Variation in Atomic and Ionic Sizes — Definition
Definition
When we talk about the 'size' of an atom or an ion, we're essentially referring to its atomic radius or ionic radius. Imagine an atom as a tiny sphere; its radius is half the distance between the nuclei of two identical atoms bonded together.
For metals, we often use the metallic radius (half the internuclear distance between two adjacent metal atoms in a metallic crystal lattice), and for non-metals, the covalent radius (half the internuclear distance between two identical atoms joined by a single covalent bond).
Ionic radius, on the other hand, is the effective distance from the nucleus to the outermost electron shell of an ion in an ionic crystal. These sizes are not fixed values but depend on the bonding environment.
\n\nNow, let's consider how these sizes change, especially for the transition elements (the d-block elements). Several factors influence the size of an atom or ion. Firstly, the **effective nuclear charge ()** is the net positive charge experienced by an electron in a multi-electron atom.
As you move across a period, the number of protons in the nucleus increases, leading to a stronger pull on the electrons, which tends to decrease the atomic size. Secondly, the shielding or screening effect is the reduction in the effective nuclear charge on the outer electrons due to the presence of inner-shell electrons.
Inner electrons 'shield' the outer electrons from the full nuclear charge. Thirdly, the number of electron shells plays a role; adding a new shell significantly increases the size as you move down a group.
Lastly, electron-electron repulsion within the same shell can cause a slight expansion. \n\nFor transition elements, these factors interact in unique ways. As you move across a transition series (e.
g., from Sc to Zn in the 3d series), you'd expect a continuous decrease in size due to increasing . However, the d-electrons, which are being added, are not very effective at shielding the outer s-electrons from the increasing nuclear charge.
This leads to an initial decrease, followed by a relatively constant size in the middle of the series, and sometimes a slight increase towards the end. This 'near constancy' is a hallmark of transition elements.
\n\nWhen moving down a group (e.g., from Sc to Y to La), you generally expect an increase in size due to the addition of new electron shells. This holds true from the 3d to the 4d series. However, a fascinating anomaly occurs when comparing the 4d and 5d series elements.
The elements of the 5d series are almost identical in size to their counterparts in the 4d series. This unexpected similarity is attributed to a phenomenon called lanthanoid contraction, which is a critical concept for NEET aspirants.
This contraction arises because of the poor shielding ability of the 4f electrons, which are filled before the 5d orbitals, leading to a greater effective nuclear charge and thus a smaller than expected size for the 5d elements.