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How to Count Atoms
How do we count the number of atoms in the chemical formulas of atoms and compounds?
Symbol
The symbol of an element represents one atom of that element. Example: The symbol for sodium is Na. This represents 1 atom of sodium.
Symbol
Try this: How many atoms are there in this symbol: Al Answer: 1 atom
Subscript
The subscript is written at the lower right of the element’s symbol. If there is more than one atom of the element in the molecule, then the subscript indicates how many atoms there are. Example: The symbol for oxygen gas is O 2. There are 2 atoms making up this molecule.
Subscript
Try this: How may atoms make up the molecule sulphur S 8 ? Answer: 8 atoms
Subscript Outside a Bracket
A subscript outside a bracket multiplies all the elements inside the bracket. Example: The symbol for barium phosphate is Ba 3 (PO 4 ) 2. In 1 formula unit of barium phosphate there are 3 atoms of barium (Ba) (2)(1) = 2 atoms of phosphorus (P) (2)(4) = 8 atoms of oxygen (O) The total is 3 + 2 + 8 = 13 atoms.
Subscript Outside a Bracket
Try this: How may atoms make up 1 formula unit of aluminum sulphate Al 2 (SO 4 ) 3 ? Answer: 2 atoms of aluminum (Al) (3)(1) = 3 atoms of sulphur (S) (3)(4) = 12 atoms of oxygen (O) The total is 2 + 3 + 12 = 17 atoms.
Coefficients for Atoms
A coefficient is the number written in front of a chemical symbol and indicates the number of atoms of that element. Example: The symbol for three atoms of carbon is written as 3C.
Coefficients for Atoms
Try this: How may atoms of lithium are there in the symbol 4Li? Answer: 4 atoms
Coefficients for Compounds
A coefficient is the number written in front of a chemical formula and indicates the number of molecules (for molecular compounds) or formula units (for ionic compounds). Example: The symbol for three molecules of glucose is written as 3C 6 H 12 O 6 . Example: The symbol for four formula units of copper (II) nitrate is written as 4Cu(NO 3 ) 2.
Coefficients for Compounds
Try this: How may formula units of iron (III) sulphate are there in the symbol 3Fe 2 (SO 4 ) 3 ? Answer: 3 formula units
Atoms in Compounds
To find the number of atoms in a compound, multiply the coefficient by the number of atoms in each element of the formula. Example: The number of atoms in three molecules of glucose 3C 6 H 12 O 6 is: (3)(6) = 18 atoms of carbon (3)(12) = 36 atoms of hydrogen (3)(6) = 18 atoms of oxygen Total number of atoms: 18 + 36 + 18 = 72 atoms
