Atomic radius describes the size of an atom, while ionic radius describes the size of an ion after electrons have been lost or gained. These radii help explain patterns in bonding, reactivity, melting points, and the structure of compounds. On the periodic table, atomic radius generally increases down a group and decreases from left to right across a period.
Understanding these trends lets students predict relative sizes without memorizing every value.
The main reason for these trends is the balance between nuclear charge, electron shielding, and the number of occupied energy levels. Moving down a group adds electron shells, so atoms get larger even though the nucleus has more protons. Moving across a period adds protons to the nucleus without adding a new shell, so the outer electrons are pulled closer.
Ions change size because cations lose electrons and shrink, while anions gain electrons and expand due to increased electron repulsion.
Key Facts
- Atomic radius generally increases down a group because atoms gain additional occupied energy levels.
- Atomic radius generally decreases from left to right across a period because effective nuclear charge increases.
- A cation is smaller than its neutral atom because it loses electrons and may lose an entire outer energy level.
- An anion is larger than its neutral atom because added electrons increase electron-electron repulsion.
- Effective nuclear charge can be estimated as Z_eff = Z - S, where Z is atomic number and S is shielding.
- In an isoelectronic series, radius decreases as nuclear charge increases, so O2- > F- > Ne > Na+ > Mg2+.
Vocabulary
- Atomic radius
- Atomic radius is a measure of the size of an atom, often defined as half the distance between the nuclei of two identical bonded atoms.
- Ionic radius
- Ionic radius is a measure of the size of an ion in a crystal or bonded structure.
- Cation
- A cation is a positively charged ion formed when an atom loses one or more electrons.
- Anion
- An anion is a negatively charged ion formed when an atom gains one or more electrons.
- Isoelectronic series
- An isoelectronic series is a group of atoms or ions that have the same number of electrons but different numbers of protons.
Common Mistakes to Avoid
- Saying atomic radius increases from left to right across a period, which is wrong because increasing nuclear charge pulls the same energy level closer to the nucleus.
- Assuming a cation is larger than its neutral atom, which is wrong because losing electrons reduces repulsion and can remove the outer shell completely.
- Assuming an anion is smaller than its neutral atom, which is wrong because added electrons increase repulsion within the electron cloud.
- Ranking isoelectronic ions only by electron count, which is wrong because ions with the same number of electrons must be compared by nuclear charge.
Practice Questions
- 1 Rank the following atoms from smallest to largest atomic radius: Mg, Al, Si, Na. Explain the periodic trend used.
- 2 The ions N3-, O2-, F-, Na+, and Mg2+ each have 10 electrons. Rank them from largest to smallest ionic radius.
- 3 Explain why Cl- is larger than Cl, but Na+ is smaller than Na, using electron shells, repulsion, and nuclear attraction.