There must be the same number of atoms belonging to each element on each side of the equation. For example,
C+O2 → CO2
On the left there is one atom of carbon (C) and two atoms in a molecule of oxygen (O2). This must balance with the right: CO2 is one atom of carbon combined with two atoms of oxygen. If the equation is not balanced, only big numbers in front of various chemicals (ie in the spaces) to balance the equation. For example,
__Fe2O3 +__C →__Fe + __CO2
Numbers in subscript (ie 2 and 3 in Fe2O3) cannot be changed because during so will change the identity of the chemical (ie Fe2O3 is different from FeO).
2.1.2 Compounds, formulae and equations