Explain why atomic radius is smaller than it's ionic radius

Answer 1

The difference between atomic radius and ionic radius primarily stems from the changes in electron configuration and the resulting effective nuclear charge that occurs when an atom becomes an ion.

1. Atomic Radius: This is defined as the distance from the nucleus of an atom to the outermost electron shell. Atomic radius generally refers to neutral atoms, which have the same number of protons and electrons.

2. Ionic Radius: This refers to the size of an ion, which can be either a cation (positively charged) or an anion (negatively charged). The ionic radius is influenced by the charge of the ion and the number of electrons relative to protons.

Reasons Why Ionic Radius is Often Larger than Atomic Radius:

1. Formation of Cations: When an atom loses electrons to form a cation (e.g., Na → Na⁺), the atomic radius decreases. This is because the loss of one or more electrons reduces electron-electron repulsion in the outer shell and allows the remaining electrons to be pulled closer to the nucleus due to the increased effective nuclear charge (more protons than electrons). Thus, cations are typically smaller than their parent atoms.

2. Formation of Anions: When an atom gains electrons to form an anion (e.g., Cl → Cl⁻), the atomic radius increases. This is because the addition of one or more electrons increases electron-electron repulsion within the outer shell, causing the electrons to spread out more. Consequently, anions are generally larger than their parent atoms.

Summary:

• The atomic radius of an element pertains to its neutral atom, while the ionic radius pertains to its state as a charged ion.
• Cations (which have lost electrons) are smaller than their neutral atoms due to reduced electron repulsion and increased effective nuclear charge.
• Anions (which have gained electrons) are larger than their neutral atoms due to increased electron repulsion.

This property of ionic radius being larger (in the case of anions) and smaller (in the case of cations) compared to atomic radius is crucial in understanding chemical reactions, ionic compounds, and the behavior of elements in various states.