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Ionic radius Knowledge-Based Study

Chemistry · The Periodic Table · Chemistry · Periodic Trends

Chemistry topic explanations on Ionic radius.

Study notes

  1. What Is Ionic Radius?

    The ionic radius is the measure of an atom's ion in a crystal lattice or ionic compound. It represents the distance from the nucleus to the outer boundary of the electron cloud in an ion. Because ions are formed when neutral atoms lose or gain valence electrons, their sizes differ significantly from their corresponding parent neutral atoms.

    Cation Formation and Size

    When a neutral metal atom loses one or more electrons, it forms a positively charged ion called a cation.

    • A cation is always smaller than its parent neutral atom.
    • Loss of a valence shell: In many representative elements, losing all valence electrons eliminates the entire outermost energy level (principal quantum number n). For example, neutral sodium has the electron configuration 2-8-1, while the sodium cation (Na+) has the configuration 2-8.
    • Increased effective nuclear charge (Zeff) per electron: With fewer electrons remaining to experience the pull of the same number of protons in the nucleus, electron-electron repulsions decrease and the remaining electrons are pulled closer to the nucleus.

    For example, the atomic radius of neutral sodium (Na) is approximately 186 pm, whereas the ionic radius of Na+ is only 102 pm.

    Anion Formation and Size

    When a neutral nonmetal atom gains one or more electrons to attain a stable valence octet, it forms a negatively charged ion called an anion.

    • An anion is always larger than its parent neutral atom.
    • Increased electron-electron repulsion: Adding electrons to the same energy level increases electrostatic repulsions among valence electrons. To minimize these repulsive forces, the electron cloud expands outward.
    • Decreased pull per electron: The positive nuclear charge (proton number, Z) remains constant while the electron count increases, meaning each electron experiences a lower effective nuclear pull.

    For example, neutral chlorine (Cl) has a covalent atomic radius of approximately 99 pm, whereas the chloride anion (Cl-) expands to an ionic radius of about 181 pm.

    Summary Comparison of Parent Atoms and Ions

    • Cation radius < Neutral atomic radius: e.g., Al3+ < Al
    • Anion radius > Neutral atomic radius: e.g., O2- > O

    Trends in the Periodic Table

    The variation of ionic radius across the periodic table follows systematic trends:

    • Down a Group: Moving down any group, ionic radii increase for ions of the same charge (e.g., Li+ < Na+ < K+ < Rb+). This increase occurs because additional principal energy levels (shells) are occupied, placing outer electrons farther from the nucleus and providing greater shielding.
    • Across a Period: When comparing ions of the same type within a period, cations generally decrease in size with increasing positive charge (e.g., Na+ > Mg2+ > Al3+). Similarly, anions decrease in size across a period as positive nuclear charge increases (e.g., P3- > S2- > Cl-).

    Isoelectronic Series

    An isoelectronic series consists of atoms or ions that possess the exact same total number of electrons and identical ground-state electron configurations.

    For example, consider the 10-electron series:

    N3-, O2-, F-, Ne, Na+, Mg2+, Al3+

    In an isoelectronic series, all species have identical electron shielding and configuration, but the number of protons increases monotonically from nitrogen (Z = 7) to aluminum (Z = 13):

    • Higher nuclear charge (Z): A greater number of protons exerts a stronger electrostatic attraction on the identical 10 electrons, pulling them closer to the core.
    • Radius order: N3- (171 pm) > O2- (140 pm) > F- (133 pm) > Na+ (102 pm) > Mg2+ (72 pm) > Al3+ (54 pm).

    Therefore, within any isoelectronic series, the species with the highest atomic number has the smallest ionic radius, and the species with the lowest atomic number has the largest ionic radius.