Themes > Science > Chemistry > General Chemistry > Matter > Atoms, Ions, and Molecules > The Size of Atoms > The Relative Size of Atoms and Their Ions

The table and figure below compare the covalent radius of neutral F, Cl, Br, and I atoms with the radii of their F^-, Cl^-, Br^-, and I^- ions. In each case, the negative ion is much larger than the atom from which it was formed. In fact, the negative ion can be more than twice as large as the neutral atom.

The only difference between an atom and its ions is the number of electrons that surround the nucleus.

Example: A neutral chlorine atom contains 17 electrons, while a Cl^- ion contains 18 electrons.

Because the nucleus can't hold the 18 electrons in the Cl^- ion as tightly as the 17 electrons in the neutral atom, the negative ion is significantly larger than the atom from which it forms.

For the same reason, positive ions should be smaller than the atoms from which they are formed. The 11 protons in the nucleus of an Na⁺ ion, for example, should be able to hold the 10 electrons on this ion more tightly than the 11 electrons on a neutral sodium atom. The table and figure below provide data to test this hypothesis. They compare the covalent radii for neutral atoms of the Group IA elements with the ionic radii for the corresponding positive ions. In each case, the positive ion is much smaller than the atom from which it forms.

The relative size of positive and negative ions has important implications for the structure of ionic compounds. The positive ions are often so small they pack in the holes between planes of adjacent negative ions. In NaCl, for example, the Na⁺ ions are so small that the Cl^- ions almost touch, as shown in the figure below.

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