Magnesium ribbon burns in air to give a white powder of Magnesium oxide.

1. Balanced chemical equation: A balanced chemical equation has the same number of each type of atom on both sides of the arrow in a chemical equation.
2. Reactants: Magnesium ($\mathrm{Mg}$) and Air represents oxygen (${\mathrm{O}}_2$).
3. When magnesium is burned in the air, it combines with oxygen gas to form magnesium oxide ($\mathrm{MgO}$).
4. The chemical equation representing the reaction is: $$\begin{gathered} \mathrm{Mg}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{∆}{\to }\mathrm{MgO}\left(\mathrm{s}\right) \\ \mathrm{Magnesium}\mathrm{Oxygen}\mathrm{Magnesium}\mathrm{oxide} \end{gathered}$$
5. But the above equation is not balanced as both sides do not have the same number of oxygen atoms.

Atom	Reactant side	Product side
Magnesium ($\mathrm{Mg}$)	1	1
Oxygen ($\mathrm{O}$)	2	1

6. Now, to balance the number of oxygen atoms, multiply MgO by 2.

$$\begin{gathered} \mathrm{Mg}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{∆}{\to }2\mathrm{MgO}\left(\mathrm{s}\right) \\ \mathrm{Magnesium}\mathrm{Oxygen}\mathrm{Magnesium}\mathrm{oxide} \end{gathered}$$

Atom	Reactant side	Product side
Magnesium ($\mathrm{Mg}$)	1	2
Oxygen ($\mathrm{O}$)	2	2

7. Next, magnesium becomes unbalanced which is made equal by adding coefficient 2 ahead of $\mathrm{Mg}$.

$$\begin{gathered} 2\mathrm{Mg}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{∆}{\to }2\mathrm{MgO}\left(\mathrm{s}\right) \\ \mathrm{Magnesium}\mathrm{Oxygen}\mathrm{Magnesium}\mathrm{oxide} \end{gathered}$$

Atom	Reactant side	Product side
Magnesium ($\mathrm{Mg}$)	2	2
Oxygen ($\mathrm{O}$)	2	2

8. Finally, the balanced equation is:

$$\begin{gathered} 2\mathrm{Mg}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{∆}{\to }2\mathrm{MgO}\left(\mathrm{s}\right) \\ \mathrm{Magnesium}\mathrm{Oxygen}\mathrm{Magnesium}\mathrm{oxide} \\ (\mathrm{White}\mathrm{powder}) \end{gathered}$$