Over the years, I have tried different strategies to help students balance chemical equations. I've found a number of students want to put numbers inside molecules when balancing. Here is a strategy I've found effective in stopping students making new and exotic chemical species!
Break 'em up and Bubble Wrap
I'll use the formation of magnesium oxide as an example.
Step One: Write out the symbol equation...
Step Two: The symbols for the atoms of each chemical are written out underneath - the breaking up part
Step Three: 'Bubble Wrap' (circle) the atoms to show they belong to each specific atom/molecule. This 'protects' the atom/molecule so that it can't be changed.
Step Four: Add bubbles where there are not enough atoms on that side of the equation - keep going until there are the same number of atoms before and after the reaction arrow.
Step Five: Count up the numbers of each bubble.
Walk through...
I can see that there are less oxygen atoms on the right-hand side, so I need to add another "bubble" of the product, MgO, underneath and write the atoms inside...
(This ensures students don't just add a Mg atom to the right-hand side).
The oxygen atoms are balanced but now there are not enough Mg atoms on the left. No problem... add a "bubble"!
I now have the same number of atoms on each side of the equation. All I need to do is add up the numbers of each "bubble"... 2 for Mg, 1 for oxygen molecule and 2 for MgO. The balanced equation is...
The method becomes a little unwieldy when dealing with larger more involved chemical species such as those found in respiration and photosynthesis - it's always good to have another method ready...
Boxing Clever
This method helps protect formula by requiring numbers to go into boxes... Draw a box in front of each of the chemicals in the equation, and use the following rules...
you can only put numbers in the boxes... nothing else can be altered!
numbers in boxes multiply the atom/molecule immediately behind it
there must be the same number of atoms on both sides of the reaction arrow
Again, the boxes are designed to protect the chemical species from any alterations.
It's then that students can start adding numbers with a pencil first (they may need to amend their numbers). I ask them to start from the left and compare the numbers of each type of atom on the left-hand side with those on the right. It may go something like this... "1 Mg on left balances 1 Mg on the right"... Moving on to oxygen, there are 2 O atoms on the left but only 1 on the right - add 2 to MgOs box etc. I ask students to do a recount of the atoms before thinking about adding another number.
Make sure you give students the opportunity to master balancing through practice.