Atomic Radius

Introduction to Periodicity

• Periodicity: The recurring and predictable pattern of element properties within the periodic table.
• Significance: Illustrates how trends such as atomic radius and electronegativity repeat consistently, facilitating the understanding of fundamental chemistry concepts like chemical bonding patterns.

Organisation of the Periodic Table

• The periodic table is structured by increasing atomic number, segmenting elements into rows (periods) and columns (groups).

• Historically, the organisation of elements evolved from being based on atomic mass to the more precise system of atomic number.

Predicting Element Properties

• Role of Periodicity:
  • Facilitates predictions such as a decrease in atomic radius across a period.
• Electron Configurations:
  • Fundamental in enabling these predictions.

Introduction to Atomic Radius

• Atomic Radius: The distance from an atom's nucleus to its outermost electron shell. It is crucial for predicting chemical reactions, understanding reactivity, and determining material properties.

Types of Atomic Radius Measurements

• Covalent Radius: Half the distance between the nuclei of two bonded atoms.

• Metallic Radius: Half the distance between the nuclei in a metallic lattice.

• Van der Waals Radius: The effective distance between non-bonded atoms affected by van der Waals forces.

Variations Across the Periodic Table

• Trends Across Periods:
  • Atomic radius decreases across periods due to increasing nuclear charge without the addition of new electron shells.
• Trends Down Groups:
  • Increases down groups as a result of added electron shells.

• Exceptions:
  • Certain elements, such as transition metals and lanthanides, exhibit deviations due to factors like the lanthanide contraction.

Factors Influencing Atomic Radius

• Nuclear Charge:
  • A stronger attraction from more protons reduces atomic size.
• Effective Nuclear Charge:
  • The net positive charge experienced by valence electrons, crucial for understanding periodic trends.
• Shielding Effect:
  • Inner electrons diminish the pull of the nucleus on outer shell electrons, impacting size.

• Electron Shell Number:
  • Additional electron shells, as one moves down a group, increase atomic size.

Examples and Practice Questions

• Worked Example:

  • Compare the atomic radii of aluminium and chlorine.

  Solution: Aluminium (atomic number 13) has an atomic radius of 143 pm, while chlorine (atomic number 17) has an atomic radius of 99 pm. Aluminium has a larger atomic radius because:

  1. It is located to the left of chlorine in period 3
  2. It has fewer protons, resulting in less nuclear attraction
  3. Its valence electrons experience more shielding from inner electrons

  Practice Question: Explain why potassium has a larger atomic radius than sodium.

  Solution: Potassium (K) has a larger atomic radius than sodium (Na) because potassium is in period 4 while sodium is in period 3. Potassium has an additional electron shell, which increases the distance from the nucleus to the outermost electrons, resulting in a larger atomic radius despite the increased nuclear charge.

Reactivity Influences of Atomic Radius

Influence on Metal Reactivity

• Larger atomic radii facilitate easier electron loss, enhancing metal reactivity.
• In Group 1 metals (alkali metals), caesium's larger radius increases reactivity compared to lithium.

Influence on Nonmetal Reactivity

• Smaller atomic radii lead to higher ionisation energies, positively affecting reactivity.

Exceptions and Anomalies

• Deviations can occur due to electron shielding, such as those between nitrogen and oxygen.
• Grasping these deviations is essential for understanding the complexities of periodic trends.

Conclusion

Understanding atomic radius and its influences on factors such as reactivity and bonding prepares students for chemistry exams and practical applications. Utilise colour-coded periodic tables and diagrams to visualise these complex concepts, promoting better comprehension and retention. Recognising atomic radius trends is vital for predicting element properties and reactions.

Glossary

• Atomic Radius: Distance from an atom's nucleus to its outermost electron shell.
• Ionisation Energy: Energy required to remove an electron from an atom.
• Electron Affinity: Energy change upon adding an electron to a neutral atom.
• Metallic Bonding: Attraction between conduction electrons and positively charged metal ions.
• Covalent Bond: Sharing of electron pairs between atoms.