3 Ways To Write Ionic Compounds

Examples of simple binary ionic compounds include potassium oxide and sodium phosphide. If the “-ide” suffix doesn’t follow a single element name, see instructions for polyatomic ions. For example, “oxide” is a simple oxygen ion, but “hydroxide” and “peroxide” are polyatomic.

For example, potassium oxide is a combination of potassium (chemical symbol K, atomic number 19) and oxygen (O, atomic number 8). Notice that the -ide ending is not part of the element name. You’re just looking for an element with the same beginning (in this case ox-).

The Group 1 elements Li, Na, K, Rb, and Cs gain a charge of 1+ (written simply as +). The Group 2 elements Mg, Ca, Sr, and Ba gain a charge of 2+. The Group 13 element Al gains a charge of 3+. The Group 15 elements N and P gain a charge of 3-. The Group 16 elements O and S gain a charge of 2-. The Group 17 elements F, Cl, Br, and I gain a charge of 1- (written as -).

For example, potassium oxide is made up of potassium ions K+{\displaystyle K^{+}} and oxygen ions O2−{\displaystyle O^{2-}}. This means that 2 potassium ions (total charge 2+) balance out the charge of 1 oxygen ion (charge 2-). Here’s a shortcut: the first ion’s charge (ignoring + or -) tells you the number of atoms of the second ion, and vice versa. For example, aluminum fluoride is made of Al3+{\displaystyle Al^{3+}} and F−{\displaystyle F^{-}} ions. The charge of Al3+{\displaystyle Al^{3+}} is 3, so there are 3 F−{\displaystyle F^{-}} atoms. The charge of F−{\displaystyle F^{-}} is 1, so there is 1 Al3+{\displaystyle Al^{3+}} atom.

For example, potassium oxide has 2 potassium atoms and 1 oxygen atom. The chemical formula is K2O{\displaystyle K_{2}O}. Aluminum fluoride has 1 aluminum atom and 3 fluorine atoms. The chemical formula is AlF3{\displaystyle AlF_{3}}.

For example, barium sulfide is made of Ba2+{\displaystyle Ba^{2+}} and S2−{\displaystyle S^{2-}} ions. Using the shortcut above, the barium ion’s charge (2) is equal to the number of sulfur ions, and the sulfur ion’s charge (2) is equal to the number of barium ions. This gives us the formula Ba2S2{\displaystyle Ba_{2}S_{2}}. However, you don’t need this many atoms to balance the charges. Write the 2 numbers as a fraction and simplify: 2 barium atoms2 sulfur atoms=11{\displaystyle {\frac {2\ barium\ atoms}{2\ sulfur\ atoms}}={\frac {1}{1}}}, so the correct formula is BaS{\displaystyle BaS}.

For example, copper sometimes forms an ion with a charge of +1, and sometimes with a charge of +2. Copper (I) chloride and copper (II) chloride are 2 different ionic compounds. The Roman numeral tells you which copper ion is part of the compound.

For example, copper (II) chloride includes the cation Cu2+{\displaystyle Cu^{2+}}, since II is the Roman numeral for 2.

For example, chloride is the name for a chlorine ion. Chlorine, like similar group 17 elements, forms ions with a charge of negative one. This is written as Cl−{\displaystyle Cl^{-}}.

To balance out the 2+ charge from a Cu2+{\displaystyle Cu^{2+}} ion, you need 2 Cl−{\displaystyle Cl^{-}} ions (2 ions x -1 charge per ion = -2). This makes an ionic compound with a net charge of +2 -2 = 0.

Copper (II) chloride has one copper atom and two chlorine atoms. The formula is CuCl2{\displaystyle CuCl_{2}}.

OH−1{\displaystyle OH^{-1}} is not an ionic compound, because it does not have a net charge of zero. It is a single ion, which can combine with ions of the opposite charge. Some elements can form polyatomic ions by themselves. For example, “peroxide” is the O22−{\displaystyle O_{2}^{2-}} ion. It contains 2 oxygen atoms, but has a total charge of -2. (This is different than oxide, the “standard” oxygen ion, which has one atom with a charge of -2. )

Cyanide: CN−{\displaystyle CN^{-}} (One carbon, one nitrogen, total charge of -1) Hydroxide: OH−{\displaystyle OH^{-}}. Nitrate: NO3−{\displaystyle NO_{3}^{-}} (One nitrogen, three oxygen, total charge of -1) Nitrite: NO2−{\displaystyle NO_{2}^{-}} Peroxide: O22−{\displaystyle O_{2}^{2-}} Sulfate: SO42−{\displaystyle SO_{4}^{2-}} Sulfite: SO32−{\displaystyle SO_{3}^{2-}}

For example, magnesium hydroxide contains a magnesium cation. Magnesium is a group 2 element that forms the ion Mg2+{\displaystyle Mg^{2+}}.

For example, to balance OH−1{\displaystyle OH^{-1}} and Mg2+{\displaystyle Mg^{2+}} , compare the charges. It takes two -1 charges to balance out one 2+ charge. This means there are two hydroxide ions and one magnesium ion.

In the case that more than one of a certain polyatomic ion is necessary to balance the charge, the entire formula for the polyatomic ion is enclosed in parentheses and the numerical subscript is placed outside the parentheses. This shows that the subscript applies to the entire polyatomic ion. [14] X Research source For example, magnesium hydroxide has the chemical formula Mg(OH)2{\displaystyle Mg(OH)_{2}}. You do not need to include the charges of the ions.