6.1 Elements and Their Ions

Learning Objectives

By the end of this section, you will be able to:

  • Define ions
  • Identify the charges of the ions formed by main group elements on the periodic table


As a recap from Chapter 3, during the formation of some compounds, atoms gain or lose electrons, and form electrically charged particles called ions (Figure 6.1a).

Two purple circles representing atoms, each with a small red central dot representing its nucleus, are pictured. The left circle labelled A shows a sodium atom, N a, which has a nucleus containing 11 protons and 12 neutrons. The atom’s surrounding electron cloud shaded purple contains 11 electrons. The right circle labelled B shows a sodium ion, N a superscript plus sign. Its nucleus contains 11 protons and 12 neutrons. The ion’s electron cloud contains 10 electrons and is smaller than that of the sodium atom in pictured in A.
Figure 6.1a: (a) A sodium atom (Na) has equal numbers of protons and electrons (11) and is uncharged. (b) A sodium cation (Na+) has lost an electron, so it has one more proton (11) than electrons (10), giving it an overall positive charge, signified by a superscripted plus sign (credit: Chemistry (OpenStax), CC BY 4.0).

You can use the periodic table to predict whether an atom will form an anion or a cation, and you can often predict the charge of the resulting ion. Atoms of many main-group metals lose enough electrons to leave them with the same number of electrons as an atom of the preceding noble gas. To illustrate, an atom of an alkali metal (group 1) loses one electron and forms a cation with a 1+ charge; an alkaline earth metal (group 2) loses two electrons and forms a cation with a 2+ charge, and so on. For example, a neutral calcium atom, with 20 protons and 20 electrons, readily loses two electrons. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. It has the same number of electrons as atoms of the preceding noble gas, argon, and is symbolized Ca2+. The name of a metal ion is the same as the name of the metal atom from which it forms, so Ca2+ is called a calcium ion.

When atoms of nonmetal elements form ions, they generally gain enough electrons to give them the same number of electrons as an atom of the next noble gas in the periodic table. Atoms of group 17 gain one electron and form anions with a 1− charge; atoms of group 16 gain two electrons and form ions with a 2− charge, and so on. For example, the neutral bromine atom, with 35 protons and 35 electrons, can gain one electron to provide it with 36 electrons. This results in an anion with 35 protons, 36 electrons, and a 1− charge. It has the same number of electrons as atoms of the next noble gas, krypton, and is symbolized Br. (A discussion of the theory supporting the favored status of noble gas electron numbers reflected in these predictive rules for ion formation is provided in a later chapter.)

Note the usefulness of the periodic table in predicting likely ion formation and charge (Figure 6.1b). Moving from the far left to the right on the periodic table, main-group elements tend to form cations with a charge equal to the group number. That is, group 1 elements form 1+ ions; group 2 elements form 2+ ions, and so on. Moving from the far right to the left on the periodic table, elements often form anions with a negative charge equal to the number of groups moved left from the noble gases. For example, group 17 elements (one group left of the noble gases) form 1− ions; group 16 elements (two groups left) form 2− ions, and so on. This trend can be used as a guide in many cases, but its predictive value decreases when moving toward the center of the periodic table. In fact, transition metals and some other metals often exhibit variable charges that are not predictable by their location in the table. For example, copper can form ions with a 1+ or 2+ charge, and iron can form ions with a 2+ or 3+ charge.

Group one (alkali metals) of the periodic table form +1 ions. Group two (alkaline earth metals) form +2 ions. Al in group 13 for +3 ion. Group 15 form -3 ions. Group 16 form -2 ions. Group 17 (halogens) form -1 ions. Group 18 (noble gases) do not form ions. Some transition metals will form ion. Some are bivalent (meaning two or more potential charges) while others are monovalent (one possible charge).
Figure 6.1b: Some elements exhibit a regular pattern of ionic charge when they form ions. Review the Periodic Table of the Elements in other formats in Appendix A (credit: Chemistry (OpenStax), CC BY 4.0).

Example 6.1a

Composition of Ions

An ion found in some compounds used as antiperspirants contains 13 protons and 10 electrons. What is its symbol?


Because the number of protons remains unchanged when an atom forms an ion, the atomic number of the element must be 13. Knowing this lets us use the periodic table to identify the element as Al (aluminum). The Al atom has lost three electrons and thus has three more positive charges (13) than it has electrons (10). This is the aluminum cation, Al3+.

Exercise 6.1a

Give the symbol and name for the ion with 34 protons and 36 electrons.

Check Your Answer[1]

The ions that we have discussed so far are called monatomic ions, that is, they are ions formed from only one atom.

Links to Interactive Learning Tools

Attribution & References

Except where otherwise noted, this page is adapted by Adrienne Richards from “3.7 Molecular and Ionic Compounds” In General Chemistry 1 & 2 by Rice University, a derivative of Chemistry (Open Stax) by Paul Flowers, Klaus Theopold, Richard Langley & William R. Robinson and is licensed under CC BY 4.0. ​Access for free at Chemistry (OpenStax)​ . / Streamlined content for student comprehension.

  1. Se2−, the selenide ion


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Enhanced Introductory College Chemistry Copyright © 2023 by Gregory Anderson; Caryn Fahey; Jackie MacDonald; Adrienne Richards; Samantha Sullivan Sauer; J.R. van Haarlem; and David Wegman is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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