What Happens to the Atoms in a Chemical Reaction
Atoms are rearranged in a chemical reaction. The atoms in the reactants become the atoms in the products, bonded in new ways. In an ordinary chemical reaction, atoms are not created or destroyed, and their nuclei do not change. This principle is called conservation of mass and is reflected in balanced chemical equations Khan Academy.
What changes during a reaction?
Chemical reactions convert one set of substances into another IUPAC Gold Book. At the atomic level, existing bonds are disrupted and new bonds form. The atoms keep their identities, but their partners and arrangements can change.
For example, hydrogen reacts with oxygen to form water:
2H₂ + O₂ → 2H₂O
The reactants contain four hydrogen atoms and two oxygen atoms. The products contain the same four hydrogen atoms and two oxygen atoms, now arranged in water molecules. The H–H and O=O bonds are disrupted as the atoms form O–H bonds.
A chemical reaction does not turn one element into another. An atom’s identity depends on its nucleus, particularly its number of protons. Chemical reactions involve changes in electrons and bonding, not changes to the nucleus Encyclopaedia Britannica.
Why must a chemical equation be balanced?
A balanced equation has the same number of atoms of each element on both sides. In the water example, the coefficients show that two hydrogen molecules react with one oxygen molecule to make two water molecules:
| Element | Reactants | Products |
|---|---|---|
| Hydrogen | 4 | 4 |
| Oxygen | 2 | 2 |
The small numbers within formulas, such as the ₂ in H₂, show how many atoms are in each molecule. The numbers placed before formulas, called coefficients, show how many molecules take part. Changing a coefficient changes the amount of a substance; changing a formula’s small numbers changes the substance itself.
Balancing an equation does not cause atoms to be conserved. It records the conservation that takes place in the reaction. If the atom counts differ, the equation is incomplete or incorrect as written.
What does conservation of mass mean?
Since the atoms are present before and after the reaction, their total mass is conserved in a closed system. A closed system is one that does not exchange matter with its surroundings. If a reaction produces a gas in an open container, for instance, that gas can escape, so the measured mass of what remains may fall. The atoms have not vanished; some of the matter has left the container.
Atoms can also change their bonding environment and oxidation state during a reaction. These changes affect how electrons are shared or transferred, but they do not turn an atom into a different element. An iron atom in a compound remains iron; a carbon atom remains carbon.
What happens to energy?
Breaking bonds requires energy, while forming bonds releases energy. The overall energy change depends on the balance between these processes. A reaction may release energy to its surroundings or absorb energy from them.
This energy change comes from the difference between the arrangements of atoms in the reactants and products. It does not mean the atoms have changed into different elements. The atoms remain the same while their bonds and surroundings change.
Can you follow a particular atom?
Yes. Chemists can use isotopes, which are forms of the same element with different numbers of neutrons, to label atoms and follow where they go during a reaction. The labeled atom can be identified in the products, even when counting the elements in a balanced equation alone cannot show which particular atom ended up where.
This method works because a chemical reaction can rearrange atoms without changing their elemental identity. It helps chemists investigate how reactions proceed and which atoms in the reactants contribute to particular parts of the products.
Key Takeaways
- Chemical reactions rearrange atoms into new substances.
- Bonds change, but the atoms’ nuclei and elemental identities remain the same.
- A balanced equation has the same number of each type of atom on both sides.
- In a closed system, the total mass of reactants and products is conserved.
- Bond breaking and bond formation account for the reaction’s energy change.
