The atomic radius describes the size of a neutral atom, typically measured as half the distance between the nuclei of two identical atoms bonded together. When an atom gains or loses electrons, it transforms into an ion, and its size, now called the ionic radius, changes significantly. Positively charged ions, known as cations, are always smaller than their parent atoms, a fact explained by examining the fundamental forces within the atomic structure.
Defining Cations and Atomic Structure
Atoms are electrically neutral because they contain an equal number of positively charged protons in the nucleus and negatively charged electrons orbiting that nucleus. An ion is an atom that has gained or lost one or more electrons, resulting in a net electrical charge. A cation forms when a neutral atom loses one or more electrons from its outermost energy shell, known as the valence shell.
The removal of negative charge leaves the atom with more protons than electrons, creating a positive charge, such as Na+ or Ca2+. For instance, a neutral sodium atom (Na) has 11 protons and 11 electrons, but it readily loses its single valence electron to become a sodium cation (Na+). This process results in a stable ion with 11 protons and only 10 electrons.
The Size Difference: Why Cations Are Smaller
The positive charge of a cation ensures that its ionic radius is smaller than the atomic radius of its neutral parent atom. The immediate physical consequence of losing one or more electrons is a contraction of the electron cloud surrounding the nucleus. This size reduction occurs for two distinct, yet related, reasons.
In many cases, the formation of a cation involves the complete removal of the outermost electron shell. The sodium atom, for example, loses its single electron from the third energy shell to become an ion that only utilizes two shells, leading to a substantial decrease in size. Even when an entire shell is not lost, the reduced number of electrons means the electron cloud occupies a smaller volume. A neutral sodium atom has an atomic radius of about 186 picometers, while its cation, Na+, has an ionic radius of only about 102 picometers, illustrating a significant shrinkage.
The Core Mechanism: Effective Nuclear Charge
The primary physical explanation for the size reduction is a phenomenon called the effective nuclear charge. This concept describes the net positive pull experienced by an atom’s outermost electrons from the nucleus, factoring in the shielding effect of the inner electrons. The nucleus’s actual positive charge is determined by the number of protons, which remains unchanged during ion formation.
When a neutral atom loses an electron to become a cation, the number of protons stays the same, but the number of electrons decreases. Consequently, the fixed positive charge of the nucleus is now distributed across fewer remaining electrons, increasing the nuclear force felt by each individual electron. This increased electrostatic attraction, or greater effective nuclear charge, pulls the remaining electron cloud inward, resulting in a smaller overall ionic radius. The tighter grip the nucleus holds on its fewer electrons is the underlying mechanism.