Ionic Radius

Definition and Periodic Trend

The ionic radius is the distance between the nucleus of an ion and its outermost electron shell. This property is crucial for understanding the behavior of ions in chemical reactions and the structure of ionic compounds.

• Periodic Trend: The ionic radius increases as the number of electrons increases.

Cation vs. Anion Ionic Radius

Ions can be classified as cations (positively charged) or anions (negatively charged), and their ionic radii differ from their neutral atoms.

Key Points

• Cations are smaller than their neutral atoms due to the loss of electrons and increased effective nuclear charge.

• Anions are larger than their neutral atoms due to the gain of electrons and decreased effective nuclear charge per electron.

• For ions of the same element, the more positive the charge, the smaller the ionic radius; the more negative the charge, the larger the ionic radius.

Examples and Applications

• Example: The lithium ion (Li+) is smaller than the neutral lithium atom because it has lost an electron.

• Example: The oxide ion (O2−) is larger than the neutral oxygen atom because it has gained two electrons.

Practice Questions

Test your understanding of ionic radius trends with the following questions:

EXAMPLE: Which of the following statements is false?

• a) The calcium ion has a smaller ionic radius than the calcium atom.

• b) The sulfide ion has a larger ionic radius than the sulfur atom.

• c) The cadmium ion has a larger ionic radius than the cadmium atom. (False)

• d) The bromide ion has a larger ionic radius than the bromine atom.

PRACTICE: Which of the following statements is false?

• a) The phosphide ion has a smaller ionic radius than the phosphorus atom.

• b) The phosphide ion has a larger ionic radius than the phosphorus atom. (True)

• c) The phosphide ion's ionic radius cannot be compared to the phosphorus atom.

• d) The phosphide ion has an equal ionic radius than the phosphorus atom.

Additional info: The correct answer for the practice question is (a), as anions (such as phosphide) are always larger than their neutral atoms due to increased electron-electron repulsion and decreased effective nuclear charge per electron.