FreeLesson 1 of 36 min read
What happens to atoms during a chemical reaction
See how bonds break, atoms rearrange and new bonds form, and why mass stays the same while energy moves in or out.
Every time a candle burns, bread bakes or iron rusts, atoms are being rearranged. If you understand what actually happens to those atoms, you can predict products, explain why mass never truly disappears, and make sense of why some reactions get hot while others get cold.
Reactants in, products out
A chemical reaction turns starting substances, the reactants, into new substances, the products. The products have different properties from the reactants. Hydrogen and oxygen are gases, but together they can make liquid water.
The key fact is this: atoms are not created or destroyed in a chemical reaction, and they do not turn into atoms of a different element. What changes is which atoms are joined to which, and how.
Break, rearrange, make
A useful way to picture any reaction:
- Bonds in the reactants break. This takes in energy.
- The atoms are now free to form new groupings.
- New bonds form between different partners. This gives out energy.
- The same atoms, in the same numbers, end up in the products.
Example: burning methane (natural gas)
Reactants: CH4 + 2O2
Products: CO2 + 2H2O
Count the atoms on each side:
Carbon: 1 on the left, 1 on the right
Hydrogen: 4 on the left, 2 x 2 = 4 on the right
Oxygen: 2 x 2 = 4 on the left, 2 + 2 = 4 on the right
The C–H bonds and O=O bonds broke. New C=O and O–H bonds formed. Same atoms, new partners.
Why mass is conserved
Because atoms are neither made nor destroyed, the total mass of the products equals the total mass of the reactants. This is the law of conservation of mass, and it is an established, well-tested fact.
If mass seems to change, look for a gas. A burning log leaves ash much lighter than the log, because carbon dioxide and water vapour escape into the air. Rusting iron gets heavier, because it combines with oxygen from the air. In a sealed container, the total mass stays the same.
Where the energy goes
Breaking bonds takes energy in; making bonds gives energy out. If making the new bonds releases more energy than breaking the old ones took, the reaction gives out energy overall. That is an exothermic reaction, like burning fuel, and the surroundings warm up.
If breaking bonds takes more energy than making them releases, the reaction takes in energy overall. That is endothermic. Mixing citric acid with baking soda in water is a common example: the mixture feels cooler as it fizzes.
What counts as a chemical change
Melting ice is a physical change: the water molecules are still H2O, just moving differently. Nuclear reactions are different again: they change the nucleus and can turn one element into another. Chemical reactions only involve the outer electrons that form bonds, so the nuclei, and therefore the elements, stay the same.
Clues that a chemical reaction may have happened:
- A gas is given off (fizzing)
- A colour change
- A temperature change
- A solid forms in a solution
- Light is given out
These are clues, not proof. Boiling water also makes bubbles. Next time you meet a reaction, write the reactants and products, count the atoms of each element on both sides, and ask which bonds broke and which formed.
Key takeaways
- In a chemical reaction, bonds break and atoms rearrange into new substances, but no atoms are created or destroyed.
- Total mass is conserved; apparent changes usually mean a gas escaped or was taken in.
- Breaking bonds takes in energy and making bonds gives it out, which decides whether a reaction is exothermic or endothermic.
Check your understanding
Written with AI help. Spotted a mistake? Tell us.