Atom Counter
Enter a chemical formula and see exactly how many atoms of each element it contains.
Atom count
Ca(OH)₂
5 atoms total
| Element | Atoms |
|---|---|
| Ca | 1 |
| O | 2 |
| H | 2 |
Different elements: 3
Total atoms: 5
Count a specific element
Oxygen: 2 atoms
How it works
From formula to atom count
Read the formula
Identify each element and its subscript.
Expand groups
For example, (OH)₂ contains O₂ and H₂.
Count the atoms
Add the atoms from every part of the formula.
Apply a coefficient
If the formula starts with a coefficient such as 2H₂O, multiply the complete formula count by 2.
Worked example: Ca(OH)₂
- Ca = 1
- O = 2 × 1 = 2
- H = 2 × 1 = 2
Total = 5 atoms
What is an atom counter?
An atom counter reads a chemical formula and tells you how many atoms of each element it contains, plus the total number of atoms.
For example, Ca(OH)₂ contains Ca = 1, O = 2, H = 2, for a total of 5 atoms. Parentheses, brackets, hydrates, and coefficients are included automatically.
Concepts
Read formulas like a counter
Subscripts tell you how many atoms
H₂O contains 2 hydrogen atoms and 1 oxygen atom. Total = 3 atoms.
Parentheses multiply a group
Ca(OH)₂ means the OH group occurs twice: Ca = 1, O = 2, H = 2.
Coefficients multiply the whole formula
2H₂O contains twice as many atoms as H₂O: H = 4, O = 2, total = 6.
Total atoms vs different elements
C₆H₁₂O₆ contains 24 atoms in total but only 3 different elements: C, H and O.
Examples
Try an example
| Formula | Atoms |
|---|---|
| 3 atoms | |
| 3 atoms | |
| 5 atoms | |
| 17 atoms | |
| 24 atoms | |
| 21 atoms | |
| 6 atoms |
FAQ
Frequently asked questions
How do I count atoms in a chemical formula?
Type the formula and read the table: each row shows one element and how many atoms of it the formula contains, with the total underneath. Parentheses, brackets, hydrates, and leading coefficients are all counted.
How many atoms are in H₂O?
3 atoms: 2 hydrogen and 1 oxygen.
How many atoms are in Ca(OH)₂?
5 atoms: Ca = 1, O = 2, H = 2. The subscript 2 outside the parentheses doubles the whole OH group.
What does a subscript mean in a chemical formula?
A subscript tells you how many atoms of the element or group immediately before it occur in the formula. H₂ means 2 hydrogen atoms; (OH)₂ means the OH group occurs twice.
What does a coefficient mean?
A coefficient multiplies the whole formula. 2H₂O is two water units: H = 4, O = 2, total = 6 atoms. Subscripts belong to the formula; coefficients multiply it.
How do I count atoms in parentheses?
Multiply everything inside the parentheses by the number outside them. In Ca(OH)₂ the 2 applies to both O and H, giving O = 2 and H = 2.
Can I count atoms in hydrates?
Yes. Enter hydrates with a middle dot, period, or asterisk, for example CuSO₄·5H₂O. The water part is expanded completely: Cu = 1, S = 1, O = 9, H = 10, total = 21 atoms.
What is the difference between total atoms and different elements?
Total atoms is the sum of every atom. Different elements is how many element types appear. C₆H₁₂O₆ has 24 total atoms but only 3 different elements: C, H and O.
Is 2H₂O the same as H₂O?
No. H₂O is one water unit with 3 atoms. 2H₂O is two water units with 6 atoms. The coefficient changes the amount, never the substance.
Why does capitalization matter in chemical formulas?
Element symbols are case-sensitive: Co is cobalt, CO is carbon plus oxygen, and lowercase co matches no element. One letter changes the entire substance.