βοΈ What is Atomic Radius?
Atomic radius refers to the typical distance from the center of the nucleus to the boundary of the surrounding cloud of electrons. Because the electron cloud doesn't have a sharp edge, atomic radius is usually defined as half the distance between the nuclei of two identical atoms bonded together.
ββ What is Ionic Radius?
Ionic radius, on the other hand, is the radius of an atom's ion. An ion is an atom or molecule that has gained or lost electrons, giving it a net electrical charge. If an atom loses electrons, it forms a positive ion (cation), and if it gains electrons, it forms a negative ion (anion). The ionic radius is the measure of the size of this ion.
π Atomic Radius vs. Ionic Radius: The Key Differences
Here's a table to highlight the key differences:
| Feature |
Atomic Radius |
Ionic Radius |
| Definition |
Half the distance between nuclei of two identical bonded atoms. |
Radius of an atom that has gained or lost electrons (an ion). |
| Charge |
Neutral (no charge). |
Charged (positive or negative). |
| Formation |
Based on the size of the neutral atom. |
Based on the gain or loss of electrons. |
| Cations (Positive Ions) |
N/A |
Smaller than the parent atom. |
| Anions (Negative Ions) |
N/A |
Larger than the parent atom. |
| Example |
Na (Sodium Atom) |
$Na^+$ (Sodium Ion) or $Cl^-$ (Chloride Ion) |
π Key Takeaways
- π Size Change: When an atom loses electrons to form a cation, its ionic radius decreases because there are fewer electrons and the remaining electrons are pulled closer to the nucleus.
- π Size Change: When an atom gains electrons to form an anion, its ionic radius increases because the increased electron repulsion causes the electron cloud to expand.
- π§ͺ Periodic Trends: Both atomic and ionic radii generally increase as you move down a group in the periodic table (due to increased electron shells) and decrease as you move from left to right across a period (due to increased nuclear charge).
- π‘ Isoelectronic Species: For isoelectronic species (atoms/ions with the same number of electrons), the ion with the greater nuclear charge will have a smaller radius. For example, $O^{2-}$, $F^-$, $Na^+$, and $Mg^{2+}$ are all isoelectronic with 10 electrons; $Mg^{2+}$ has the largest nuclear charge and thus the smallest radius.