Using only a periodic table as a guide, arrange each of the...

Using only a periodic table as a guide, arrange each of the following series of species in order of increasing size.
(a) $\mathrm{Li}, \mathrm{Be}^{2+}, \mathrm{Be}$
(b) $\mathrm{S}, \mathrm{S}^{2-}, \mathrm{Cl}$
(c) $\mathrm{O}, \mathrm{S}, \mathrm{Si}$

Key Concepts

Periodic Trends

Periodic trends are predictable patterns observed in the properties of elements as one moves across or down the periodic table. Size, or radius, is a key periodic trend that depends on both the number of electron shells and the effective nuclear charge, allowing predictions about relative sizes of atoms and ions using only the periodic table.

Isoelectronic Species

Isoelectronic species are atoms or ions that have the same number of electrons. When comparing their sizes, the species with the higher nuclear charge will generally be smaller because the increased positive charge more strongly attracts the same number of electrons, pulling them closer to the nucleus.

Effective Nuclear Charge

Effective nuclear charge is the net positive charge experienced by electrons in an atom. It accounts for the shielding effect of inner-shell electrons. Higher effective nuclear charge pulls electrons closer to the nucleus, leading to a smaller atomic or ionic size, which explains trends across periods or within isoelectronic series.

Atomic and Ionic Radii

Atomic and ionic radii refer to the sizes of atoms and ions, respectively. In the periodic table, atomic radii generally decrease from left to right due to increasing nuclear charge attracting electrons closer, and increase down a group due to added electron shells. For ions, sizes can be significantly altered compared to their neutral atoms; cations are typically smaller than their parent atoms because they have lost electrons, while anions are larger due to added electron–electron repulsion.

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