How to count atoms 1.The symbol of an element represents one atom of that element. Ex: Ba, Cl, ….etc. 2.A subscript is a number written at the lower right corner behind the symbol of an element. If there is more than one atom of the element, then a subscript is used to indicate the number of atoms. Ex: Cl 2 = 2 atoms 1.The symbol of an element represents one atom of that element. Ex: Ba, Cl, ….etc. 2.A subscript is a number written at the lower right corner behind the symbol of an element. If there is more than one atom of the element, then a subscript is used to indicate the number of atoms. Ex: Cl 2 = 2 atoms

H 2 SO 4 = 2 H 1 S 4 O

3. A subscript outside a bracket multiplies all the elements inside the brackets. Ca 3 (PO 4 ) 2 = Ca= 3 P=2 O=8 Ca 3 (PO 4 ) 2 = Ca= 3 P=2 O=8

A coefficient is a number written in front of a chemical symbol and indicates the number of atoms of that elements or number of molecules 3 C= 3 carbon atoms 2 Na SO 4 = 2 Na 2 S 8 O 3 C= 3 carbon atoms 2 Na SO 4 = 2 Na 2 S 8 O

Practice problems: 2H 2 O _____molecules of water _____ H ( Hydrogen) _____ O ( oxygen)

3 Na 2 SO 4 _____molecules of Na 2 SO 4 _____ S ( sulphur) _____ O ( Oxygen)

4Pb(NO 3 ) 2 ____ molecules of Pb(NO 3 ) 2 ____N ( Nitrogen) ____O ( Oxygen)