Question

a. Distinguish between a cation and an anion. b. How does the size of each compare with the size of the neutral atom from which it is formed?

Short answer

Cations are positively charged and smaller than their neutral atoms, while anions are negatively charged and larger than their neutral atoms.

Step-by-step solution

- Understanding Cations

Cations are positively charged ions formed when an atom loses one or more electrons. This loss results in more protons than electrons, giving the ion a positive charge.

- Understanding Anions

Anions are negatively charged ions formed when an atom gains one or more electrons. This gain results in more electrons than protons, giving the ion a negative charge.

- Size Comparison of Cations

Cations are smaller than their neutral atoms because losing electrons reduces electron-electron repulsion and allows the remaining electrons to be pulled closer to the nucleus.

- Size Comparison of Anions

Anions are larger than their neutral atoms because gaining electrons increases electron-electron repulsion, causing the electron cloud to expand.

Key concepts

Cations

Cations are positively charged ions. They form when an atom loses one or more electrons. This electron loss means there are now more protons than electrons in the atom.
Protons have a positive charge, so the loss of electrons, which are negatively charged, leaves the ion with an overall positive charge.

• Example: A sodium atom loses one electron to become a Na⁺ cation

Cations are crucial in many chemical reactions and play a significant role in the functioning of batteries, where they move toward the negative electrode.

Anions

Anions are negatively charged ions. They form when an atom gains one or more electrons. This increase in electrons means there are now more electrons than protons in the atom.
Given that electrons carry a negative charge, the gain of electrons results in a negatively charged ion.

• Example: A chlorine atom gains one electron to become a Cl^- anion

Anions are found in various compounds and are essential for many biochemical processes. They help balance out the charge in salts and are involved in ion exchange processes.

Ion Size Comparison

The size of ions compared to their neutral atoms can vary significantly, depending on whether they are cations or anions.

• Cation Size: Cations are typically smaller than their neutral atoms. When an atom loses electrons to form a cation, there is less electron-electron repulsion. This allows the remaining electrons to be pulled closer to the nucleus, resulting in a smaller ionic radius.
• Anion Size: Anions are generally larger than their neutral atoms. The addition of electrons increases electron-electron repulsion, which causes the electron cloud to expand. This results in a larger ionic radius for anions than their original neutral atoms.

By understanding these size differences, we can predict properties such as ionic bonding strength and coordination in various compounds.