Atoms, Elements and Chemical Reactions: Grade 7 Study Guide
All matter is made of atoms. In chemical reactions, atoms are rearranged to make new substances.
An atom is the smallest unit of an element that keeps its properties. It has a nucleus of protons (positive) and neutrons (no charge) surrounded by electrons (negative). The number of protons is the atomic number and identifies the element. An element is made of only one kind of atom (oxygen, iron). A molecule is two or more atoms joined together, and a compound is a molecule made of different elements, such as water (H₂O) or carbon dioxide (CO₂).
The periodic table arranges elements by atomic number. Elements in the same column (group) have similar properties.
Parts of an atom
| Particle | Charge | Location |
|---|---|---|
| Proton | +1 | nucleus |
| Neutron | 0 | nucleus |
| Electron | −1 | around the nucleus |
Chemical reactions
In a chemical reaction, the starting substances (reactants) form new substances (products) with different properties. Signs: a gas forms, a precipitate (solid) appears, the color changes or heat or light is released. Atoms are not created or destroyed, so the total mass stays the same (conservation of mass). Example: 2H₂ + O₂ → 2H₂O. A chemical equation must have the same number of each atom on both sides.
H₂ + O₂ → H₂O.
O is unbalanced.
2H₂ + O₂ → 2H₂O.
4 H and 2 O on each side.
Changes in energy
Reactions that release heat are exothermic (burning, hand warmers); those that absorb heat are endothermic (cold packs). Synthetic materials such as plastics, medicines and fertilizers come from chemical reactions of natural materials, with benefits and environmental costs that need careful decisions.
Worked example and common misconceptions
Example: Baking soda and vinegar fizz because they form carbon dioxide gas. In a closed bag, the total mass before and after is the same. Common misconceptions: thinking atoms disappear when something burns (the gases escape), mixing up a mixture with a compound and thinking subscripts and coefficients are interchangeable. Wear safety goggles for experiments, and only do them with adult supervision.
Check your understanding: model answers
1. The number of protons in the atom. 2. A compound. 3. H₂ + Cl₂ → 2HCl.
✨ Key Takeaways
- Atoms have protons, neutrons and electrons.
- The atomic number identifies the element.
- Reactions rearrange atoms.
- Mass is conserved.
💬 Questions for Discussion & Reflection
- What does the atomic number tell you?
- Is H₂O an element or compound?
- Balance: H₂ + Cl₂ → HCl.
Print a periodic table study sheet →