Electron Shells and the Sizes of Ions

Topic: The Periodic Table Variations Of Chemical Properties With Group And Row

Ionic radius is the radius of an ion, regardless of whether it is an anion or a cation.

While neither atoms nor ions have sharp boundaries, it is useful to treat them as if they are hard spheres with radii. In this way, the sum of ionic radii of a cation and an anion can give us the distance between the ions in a crystal lattice. Ionic radii are typically given in units of either picometer (pm) or Angstroms (Å), with 1 Å = 100 pm. Typical values range from 30 pm (0.3 Å) to over 200 pm (2 Å).

Ions may be larger or smaller than the neutral atom, depending on the ion’s charge. When an atom loses an electron to form a cation, the lost electron no longer contributes to shielding the other electrons from the charge of the nucleus; consequently, the other electrons are more strongly attracted to the nucleus, and the radius of the atom gets smaller. Similarly, when an electron is added to an atom, forming an anion, the added electron repels other electrons, increasing the size of the atom. The ionic radius is not a fixed property of a given ion; rather, it varies with coordination number, spin state, and other parameters.

Ionic radii increase upon descending a group and decrease going across a period. Note that this only applies if the elements are the same type of ion, either cations or anions. For example, while neutral lithium is larger than neutral fluorine, the lithium cation is much smaller than the fluorine anion, due to the lithium cation having a different highest electron shell.


Practice Questions

 

Khan Academy

 

MCAT Official Prep (AAMC)

Official Guide C/P Section Passage 3 Question 13

Practice Exam 2 C/P Section Passage 7 Question 38

 

Key Points

• The ionic radius is the distance between the nucleus and the electron in the outermost shell of an ion.

• When an atom loses an electron to form a cation, the lost electron no longer contributes to shielding the other electrons from the charge of the nucleus; consequently, the other electrons are more strongly attracted to the nucleus, and the radius of the atom gets smaller.

• When an electron is added to an atom, forming an anion, the added electron repels other electrons, increasing the size of the atom.


Key Terms

Electron shell: The collective states of all electrons in an atom having the same principal quantum number (visualized as an orbit in which the electrons move).

Cation: A positively charged ion, as opposed to an anion.

Ion: An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.

Anion: A negatively charged ion, as opposed to a cation.



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