What is atomic radius?

Short Answer

Atomic radius is the distance from the nucleus of an atom to the outermost electron. It gives an idea of the size of an atom.

The atomic radius varies across the periodic table: it decreases across a period due to increasing nuclear charge pulling electrons closer, and it increases down a group as new electron shells are added, making the atom larger.

Detailed Explanation :

Definition of Atomic Radius

Atomic radius is defined as the distance from the nucleus of an atom to its outermost electron shell. It is a measure of the size of an atom, although the exact boundary of an atom is not sharply defined due to the electron cloud.

  • Measured in picometers (pm) or angstroms (Å).
  • Helps in understanding bond lengths, reactivity, and physical properties of elements.

Factors Affecting Atomic Radius

  1. Nuclear Charge (Number of Protons)
    • Across a period, atomic number increases → more protons → greater pull on electrons → smaller radius.
  2. Number of Electron Shells
    • Down a group, new shells are added → electrons are farther from the nucleus → larger radius.
  3. Electron-Electron Repulsion
    • Inner electrons shield outer electrons from nuclear charge, influencing atomic size.
  4. Subatomic Particle Distribution
    • Distribution of electrons in different orbitals (s, p, d, f) also affects the radius.

Trends in the Periodic Table

  1. Across a Period (Left to Right)
    • Atomic radius decreases across a period.
    • Reason: Increased nuclear charge pulls electrons closer.
    • Example: Lithium (Li) → 152 pm, Fluorine (F) → 64 pm
  2. Down a Group (Top to Bottom)
    • Atomic radius increases down a group.
    • Reason: Additional electron shells increase distance between nucleus and outer electrons.
    • Example: Lithium (Li) → 152 pm, Cesium (Cs) → 265 pm

Types of Atomic Radius

  1. Covalent Radius
    • Half the distance between nuclei of two bonded atoms.
  2. Metallic Radius
    • Half the distance between adjacent metal atoms in a metallic lattice.
  3. Van der Waals Radius
    • Distance between non-bonded atoms in a crystal or molecule.

Importance of Atomic Radius

  1. Predicts Reactivity
    • Smaller radius → stronger nuclear pull → affects ionization and electronegativity.
    • Larger radius → more reactive metals in Group 1.
  2. Bond Formation
    • Determines bond lengths in molecules and compounds.
  3. Periodic Trends Analysis
    • Helps explain trends in ionization energy, electronegativity, and metallic character.
  4. Chemical and Physical Properties
    • Influences melting point, boiling point, and density of elements.

Examples of Atomic Radius Trends

  1. Period 2 Elements:
    • Lithium (152 pm) → Beryllium (112 pm) → Boron (87 pm) → Carbon (67 pm) → Oxygen (60 pm) → Fluorine (64 pm)
    • Shows decrease across the period.
  2. Group 1 Elements:
    • Lithium (152 pm), Sodium (186 pm), Potassium (227 pm), Rubidium (248 pm), Cesium (265 pm)
    • Shows increase down the group.

Applications

  • Predicting ionic sizes and chemical reactivity.
  • Determining bond lengths and molecular structures.
  • Explaining trends in electropositivity, electronegativity, and metallic/non-metallic character.
  • Used in material science, nanotechnology, and chemical engineering.
Conclusion

Atomic radius measures the size of an atom from its nucleus to the outermost electron. It decreases across a period due to increasing nuclear charge and increases down a group due to addition of electron shells. Understanding atomic radius helps explain periodic trends, reactivity, bonding, and physical properties, making it a fundamental concept in chemistry.