Atomic radius is not a single directly measurable hard boundary; several operational definitions are used, such as covalent, metallic and van der Waals radii.
Atomic radius is not a single directly measurable hard boundary; several operational definitions are used, such as covalent, metallic and van der Waals radii.
The definition must be stated before comparing values. Periodic trends are useful, but different radius definitions should not be mixed.
A sodium atom is larger than a chlorine atom in the same period because effective nuclear attraction increases across the row.
Atomic radius is not a single directly measurable hard boundary; several operational definitions are used, such as covalent, metallic and van der Waals radii.
The definition must be stated before comparing values. Periodic trends are useful, but different radius definitions should not be mixed.
Atomic radius is not a single directly measurable hard boundary; several operational definitions are used, such as covalent, metallic and van der Waals radii.
The definition must be stated before comparing values. Periodic trends are useful, but different radius definitions should not be mixed.
For two identical atoms joined by a covalent bond, a covalent radius is commonly related to half the internuclear distance. A van der Waals radius describes a larger contact distance for atoms that are not covalently bonded. Metallic and ionic radii are defined in still other structural contexts.
Because the definition changes with the physical situation, a table of radii must be read with its method attached. Comparing one covalent-radius dataset with one van der Waals-radius dataset as if they were interchangeable can produce fake “exceptions”.
Moving from left to right across a period adds protons to the nucleus and adds electrons mainly to the same principal shell. Inner electrons screen part of the nuclear charge, but the effective attraction felt by the valence electrons generally increases. Their distribution is pulled inward, so characteristic atomic radii usually decrease across a period.
This is a trend, not a smooth mathematical line. Electron configuration, the particular radius definition and bonding environment all affect measured or tabulated values.
Moving down a group adds an occupied principal shell. Valence electrons are then found in states whose characteristic extent is farther from the nucleus, and inner shells provide substantial screening. The increase in shell number usually dominates the simultaneous increase in nuclear charge, so atoms generally become larger down a group.
Size helps explain why ionization energy and bonding behavior change across the periodic table. A valence electron that is, on average, farther from the nucleus and more strongly screened is usually easier to remove than one held in a compact distribution by a larger effective nuclear attraction.
The same logic also explains why cations are typically smaller than their parent atoms and anions are typically larger, although ionic radii depend strongly on charge and coordination environment.
The phrase “chlorine is smaller than sodium” is useful when both values come from a comparable radius convention. It is less meaningful when the values come from unrelated definitions or structures. Radius is therefore best treated as a model-dependent size descriptor that becomes powerful when comparisons are made consistently.
Between Na and Cl in period 3, which generally has the larger covalent/atomic size, and why?
Na is generally larger. Across the period, electrons are added to the same main shell while effective nuclear attraction increases, pulling the valence distribution inward toward Cl.