Ion Composition and Formation
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+.
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.
Monatomic and Polyatomic Ions
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 the table below.
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 | \(\text{CH}_{3}\text{COOH}\) |
| cyanide | \(\text{CN}^{−}\) | hydrocyanic acid | \(\text{HCN}\) |
| azide | \(\text{N}_{3}^{−}\) | hydrazoic acid | \(\text{HN}_{3}\) |
| carbonate | \(\text{CO}_{3}^{2−}\) | carbonic acid | \(\text{H}_{2}\text{CO}_{3}\) |
| bicarbonate | \(\text{HCO}_{3}^{−}\) | ||
| nitrate | \(\text{NO}_{3}^{−}\) | nitric acid | \(\text{HNO}_{3}\) |
| nitrite | \(\text{NO}_{2}^{−}\) | nitrous acid | \(\text{HNO}_{2}\) |
| sulfate | \(\text{SO}_{4}^{2−}\) | sulfuric acid | \(\text{H}_{2}\text{SO}_{4}\) |
| hydrogen sulfate | \(\text{HSO}_{4}^{−}\) | ||
| sulfite | \(\text{SO}_{3}^{2−}\) | sulfurous acid | \(\text{H}_{2}\text{SO}_{3}\) |
| hydrogen sulfite | \(\text{HSO}_{3}^{−}\) | ||
| phosphate | \(\text{PO}_{4}^{3−}\) | phosphoric acid | \(\text{H}_{3}\text{PO}_{4}\) |
| hydrogen phosphate | \(\text{HPO}_{4}^{2−}\) | ||
| dihydrogen phosphate | \(\text{H}_{2}\text{PO}_{4}^{−}\) | ||
| perchlorate | \(\text{ClO}_{4}^{−}\) | perchloric acid | \(\text{HClO}_{4}\) |
| chlorate | \(\text{ClO}_{3}^{−}\) | chloric acid | \(\text{HClO}_{3}\) |
| chlorite | \(\text{ClO}_{2}^{−}\) | chlorous acid | \(\text{HClO}_{2}\) |
| hypochlorite | \(\text{ClO}^{−}\) | hypochlorous acid | \(\text{HClO}\) |
| chromate | \(\text{CrO}_{4}^{2−}\) | chromic acid | \(\text{H}_{2}\text{Cr}_{2}\text{O}_{4}\) |
| dichromate | \(\text{Cr}_{2}\text{O}_{7}^{2−}\) | dichromic acid | \(\text{H}_{2}\text{Cr}_{2}\text{O}_{7}\) |
| permanganate | \(\text{MnO}_{4}^{−}\) | permanganic acid | \(\text{HMnO}_{4}\) |
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 \(\text{ClO}_{4}^{−},\) chlorate is \(\text{ClO}_{3}^{−},\) chlorite is \(\text{ClO}_{2}^{−}\) and hypochlorite is \(\text{ClO}^{−}.\) Unfortunately, the number of oxygen atoms corresponding to a given suffix or prefix is not consistent; for example, nitrate is \(\text{NO}_{3}^{−}\) while sulfate is \(\text{SO}_{4}^{2−}.\) This will be covered in more detail when we discuss nomenclature.
The nature of the attractive forces that hold atoms or ions together within a compound is the basis for classifying chemical bonding. When electrons are transferred and ions form, ionic bonds result. Ionic bonds are electrostatic forces of attraction, that is, the attractive forces experienced between objects of opposite electrical charge (in this case, cations and anions).
When electrons are “shared” and molecules form, covalent bonds result. Covalent bonds are the attractive forces between the positively charged nuclei of the bonded atoms and one or more pairs of electrons that are located between the atoms. Compounds are classified as ionic or molecular (covalent) on the basis of the bonds present in them.
This lesson is part of:
Atoms, Molecules and Ions