# 3.1 Ionic and covalent compounds  (Page 2/13)

 Page 2 / 13 Some elements exhibit a regular pattern of ionic charge when they form ions.

## 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, Al 3+ .

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

Se 2− , the selenide ion

## 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 Mg 2+ , 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 N 3− , and it is called a nitride ion.

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.

Al will form a cation with a charge of 3+: Al 3+ , an aluminum ion. Carbon will form an anion with a charge of 4−: C 4− , 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 [link] . 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.

Common Polyatomic Ions
Name Formula Related Acid Formula
ammonium ${\text{NH}}_{4}{}^{+}$
hydronium ${\text{H}}_{3}{\text{O}}^{+}$
oxide ${\text{O}}^{2-}$
peroxide ${\text{O}}_{2}{}^{2-}$
hydroxide ${\text{OH}}^{-}$
acetate ${\text{CH}}_{3}{\text{COO}}^{-}$ acetic acid CH 3 COOH
cyanide CN hydrocyanic acid HCN
azide ${\text{N}}_{3}{}^{-}$ hydrazoic acid HN 3
carbonate ${\text{CO}}_{3}{}^{2-}$ carbonic acid H 2 CO 3
bicarbonate ${\text{HCO}}_{3}{}^{-}$
nitrate ${\text{NO}}_{3}{}^{-}$ nitric acid HNO 3
nitrite ${\text{NO}}_{2}{}^{-}$ nitrous acid HNO 2
sulfate ${\text{SO}}_{4}{}^{2-}$ sulfiric acid H 2 SO 4
hydrogen sulfate ${\text{HSO}}_{4}{}^{-}$
sulfite ${\text{SO}}_{3}{}^{2-}$ sulfurous acid H 2 SO 3
hydrogen sulfite ${\text{HSO}}_{3}{}^{-}$
phosphate ${\text{PO}}_{4}{}^{3-}$ phosphoric acid H 3 PO 4
hydrogen phosphate ${\text{HPO}}_{4}{}^{2-}$
dihydrogen phosphate ${\text{H}}_{2}{\text{PO}}_{4}{}^{-}$
perchlorate ${\text{ClO}}_{4}{}^{-}$ perchloric acid HClO 4
chlorate ${\text{ClO}}_{3}{}^{-}$ chloric acid HClO 3
chlorite ${\text{ClO}}_{2}{}^{-}$ chlorous acid HClO 2
hypochlorite ClO hypochlorous acid HClO
chromate ${\text{CrO}}_{4}{}^{2-}$ chromic acid H 2 Cr 2 O 4
dichromate ${\text{Cr}}_{2}{\text{O}}_{7}{}^{2-}$ dichromic acid H 2 Cr 2 O 7
permanganate ${\text{MnO}}_{4}{}^{-}$ permanganic acid HMnO 4

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