smaller than their parent atoms; anions are larger than their parent atoms.
Factors Influencing	Effective nuclear charge, shielding effect, number of shells, electron-electron repulsion.	Effective nuclear charge, shielding effect, number of shells, electron-electron repulsion, and importantly, the charge of the ion (number of electrons lost/gained).
Trends (General)	Decreases across a period (with d-block exceptions), increases down a group (with lanthanoid contraction exception).	For a given charge, generally decreases across a period. Higher positive charge means smaller ionic radius for the same element.

Atomic radius describes the size of a neutral atom, typically measured in its elemental form (metallic or covalent radius). Ionic radius, conversely, refers to the size of an atom after it has gained or lost electrons to form an ion. A crucial distinction is that cations are always smaller than their parent atoms due to increased effective nuclear charge on fewer electrons, while anions are larger due to increased electron-electron repulsion and decreased effective nuclear charge per electron. Both are influenced by similar fundamental factors like nuclear charge and shielding, but the ionic charge introduces an additional layer of complexity to ionic radius trends.