Why is it difficult to define the atomic radius of an isolated atom?

Defining the atomic radius of an isolated atom is challenging because the electron cloud surrounding the nucleus does not have a sharp, well-defined boundary. According to quantum mechanics, there's a probability distribution for finding electrons, which extends infinitely, albeit with rapidly decreasing probability, away from the nucleus. This means there's no distinct 'edge' to measure. Therefore, atomic radius is typically defined based on the internuclear distance between two atoms in a bonded or non-bonded state, providing a practical and measurable approximation of atomic size.

What is the difference between covalent radius and van der Waals radius?

The covalent radius is half the internuclear distance between two identical atoms that are chemically bonded (sharing electrons) in a molecule. It represents the effective size of an atom when it forms a covalent bond. The van der Waals radius, conversely, is half the internuclear distance between two identical non-bonded atoms belonging to adjacent molecules that are in closest contact. It reflects the size of an atom when it is not chemically bonded but interacting through weak intermolecular forces. Consequently, van der Waals radii are always larger than covalent radii for the same element, as they account for the non-overlapping electron clouds of separate entities.

Why are cations smaller than their parent atoms?

Cations are formed when an atom loses one or more electrons. This loss of electrons typically results in the removal of the outermost electron shell, significantly reducing the atom's size. Even if the outermost shell is not completely lost, the remaining electrons experience a stronger effective nuclear charge because the number of protons (nuclear charge) remains constant while the number of electrons decreases. This increased attraction pulls the electron cloud closer to the nucleus, leading to a smaller ionic radius compared to the neutral parent atom.

Why are anions larger than their parent atoms?

Anions are formed when an atom gains one or more electrons. This addition of electrons increases the electron-electron repulsion within the outermost electron shell. To minimize this increased repulsion, the electron cloud expands, pushing the electrons further away from the nucleus. Since the nuclear charge (number of protons) remains the same, it cannot effectively pull the expanded electron cloud as tightly. This results in a larger ionic radius for the anion compared to its neutral parent atom.

Explain the trend of atomic radius across a period.

Across a period (from left to right) in the periodic table, the atomic radius generally decreases. This is because as we move across a period, electrons are added to the same principal energy shell. Simultaneously, the nuclear charge (number of protons) increases. The increased positive charge of the nucleus exerts a stronger attractive force on the valence electrons. Although there is some increase in electron-electron repulsion, the effect of the increasing nuclear charge (and thus increasing effective nuclear charge) dominates, pulling the electron cloud closer to the nucleus and reducing the atomic size.

Atomic Radius and Ionic Radius

Chemistry

NEET UG

Version 1Updated 21 Mar 2026

Explore This Topic

Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

Atomic radius is fundamentally defined as half the distance between the nuclei of two identical atoms bonded together, either covalently or metallically. For non-bonded atoms, it refers to half the internuclear distance between two non-bonded atoms of the same element when they are in contact. Ionic radius, on the other hand, represents the effective distance from the center of the nucleus to the …

Quick Summary

Atomic radius and ionic radius are fundamental periodic properties that describe the size of atoms and ions, respectively. Atomic radius is not a fixed value but is defined based on the internuclear distance in different bonding scenarios: covalent radius (half the bond length in a covalent molecule), metallic radius (half the internuclear distance in a metal lattice), and van der Waals radius (half the distance between non-bonded atoms in contact).

Generally, $r_{vdW} > r_{met} > r_{cov}$ . Key factors influencing atomic radius are nuclear charge (pulls electrons closer), number of electron shells (increases size), and shielding effect (reduces effective nuclear charge).

Across a period, atomic radius decreases due to increasing effective nuclear charge. Down a group, it increases due to the addition of new electron shells. Ionic radius refers to the size of an ion. Cations (positive ions) are always smaller than their parent atoms due to electron loss and increased effective nuclear charge.

Anions (negative ions) are always larger than their parent atoms due to electron gain and increased electron-electron repulsion. For isoelectronic species, ionic radius decreases with increasing nuclear charge.

These radii are crucial for understanding chemical bonding, crystal structures, and reactivity.

Vyyuha

Your 6-Month Blueprint, Updated Nightly

AI analyses your progress every night. Wake up to a smarter plan. Every. Single.…

Key Concepts

Periodic Trend of Atomic Radius Across a Period

As you move from left to right across a period, the atomic radius generally decreases. This is because…

Comparison of Cation, Anion, and Parent Atom Size

The size relationship between an atom and its ions is a critical concept. Cations are always smaller than…

Isoelectronic Species and Ionic Radius

Isoelectronic species are atoms or ions that possess the same number of electrons. For such a series, the…

Atomic Radius: — Half internuclear distance.

- Covalent ( $r_{cov}$ ): Bonded atoms, electron overlap. - Metallic ( $r_{met}$ ): Metal lattice. - Van der Waals ( $r_{vdW}$ ): Non-bonded atoms, no overlap. - Order: $r_{vdW} > r_{met} > r_{cov}$ .

Ionic Radius: — Radius of an ion.

- Cation ( $M^+$ ): Smaller than parent atom ( $M$ ). - Anion ( $X^-$ ): Larger than parent atom ( $X$ ).

Trends:

- Across Period (L to R): Atomic/Ionic radius $\downarrow$ (due to $\uparrow Z_{eff}$ ). - Down Group (T to B): Atomic/Ionic radius $\uparrow$ (due to $\uparrow$ shells).

Isoelectronic Species: — Same number of electrons. Size

\propto \frac{1}{\text{Nuclear Charge (Z)}}

Factors: — Nuclear charge (Z), No. of shells (n), Shielding effect, Electron-electron repulsion.

Cats Shrink, Ants Grow. Period Decreases, Group Increases. Iso-Nuclear Shrinks.