Ionic Radius: The Changing Size of an Atom
What is an Ion?
Before we dive into size, let's understand what an ion is. An atom is typically neutral, meaning the number of protons (positive charges) in the nucleus equals the number of electrons (negative charges) orbiting it. An ion forms when an atom gains or loses one or more electrons, resulting in a net electrical charge.
- Cation (+ charge): Formed when an atom loses one or more electrons. Since electrons are lost, the number of protons exceeds the number of electrons, creating a net positive charge. Example: A sodium atom (Na) loses one electron to become a sodium cation (Na^+).
- Anion (- charge): Formed when an atom gains one or more electrons. The number of electrons now exceeds the number of protons, creating a net negative charge. Example: A chlorine atom (Cl) gains one electron to become a chloride anion (Cl^-).
Why Cations Are Smaller and Anions Are Larger
The key to understanding the change in size lies in the balance of forces within the atom. The nucleus pulls the electron cloud inward, while the electrons themselves repel each other.
Why Cations Shrink: Imagine a sodium atom with 11 protons and 11 electrons. When it loses one electron to become Na^+, it now has 11 protons but only 10 electrons. The positive nuclear charge remains the same, but there are fewer electrons repelling each other. The nucleus can now pull the remaining electrons closer, resulting in a much smaller ionic radius.
Why Anions Grow: Now consider a chlorine atom with 17 protons and 17 electrons. When it gains one electron to become Cl^-, it has 17 protons and 18 electrons. The nuclear charge is unchanged, but the added electron increases the repulsion between electrons. This increased repulsion causes the electron cloud to spread out, making the anion larger than the original atom.
Trends in the Periodic Table
The periodic table is a powerful tool for predicting and understanding ionic radii. The size of ions follows two main trends: one going down a group and one going across a period.
| Trend Direction | Explanation | Example |
|---|---|---|
| Down a Group | Ionic radius increases. As you move down a column, each element has an additional electron shell. This extra shell, farther from the nucleus, makes the ion larger. | In Group 1: Li^+ < Na^+ < K^+ < Rb^+ |
| Across a Period (for Cations) | Ionic radius decreases. Moving from left to right, cations have the same electron configuration but an increasing nuclear charge. The stronger pull from the nucleus shrinks the ion. | Period 3: Na^+ > Mg^2+ > Al^3+ |
| Across a Period (for Anions) | Ionic radius also decreases. Similar to cations, anions across a period have the same number of electrons but increasing nuclear charge, pulling the electron cloud inward. | Period 3: P^3- > S^2- > Cl^- |
A Closer Look: Sodium and Chlorine
Let's follow the journey of a sodium atom and a chlorine atom as they form the compound sodium chloride, or table salt. This is a perfect example to see ionic radius in action.
Sodium (Na): A neutral sodium atom has 11 protons and 11 electrons. Its atomic radius is about 186 pm[1]. It readily loses its single outer electron to achieve a stable electron configuration. After losing that electron, it becomes the Na^+ cation. The loss of an entire electron shell causes a dramatic decrease in size. The ionic radius of Na^+ is only about 102 pm.
Chlorine (Cl): A neutral chlorine atom has 17 protons and 17 electrons. Its atomic radius is about 99 pm. It is one electron short of a stable configuration, so it eagerly accepts the electron that sodium lost. Upon gaining that electron, it becomes the Cl^- anion. The added electron increases electron-electron repulsion, causing the electron cloud to expand. The ionic radius of Cl^- is about 181 pm.
So, in table salt, the small Na^+ cation fits neatly into the spaces between the much larger Cl^- anions, forming a strong and stable crystal lattice.
| Species | Type | Approximate Radius (pm) | Size Change |
|---|---|---|---|
| Sodium Atom (Na) | Atom | 186 | - |
| Sodium Ion (Na^+) | Cation | 102 | Decrease (~45%) |
| Chlorine Atom (Cl) | Atom | 99 | - |
| Chloride Ion (Cl^-) | Anion | 181 | Increase (~83%) |
Common Mistakes and Important Questions
Q: Is the nucleus getting bigger or smaller when an ion forms?
Q: Why is a cation always smaller than its atom, even if it has multiple electron shells?
Q: How does the size of an ion affect the properties of a compound?
Footnote
[1] pm (picometer): A unit of length equal to one trillionth of a meter (1 x 10^-12 m). It is commonly used to express atomic and ionic sizes.