Chemistry Basics
Chemistry is the study of matter — what it is made of, how it behaves, and how it changes. Understanding chemistry explains everything from cooking to medicine to the air we breathe.
Atomic Structure
Every element is made of atoms. An atom has three types of particles:
| Particle | Charge | Location |
|---|---|---|
| Proton | Positive (+) | Nucleus |
| Neutron | Neutral (0) | Nucleus |
| Electron | Negative (−) | Orbits nucleus in shells |
The atomic number = number of protons (defines the element). The mass number = protons + neutrons. Atoms of the same element with different neutron counts are called isotopes.
The Periodic Table
The periodic table organizes all 118 known elements by atomic number. Key features:
- Periods (rows) — elements in the same row have the same number of electron shells
- Groups (columns) — elements in the same column have similar chemical properties
- Metals — left/center (conduct electricity, malleable)
- Nonmetals — upper right (poor conductors, many are gases)
- Noble gases — Group 18; very stable, rarely react
Valence Electrons
The electrons in the outermost shell determine how an element reacts. Elements in the same group have the same number of valence electrons.
Chemical Bonds
Atoms bond to become more stable by achieving a full outer electron shell.
| Bond Type | How It Forms | Example |
|---|---|---|
| Ionic | One atom transfers electrons to another | NaCl (table salt) |
| Covalent | Atoms share electrons | H₂O (water), CO₂ |
| Metallic | Electrons flow freely between metal atoms | Iron, copper wire |
Types of Chemical Reactions
| Type | Pattern | Example |
|---|---|---|
| Synthesis | A + B → AB | 2H₂ + O₂ → 2H₂O |
| Decomposition | AB → A + B | 2H₂O → 2H₂ + O₂ |
| Single Replacement | A + BC → AC + B | Zn + 2HCl → ZnCl₂ + H₂ |
| Double Replacement | AB + CD → AD + CB | NaCl + AgNO₃ → AgCl + NaNO₃ |
| Combustion | CₓHᵧ + O₂ → CO₂ + H₂O | CH₄ + 2O₂ → CO₂ + 2H₂O |
Law of Conservation of Mass
Atoms are never created or destroyed in a chemical reaction — only rearranged. A balanced equation has equal atoms on both sides.
Quick Quiz
Test what you just learned. Choose the best answer for each question.