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Elements are composed of atoms, and each atom contains a nucleus surrounded by electrons. The nucleus, made of protons and neutrons, has a positive charge due to the presence of protons. Electrons, negatively charged particles, move in different energy levels (or orbitals) around the nucleus. The size of an atom is measured by its atomic radius, which is the distance from the nucleus to the outermost electron in the electron cloud. The number of electron shells and the effective nuclear charge (the net positive charge experienced by an electron) influence an atom's size.

Anions are negatively charged ions formed when an atom gains one or more electrons in its outermost electron shell. This electron gain leads to an increase in the negative charge of the atom, making it an anion. The process usually occurs in elements with higher electronegativity, which have a stronger tendency to attract electrons and form negative ions.

When an atom gains an electron to become an anion, the number of electron shells remains the same, but the additional electron(s) cause an increase in electron-electron repulsion within the outermost shell. This repulsion forces the electrons to spread out and occupy more space, which results in an expansion of the electron cloud and an increase in the atomic radius. Furthermore, the effective nuclear charge decreases when an atom becomes an anion because there are now more negatively charged electrons than positively charged protons. The reduced net positive charge from the nucleus means that the electrons in the outermost shell are less strongly attracted towards the nucleus, allowing them to spread out and increase the atomic radius further.

Anions are always larger than the atoms from which they are formed because the additional electron(s) increase electron-electron repulsion within the outermost shell and cause a decrease in the effective nuclear charge. The combination of these factors leads to an expansion of the electron cloud and an increase in atomic radius, making anions larger than their corresponding neutral atoms.