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Bot GPT-4o The radius of an atom or ion, often referred to as the atomic radius or ionic radius, is a measure of the size of the atom or ion. Here's what you need to know about each:
### Atomic Radius
- **Definition**: The atomic radius is the distance from the center of an atom's nucleus to the outermost electron shell.
- **Trend in the Periodic Table**:
- **Across a Period**: The atomic radius decreases from left to right across a period. This is because as the number of protons in the nucleus increases, the effective nuclear charge experienced by the electrons also increases, pulling the electron cloud closer to the nucleus.
- **Down a Group**: The atomic radius increases as you move down a group. This is because additional electron shells are being added, which are further from the nucleus.
### Ionic Radius
- **Definition**: The ionic radius is the radius of an atom's ion, which can be larger or smaller than the atomic radius depending on whether the ion is positively charged (cation) or negatively charged (anion).
- **Cations**: When an atom loses electrons to form a cation, it usually becomes smaller than its neutral atom. This contraction occurs because the loss of electrons results in a stronger effective nuclear charge acting on the remaining electrons.
- **Anions**: When an atom gains electrons to form an anion, it usually becomes larger than its neutral atom. This expansion happens because the addition of extra electrons results in increased electron-electron repulsion and a lesser effective nuclear charge experienced per electron.
### Specific Values
- **Atomic Radii**: Typically range from about 30 pm (picometers) to 300 pm.
- **Ionic Radii**: Varies significantly but can be typically around 30 pm to 200 pm for cations and 140 pm to 230 pm for anions.
### Examples
- **Helium (He)**: Atomic radius is about 31 pm.
- **Sodium (Na)**: Atomic radius is about 186 pm; the ionic radius when it forms Na⁺ is about 102 pm.
- **Chlorine (Cl)**: Atomic radius is about 99 pm; the ionic radius when it forms Cl⁻ is about 181 pm.
These radii are not fixed distances but rather approximate ranges given the probabilistic nature of electron clouds. Measurements can vary slightly depending on the method used and the specific context of the atom or ion (e.g., within a molecule, crystal lattice, etc.).