Predicting Formulas of Binary Compounds

Just as a different set of rules is used to name ionic vs. covalent compounds, two sets of rules are required to go from a name to a formula.

Ionic Compounds

The name of an ionic compound indicates which elements are present. For example, calcium bromide contains calcium (Ca) and bromine (Br). However, the name alone does NOT tell how many of each atom are present (strictly speaking, the simplest ratio). In the case of calcium bromide, the correct formula is CaBr2, not CaBr. To determine these formulas, the following set of rules can be used.

  1. Determine the expected charge of the metal ion. This can be done for many elements by examination of the periodic table. Elements in the first column (Li, Na, K, Rb, Cs) have a +1 charge in ionic compounds, elements in the second column (Be, Mg, Ca, Sr, Ba) have a +2 charge, and elements in column 13 (Al, Ga, In, Tl) typically have a +3 charge in ionic compounds. Unfortunately, this procedure doesn't work for transition metals.

  2. Determine the expected charge of the non-metal using the a similar procedure. Elements in column 17 (F, Cl, Br, I) have a -1 charge in ionic compounds, elements in column 16 (O, S, Se, Te) have a -2 charge, and elements in column 15 (N, P, As, Sb) often have a -3 charge in ionic compounds.

  3. The total positive charge of the metal must equal the total negative charge of the non-metal. This is achieved by altering the subscripts (changing the formula). For example, in calcium bromide, calcium exists as Ca+2 and bromine exists as Br-1. To make these charges balance, two Br-1 are needed for every Ca+2, so the formula becomes CaBr2. See the additional examples given below.

Name Cation Charge Anion Charge Charge Balance Formula
lithium iodide Li+1 I-1 |+1| = |-1| LiI
potassium sulfide K+1 S-2 2 x |+1| = |-2| K2S
magnesium oxide Mg+2 O-2 |+2| = |-2| MgO
aluminum oxide Al+3 O-2 2 x |+3| = 3 x |-2| Al2O3

Covalent Compounds

In covalent compounds, none of the atoms carries any charge. If the name of a compound is given, the formula can be determined very simply from the name because the name indicates how many of each atom are present. For example, dinitrogen oxide has the formula N2O.

A more difficult problem is to determine the most likely formula for a compound formed from the combination of any two non-metals. For example, "what is the most probable formula for a compound containing only silicon and oxygen?". To answer this question, the following set of rules can be used. While these rules are actually a rather extreme oversimplification, they work for a surprising large percentage of molecules.

  1. Determine the number of bonds each atom wants to have. This is determined from an element's position on the periodic table. Elements in column 17 (F, Cl, Br, I) want 1 bond, elements in column 16 (O, S, Se, Te) want 2 bonds, elements in column 15 (N, P, As, Sb) want 3 bonds, and elements in column 14 (C, Si, Ge) want 4 bonds. Hydrogen (H) wants 1 bond.
  2. The atom that wants the most bonds is placed in the center of the structure.
  3. Lines are drawn to show bonds between atoms. A single line between two atoms counts as one bond for each atom. A closely spaced pair of lines is used to represent two bonds between the same two atoms (a double bond), and a set of three lines represents three bonds (a triple bond).

These rules can also be applied to elements. In the examples given below, all of the atoms have the "correct" number of bonds.

Elements # Bonds each wants Structural Formula
Br Br: 1 Br–Br
O O: 2 O=O
N N: 3 NºN
H and O H: 1
O: 2		 H–O–H
C and O C: 4
O: 2		 O=C=O
N and H N: 3
H: 1		 nh3.gif (977 bytes)