Sodium (Na) has a greater atomic size than Lithium (Li). This difference is a direct consequence of periodic trends, which govern the structure and properties of elements. Both elements belong to Group 1 of the periodic table, but their distinct locations result in a measurable difference in their atomic dimensions.
Understanding Atomic Radius
Atomic radius is the measure used to define the size of an atom. Since atoms do not have a fixed boundary, their size is typically determined by measuring the distance between the nuclei of two bonded, identical atoms and taking half of that distance (metallic radius for metals like Lithium and Sodium). Atomic size is a balance between the attractive positive charge of the nucleus and the repulsive negative forces between the orbiting electrons.
The effective size is determined by the distance of the outermost electrons (valence electrons) from the central nucleus. A stronger attraction between the nucleus and the valence electrons pulls the electron cloud inward, resulting in a smaller atom. Conversely, a weaker attraction allows the electron cloud to expand outward, creating a larger atomic radius. Atomic radii are typically measured in picometers (pm).
How Electron Shells Determine Size
The main factor determining the increase in atomic size when moving down a group on the periodic table is the addition of electron shells. Electrons exist in distinct energy levels, labeled by the Principal Quantum Number (\(n\)). Each step down a group adds a new, filled shell, placing the outermost valence electrons progressively farther from the nucleus.
The presence of these inner-shell electrons introduces electron shielding, or the screening effect. Inner electrons effectively block a portion of the nucleus’s positive charge from reaching the valence electrons. This acts as a partial barrier, reducing the net attractive force (effective nuclear charge) experienced by the valence electrons. When valence electrons are not pulled as strongly toward the center, the electron cloud expands, leading to a larger atomic size.
Applying the Trend to Sodium and Lithium
Lithium (Li) and Sodium (Na) illustrate the effect of adding electron shells, as both are in Group 1. Lithium is located in Period 2 and has electrons occupying two principal quantum shells. Its single valence electron resides in the second shell (\(n=2\)), shielded only by the two electrons in the innermost first shell.
Sodium is in Period 3 and has electrons occupying three principal quantum shells. Its single valence electron is located in the third shell (\(n=3\)), one full shell deeper than Lithium’s. This valence electron is shielded by ten inner-shell electrons (two in the first shell and eight in the second), providing significantly more screening than Lithium’s two inner electrons.
The greater distance of the valence electron from the nucleus, combined with the stronger shielding effect from the additional inner shell, causes Sodium’s electron cloud to be substantially larger. The atomic radius of Lithium is approximately 152 picometers (pm), while Sodium’s is approximately 186 pm. The increase in principal quantum shells is the dominant factor that overrides the increased nuclear charge, resulting in Sodium’s greater atomic size.