Atomic radius describes the size of an atom. It is often considered the distance from an atom’s nucleus to its outermost electron orbital. Understanding atomic size is significant in chemistry because it influences how atoms interact with one another and form chemical bonds. It also influences various physical and chemical properties of elements, such as reactivity, electronegativity, and bond formation.
What Atomic Radius Represents
Atomic radius is not a fixed boundary, but a measure of an atom’s electron cloud size. Electrons do not have sharply defined orbits; instead, their positions are described as probability distributions that gradually taper off from the nucleus. To standardize its measurement, atomic radius is often defined as one-half the distance between the nuclei of two identical atoms bonded together. For example, in a hydrogen molecule (H2), the distance between the two hydrogen nuclei is 74 picometers, meaning the atomic radius of a single hydrogen atom is 37 picometers.
The Impact of Electron Shells
A primary reason for an increase in atomic radius is the addition of electron shells. As one moves down a group in the periodic table, new electron shells are added to the atoms. Each new shell places the outermost electrons further from the nucleus, leading to a larger atomic size. For instance, comparing lithium (Li) to sodium (Na) in the first group, sodium has an additional electron shell compared to lithium, making it larger.
Understanding Electron Shielding
Electron shielding, also known as the screening effect, contributes to the increase in atomic size. Inner-shell electrons, those closer to the nucleus, repel the outer valence electrons. This repulsion, combined with the attraction of the inner electrons to the nucleus, effectively “shields” the outer electrons from the full attractive pull of the positive nucleus.
This phenomenon reduces the “effective nuclear charge” that the outermost electrons experience. With a diminished pull from the nucleus, the valence electrons are less tightly held and spread out over a larger volume, increasing the atomic radius. The more electron shells an atom possesses, the greater the shielding effect, as there are more inner electrons to block the nuclear charge from the outer electrons.
Electron Repulsion in Ion Formation
The formation of an anion (a negatively charged ion) also increases atomic radius compared to the neutral atom. When a neutral atom gains one or more electrons, the total number of electrons in its electron cloud increases. These additional electrons introduce greater electron-electron repulsion within the atom’s electron cloud.
This increased repulsion causes the electron cloud to expand, leading to a larger ionic radius for the anion. Even though the nuclear charge remains constant, the added electrons and their repulsive forces cause existing electrons to spread out, resulting in a larger species. This expansion is distinct from periodic trends and is a direct consequence of the altered electron-to-proton ratio in the newly formed ion.