To understand balancing equations, you must be able to complete two steps:
I'll try to illustrate this for the following chemical reaction:
sodium + chlorine ----> sodium chloride
Step 1: You must be able to write the correct formulas for each of the products and reactants. This step involves making sure the subscripts for each reactant and product are correct.
The formulas for the reactants and products are:
sodium - an element is represented by Na
chlorine - one of the seven diatomic elements (that means that the atoms double up in nature - H2 hydrogen, N2 nitrogen, O2 oxygen, F2 fluorine, Cl2 chlorine, Br2 bromine, and I2 iodine) - Cl2
sodium chloride - NaCl
Step 2: Balance the equation (make sure that there are the same number of each type of atom on both sides of the equation) by adjusting the coefficients (the numbers in front of the formulas). DO NOT CHANGE THE SUBSCRIPTS FROM STEP 1.
Unbalanced equation from Step 1:
Na + Cl2 ---> NaCl
It is unbalanced because there are two chlorine atoms on the left and only one on the right.
Adjust the coefficients so that there are the same numbers of each atom on both sides.
2 Na + Cl2 ---> 2 NaCl
Caution: a very common error for beginning students is to attempt to balance an equation by changing the subscripts instead of the coefficients. In this case, a student might write
Na + Cl2 ---> NaCl2 or
Na + Cl ---> NaCl
Although these appears to be balanced (same numbers of each atom on both sides) they are incorrect because in the first case,sodium chloride in nature exists not as NaCl2, but NaCl, and in the second case, Chlorine atoms are diatomic (doubled up) in nature.