3.3: Types of ions

In ordinary chemical reactions, the nucleus of each atom (and thus the identity of the element) remains unchanged. Electrons, however, can be added to atoms by transfer from other atoms, lost by transfer to other atoms, or shared with other atoms. The transfer and sharing of electrons among atoms govern the chemistry of the elements. During the formation of some compounds, atoms gain or lose electrons, and form electrically charged particles called ions (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): (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.

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 of this text.)

Note the usefulness of the periodic table in predicting likely ion formation and charge (Figure \(\PageIndex{2}\)). 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.

Figure \(\PageIndex{2}\): Some elements exhibit a regular pattern of ionic charge when they form ions.

Example \(\PageIndex{1}\): Composition of Ions

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

Solution

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 \(\PageIndex{1}\)

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

Answer

Se²⁻, the selenide ion

Example \(\PageIndex{2}\): Formation of Ions

Magnesium and nitrogen react to form an ionic compound. Predict which forms an anion, which forms a cation, and the charges of each ion. Write the symbol for each ion and name them.

Solution

Magnesium’s position in the periodic table (group 2) tells us that it is a metal. Metals form positive ions (cations). A magnesium atom must lose two electrons to have the same number electrons as an atom of the previous noble gas, neon. Thus, a magnesium atom will form a cation with two fewer electrons than protons and a charge of 2+. The symbol for the ion is Mg²⁺, and it is called a magnesium ion.

Nitrogen’s position in the periodic table (group 15) reveals that it is a nonmetal. Nonmetals form negative ions (anions). A nitrogen atom must gain three electrons to have the same number of electrons as an atom of the following noble gas, neon. Thus, a nitrogen atom will form an anion with three more electrons than protons and a charge of 3−. The symbol for the ion is N³⁻, and it is called a nitride ion.

Exercise \(\PageIndex{2}\)

Aluminum and carbon react to form an ionic compound. Predict which forms an anion, which forms a cation, and the charges of each ion. Write the symbol for each ion and name them.

Answer

Al will form a cation with a charge of 3+: Al³⁺, an aluminum ion. Carbon will form an anion with a charge of 4−: C⁴⁻, a carbide ion.

The ions that we have discussed so far are called monatomic ions, that is, they are ions formed from only one atom. We also find many polyatomic ions. These ions, which act as discrete units, are electrically charged molecules (a group of bonded atoms with an overall charge). Some of the more important polyatomic ions are listed in Table \(\PageIndex{1}\). Oxyanions are polyatomic ions that contain one or more oxygen atoms. At this point in your study of chemistry, you should memorize the names, formulas, and charges of the most common polyatomic ions. Because you will use them repeatedly, they will soon become familiar.

Table \(\PageIndex{1}\): Common Polyatomic Ions

Name	Formula	Related Acid	Formula
ammonium	\(\ce{NH4+}\)
hydronium	\(\ce{H_3O^+}\)
oxide	\(\ce{O^{2-}}\)
peroxide	\(\ce{O_2^{2-}}\)
hydroxide	\(\ce{OH^-}\)
acetate	\(\ce{CH_3COO^-}\)	acetic acid	\(\ce{CH_3COOH}\)
cyanide	\(\ce{CN^-}\)	hydrocyanic acid	\(\ce{HCN}\)
azide	\(\ce{N_3^-}\)	hydrazoic acid	\(\ce{HN_3}\)
carbonate	\(\ce{CO_3^{2-}}\)	carbonic acid	\(\ce{H_2CO_3}\)
bicarbonate	\(\ce{HCO_3^-}\)
nitrate	\(\ce{NO_3^-}\)	nitric acid	\(\ce{HNO_3}\)
nitrite	\(\ce{NO_2^-}\)	nitrous acid	\(\ce{HNO_2}\)
sulfate	\(\ce{SO_4^{2-}}\)	sulfuric acid	\(\ce{H_2SO_4}\)
hydrogen sulfate	\(\ce{HSO_4^-}\)
sulfite	\(\ce{SO_3^{2-}}\)	sulfurous acid	\(\ce{H_2SO_3}\)
hydrogen sulfite	\(\ce{HSO_3^-}\)
phosphate	\(\ce{PO_4^{3-}}\)	phosphoric acid	\(\ce{H_3PO_4}\)
hydrogen phosphate	\(\ce{HPO_4^{2-}}\)
dihydrogen phosphate	\(\ce{H_2PO_4^-}\)
perchlorate	\(\ce{ClO_4^-}\)	perchloric acid	\(\ce{HClO_4}\)
chlorate	\(\ce{ClO_3^-}\)	chloric acid	\(\ce{HClO_3}\)
chlorite	\(\ce{ClO_2^-}\)	chlorous acid	\(\ce{HClO_2}\)
hypochlorite	\(\ce{ClO^-}\)	hypochlorous acid	\(\ce{HClO}\)
chromate	\(\ce{CrO_4^{2-}}\)	chromic acid	\(\ce{H_2CrO_4}\)
dichromate	\(\ce{Cr_2O_7^{2-}}\)	dichromic acid	\(\ce{H_2Cr_2O7}\)
permanganate	\(\ce{MnO_4^-}\)	permanganic acid	\(\ce{HMnO_4}\)

Note that there is a system for naming some polyatomic ions; -ate and -ite are suffixes designating polyatomic ions containing more or fewer oxygen atoms. Per- (short for “hyper”) and hypo- (meaning “under”) are prefixes meaning more oxygen atoms than -ate and fewer oxygen atoms than -ite, respectively. For example, perchlorate is \(\ce{ClO4-}\), chlorate is \(\ce{ClO3-}\), chlorite is \(\ce{ClO2-}\) and hypochlorite is ClO⁻. Unfortunately, the number of oxygen atoms corresponding to a given suffix or prefix is not consistent; for example, nitrate is \(\ce{NO3-}\) while sulfate is \(\ce{SO4^{2-}}\). This will be covered in more detail in the next module on nomenclature.

Summary

Metals (particularly those in groups 1 and 2) tend to lose the number of electrons that would leave them with the same number of electrons as in the preceding noble gas in the periodic table. By this means, a positively charged ion is formed. Similarly, nonmetals (especially those in groups 16 and 17, and, to a lesser extent, those in Group 15) can gain the number of electrons needed to provide atoms with the same number of electrons as in the next noble gas in the periodic table. Thus, nonmetals tend to form negative ions. Positively charged ions are called cations, and negatively charged ions are called anions. Ions can be either monatomic (containing only one atom) or polyatomic (containing more than one atom).

Glossary

monatomic ion

ion composed of a single atom

polyatomic ion

ion composed of more than one atom

oxyanion

polyatomic anion composed of a central atom bonded to oxygen atoms

Contributors and Attributions

Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110).