Atoms are the fundamental building blocks of all matter, containing a dense, positively charged nucleus surrounded by a cloud of negatively charged electrons. These electrons are organized in energy levels or shells around the nucleus. The size of an atom, referred to as its atomic radius, is determined by the outermost boundary of this electron cloud. Atoms are typically electrically neutral, meaning they contain an equal number of protons and electrons. However, atoms frequently gain or lose electrons to achieve a more stable configuration, forming charged particles known as ions. This process significantly alters the atom’s original size.
Defining Anions and the Change in Radius
An atom becomes an anion when it gains one or more electrons, resulting in a net negative electrical charge. For example, a neutral chlorine atom gains a single electron to become a chloride ion with a charge of -1. When a neutral atom transforms into an anion, its radius invariably increases. The resulting ionic radius of the anion is always larger than the atomic radius of the parent atom. For instance, a neutral sulfur atom (104 picometers) expands to a sulfide ion (about 170 picometers) after gaining two electrons. This size increase is a direct consequence of the extra electron occupying space within the outermost electron shell, causing the entire electron cloud to swell outward.
The Mechanism of Radius Expansion
The expansion of the electron cloud in an anion is driven by two main physical effects: increased electron-electron repulsion and a change in the effective nuclear pull.
Increased Electron-Electron Repulsion
The first reason is the increased repulsion between the electrons themselves. When an electron is added to the atom’s outermost shell, the total number of negative charges increases. Since all electrons carry the same negative charge, they repel each other strongly. This increased electrostatic repulsion forces the electrons to spread out further in space, pushing the entire boundary of the electron cloud away from the nucleus.
Reduced Effective Nuclear Charge
The second effect relates to the balance of charge within the atom, known as the effective nuclear charge. The nucleus contains a fixed number of positively charged protons, which exert an attractive pull on all the surrounding electrons. This positive charge remains constant during ion formation.
When an extra electron is added, the fixed positive charge of the nucleus must now hold onto a greater number of negative charges. The existing electrons partially shield the outer electrons from feeling the full attractive force of the nucleus. Because the shielding effect increases with the addition of a new electron, the pull felt by each individual electron in the outermost shell becomes weaker. This reduced inward pull, combined with the enhanced outward push from electron-electron repulsion, allows the electron cloud to expand, resulting in a larger ionic radius.
How Cation Formation Differs
The change in size when an atom forms a cation provides a strong contrast to anion formation. A cation is formed when a neutral atom loses one or more electrons, resulting in a net positive charge. Unlike anions, the radius of a cation always decreases, making it smaller than its parent atom. This reduction in size is due to the opposite of the anion-forming mechanisms.
When an atom loses its outermost electron, electron-electron repulsion is reduced because there are fewer negative charges in the cloud. The remaining electrons are held by the same number of protons in the nucleus, but with less competition for the attractive force. This means the effective nuclear charge felt by each remaining electron increases.
The stronger net pull from the nucleus draws the remaining electron cloud inward, causing the cation to shrink. For instance, a neutral aluminum atom with a radius of 118 picometers shrinks to an ionic radius of only 68 picometers when it loses three electrons to form the aluminum ion.