Count the atoms
Same atoms on both sides, or not?
Check the recipe
A recipe needs the same ingredients before and after: nothing can appear from thin air. An equation should do the same with atoms.
Look at Mg + O₂ → MgO. How many oxygen atoms are on the left, and how many on the right?
What this lesson covers
The idea
A chemical equation is balanced when the number of atoms of each element is the same on the LHS and RHS; otherwise it is unbalanced, and such an equation is a skeletal chemical equation.
Check the recipe
A recipe needs the same ingredients before and after: nothing can appear from thin air. An equation should do the same with atoms.
Look at Mg + O₂ → MgO. How many oxygen atoms are on the left, and how many on the right?
- 2 on the left, 1 on the right
- 1 on the left, 1 on the right
- 2 on each side
Count and decide
Look at the atom table. Left = LHS atoms, right = RHS atoms. Tap Balanced only if every element matches.
Balanced or skeletal
A chemical equation is balanced when the number of atoms of each element is the same on the LHS and RHS. If not, it is unbalanced, and such an equation is a skeletal chemical equation.
Mg + O₂ → MgO is the skeletal equation for the burning of magnesium in air: oxygen does not match.
- Element | Atoms in Mg + O₂ → MgO: LHS | RHS
- Mg | 1 | 1
- O | 2 | 1
Notes
A chemical equation is balanced when the number of atoms of each element is the same on the LHS and RHS; otherwise it is unbalanced, and such an equation is a skeletal chemical equation.
Check yourself
Count the atoms. Is CaO + H₂O → Ca(OH)₂ balanced?
Which of these is a skeletal equation?
How many oxygen atoms are on the RHS of Fe + H₂O → Fe₃O₄ + H₂ ?
Answer: 4
Fe₃O₄ has 3 Fe and 4 O. H₂ has no oxygen. So 4 oxygen atoms on the RHS (and only 1 on the LHS).
- Yes: Ca 1 and 1, O 2 and 2, H 2 and 2 — correct. Yes! Ca(OH)₂ has one Ca, two O and two H. Every element matches.
- No: O is 2 on the left but 1 on the right. In Ca(OH)₂ the 2 outside the bracket applies to O and H both: two O, two H.
- No: there are 3 formulae on the left. There are only two on the left. We count atoms of each element, not formulae.
- C + O₂ → CO₂. C: 1 and 1. O: 2 and 2. This one is balanced.
- H₂ + O₂ → H₂O — correct. Yes! H: 2 and 2, but O: 2 on the left and only 1 on the right.
- 2H₂ + O₂ → 2H₂O. H: 4 and 4. O: 2 and 2. This one is balanced.