A chemical reaction rearranges atoms to form new substances. Reactants appear before the arrow in an equation; products appear after it. The arrow means forms, rather than equals. A word equation names substances, while a symbol equation uses chemical formulas. For example, hydrogen + oxygen → water describes the same substances as H2+O2→H2O, although that symbol equation still needs balancing.
A subscript belongs to a formula: H2O contains two hydrogen atoms and one oxygen atom per molecule. A coefficient multiplies the whole formula: 3H2O contains six hydrogen atoms and three oxygen atoms. In ordinary chemical reactions, the types of atoms remain unchanged; bonds and arrangements change. Use atom counts to check any proposed particle picture.
Example 1
Count atoms represented by 2CO2.
The coefficient 2 multiplies the entire formula. Carbon: 2×1=2 atoms. Oxygen: 2×2=4 atoms.
Example 2
Identify reactants and product in magnesium + oxygen → magnesium oxide.
Magnesium and oxygen are the starting substances, so they are reactants. Magnesium oxide is formed, so it is the product.
Watch out: Ordinary chemical reactions rearrange existing atoms. Changing an element requires a nuclear change.