Identify the substance and its chemical formula
Find the number of moles if needed
Use Avogadro’s number: 6.022 × 10^23 atoms per mole
Multiply moles by Avogadro’s number to get total atoms
For a compound, count atoms per formula unit
Multiply atoms per formula unit by the number of formula units
For a molecule, multiply the number of molecules by atoms in each molecule
For a sample by mass, convert mass to moles using molar mass first
For a sample by particles, use the particle count directly
For an element in atomic form, the number of particles equals the number of atoms
For a diatomic element, multiply molecules by 2 to get atoms
For a polyatomic compound, add the atoms of each element in the formula
Use subscripts in the formula to determine atoms of each element
Use parentheses and multiply atoms inside by the subscript outside
Sum all atoms of all elements for the total number of atoms