_ H₂ + O2
H20
balance equation

2H2 + O2 = 2H2O

Explanation:

From the original equation, you first need to write each component separately.

Left side: H = 2 ; O = 2 (number based on the subscript)

Right side: H = 2 ; O = 1 (number based on the subscripts; no subscript means that the element is just 1)

Notice that the number of H is already balance but the number of O is not. In order to balance the O, you need to multiply the element by 2, but you CANNOT do this by simply changing the subscript.

Left side: H = 2 ; O = 2

Right side: H = 2 x 2 = 4 ; O = 1 x 2 = 2

Now, notice that the number of O is now balance (both are 2) but the number of H is not (since  

You also multiply the left side H by 2. Hence,

Left side: H = 2 x 2 = 4 ; O = 2

Right side: H = 2 x 2 = 4 ; O = 1 x 2 = 2

The equation is now balanced.

it is the most commonly accepted one