- 1). Underline each element in a formula, if you are working with more than one element.
- 2). Add the number of elements together. The sum is equivalent to the number of atoms. For example, the formula NaOH has three elements: Na, O and H, for a total of three atoms.
- 3). Add numbers appearing after elements. For example, in the element Na4, the number of atoms in the element is 4. In the formula Fe2S3, the element Fe has two atoms, and the element S has three atoms, for a total of five atoms.
- 4). When an element has numbers appearing before and after it, multiply them. For example, in the element 2N4, 2 and 4 should be multiplied. The total number of N atoms is eight.
- 5). For a more complicated formula, such as 2(NH4)3, take the number on the right side of the parenthesis and multiply it by each element. You should arrive at 2(N3H12). Next, multiply the number on the left side of the parentheses by the number associated with each element. The number of N atoms is 6, and the number of H atoms is 24. Added together, the total number of atoms for the formula is 30.
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