Atomic & Ionic Size Comparison
Atomic size grows down a group (more shells) and shrinks across a period (rising nuclear charge pulling on the same shells). For ions: cations are smaller than their parent atoms, anions larger, and in an isoelectronic series the species with more protons is smaller.
Click the larger species, or rank them smallest to largest, in the problems below.
Common Questions
How does atomic size change across a period?
Atomic radius decreases from left to right across a period. Adding protons increases nuclear charge, pulling electrons closer to the nucleus, even though electrons are added to the same shell.
How does atomic size change down a group?
Atomic radius increases going down a group. Each row adds a new electron shell, placing the outermost electrons farther from the nucleus despite increasing nuclear charge.
Why are cations smaller than their parent atoms?
Removing electrons reduces electron-electron repulsion, allowing the remaining electrons to be pulled closer to the nucleus. Also, cations often lose an entire outer shell, dramatically shrinking the radius.
Why are anions larger than their parent atoms?
Adding electrons increases electron-electron repulsion in the outer shell, causing the electron cloud to expand. The same nuclear charge must hold more electrons, resulting in a larger radius.