The key to writing the formula for a simple compound (i.e. one with two elements), is to determine whether the compound is ionic or covalent. We can do this by determining whether each element in the compound is a metal or a nonmetal.
You can determine whether an element is a metal or a nonmetal by where it is located on the periodic table. Look at the first periodic table in the graphic shown below. The elements that are colored yellow are metals and the elements that are colored red are nonmetals.
- If the first element in the compound is a metal, the compound will be ionic. An ionic compound is generally made of a metal and nonmetal, with the metal symbol appearing in front of the nonmetal.
- If the first element in the compound is a nonmetal, the compound will be covalent.
Let's look at your compound and determine whether it is ionic or covalent. The first element is calcium (Ca). Locate this element on the periodic table shown below. Notice that the first element (Ca) is a metal. So, your compound is ionic.
In order to write the formula for an ionic compound, you must know each element's charge number. The charge number indicates how many electrons are lost or gained when the element forms an ion. For many elements, you can determine the charge number from the location of the element on the periodic table. Look at the second periodic table in the graphic shown below. The charge numbers of some of the elements have been written at the top of each column of the periodic table. The charge numbers are not normally found on most periodic tables; you will need to memorize the charge numbers for each column.
Let's determine the charge numbers for your compound. Calcium is in a column with a "2" at the top, so its charge number is 2. Chlorine is in column with a "1" at the top, so its charge number is 1. Specifically, its charge is negative one as the charges starting from the left side of the table are negative.
To write the formula for calcium chloride, follow these steps:
- Write the charge number at the top of each element: ~`Ca^2Cl^1`
- Switch the charge numbers: ~`Ca^1Cl^2`
- Bring down the numbers as subscripts: ~`Ca_1Cl_2`
- Eliminate any subscripts that are "1's": ~`CaCl_2` (You can also eliminate subscripts if they are identical.)