Atoms change their size when they turn into charged ions. A metal atom loses electrons to become a positive ion, called a cation. This makes the cation much smaller than the original atom.
Losing electrons often removes the entire outer shell. The remaining electrons also feel a stronger pull from the positive protons in the nucleus. This stronger pull shrinks the electron cloud tightly around the center.
A nonmetal atom gains electrons to become a negative ion, called an anion. This makes the anion larger than the original atom. The extra electrons repel each other strongly.
This repulsion pushes the electron cloud further outward. Students often think ions stay the exact same size as neutral atoms. That is not true because losing or gaining electrons changes the balance of forces.
Scientists cannot measure a single ion by itself. Instead, they use X-rays to measure the total distance between two joined ions in a solid crystal. Then they divide that distance to find the radius of each specific ion.
Comparing sizes helps chemists predict how crystals form and how strongly they hold together. Sometimes different ions have the exact same number of electrons. In this case, the ion with more protons will be smaller. The extra protons pull that same number of electrons much closer to the nucleus.