3.3: Ions of Some Common Elements

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Learning Objectives

Become familiar with the charge of some common ions.
Use the periodic table to predict ion charge.

In many cases, elements that belong to the same group (vertical column) on the periodic table form ions with the same charge because they have the same number of valence electrons. Thus, as introduced previously, the periodic table becomes a tool for remembering the charges on many ions. For example, all ions made from alkali metals, the first column on the periodic table, have a +1 charge. Ions made from alkaline earth metals, the second group on the periodic table, have a +2 charge. On the other side of the periodic table, the next-to-last column, the halogens, form ions having a −1 charge. Figure \(\PageIndex{1}\) shows how the charge on many ions can be predicted by the location of an element on the periodic table.

Some elements, especially transition metals, can form ions with variable charges. Figure \(\PageIndex{1}\) shows the characteristic charges for some of these ions. Notice that there is no simple pattern for transition metal ions (or for the larger main group elements) as there is with the main group ions. This is because the transition metals have electrons in d subshell and do not follow the octet rule. In order for an element such as iron (Fe) to achieve the same noble gas configuration of argon (Ar), it would need to lose 6 electrons in the 3d subshell and 2 electrons in the 4s subshell. An iron ion with a charge of +8 is not very likely, therefore, the octet rule is not applicable to transition elements.

Note

For a multiply-charged ion, the correct convention is to write the charge number first followed by the sign. For example, the barium cation is written Ba²⁺, not Ba⁺².

Figure \(\PageIndex{1}\): Common ions formed by specific elements on the periodic table. The charge that a representative (main group element) acquires when it becomes an ion is related to the location on the periodic table. Note that some metal atoms, specifically the transition elements, commonly form ions with variable charges.

Example \(\PageIndex{1}\)

Which of these ions is not likely to form?

Mg⁺
K⁺

Solution

(a) Mg is in Group 2A and has two valence electrons. It achieves octet by losing two electrons to form Mg²⁺cation. Losing only one electron to form Mg⁺does not make an octet, hence, Mg⁺is not likely to form.

Exercise \(\PageIndex{2}\)

Which of these ions is not likely to form?

S³⁻
N³⁻

Answer: (a) S is in Group 6A and has six valence electrons. It achieves octet by gaining two electrons to form S²⁻anion. Gaining three electrons to form S³⁻does not make it octet, hence, S³⁻is not likely to form.