⚗️ What is Chemistry?
Chemistry is the study of matter, its properties, and how it changes. It's often called the "central science" because it connects physics with biology and other natural sciences. Understanding chemistry helps us comprehend everything from cooking to medicine to materials science.
Why Study Chemistry?
- Medicine: Drug development, understanding how medications work
- Environment: Pollution, climate change, clean energy solutions
- Materials: Plastics, metals, semiconductors, new technologies
- Daily Life: Cooking, cleaning, nutrition, cosmetics
⚛️ Atomic Structure
Parts of an Atom
| Particle | Charge | Mass (amu) | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | ~0 (1/1836) | Electron cloud |
Key Atomic Definitions
- Atomic Number (Z): Number of protons (defines the element)
- Mass Number (A): Protons + Neutrons
- Isotopes: Same element, different neutrons
- Ion: Atom with gained/lost electrons (charged)
Electron Configuration
Electrons fill orbitals in a specific order based on energy levels:
Orbital Capacity
- s orbital: 2 electrons
- p orbitals: 6 electrons (3 orbitals × 2)
- d orbitals: 10 electrons (5 orbitals × 2)
- f orbitals: 14 electrons (7 orbitals × 2)
Electron configuration: 1s² 2s² 2p²
6 total electrons fill: 2 in 1s, 2 in 2s, 2 in 2p
📊 The Periodic Table
Periodic Trends
| Property | Across Period (→) | Down Group (↓) |
|---|---|---|
| Atomic Radius | Decreases | Increases |
| Ionization Energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
| Metallic Character | Decreases | Increases |
Important Groups
- Group 1 - Alkali Metals: Li, Na, K... (1 valence e⁻, very reactive)
- Group 2 - Alkaline Earth: Be, Mg, Ca... (2 valence e⁻)
- Group 17 - Halogens: F, Cl, Br... (7 valence e⁻, very reactive)
- Group 18 - Noble Gases: He, Ne, Ar... (8 valence e⁻, stable/inert)
For main group elements: Group number = Number of valence electrons
Example: Group 14 (Carbon) has 4 valence electrons
🔗 Chemical Bonding
Types of Chemical Bonds
| Bond Type | How It Forms | Between | Example |
|---|---|---|---|
| Ionic | Transfer of electrons | Metal + Nonmetal | NaCl |
| Covalent | Sharing of electrons | Nonmetal + Nonmetal | H₂O, CO₂ |
| Metallic | Sea of delocalized e⁻ | Metal + Metal | Fe, Cu |
Ionic Bonds
Characteristics of Ionic Compounds
- High melting/boiling points
- Conduct electricity when dissolved or molten
- Form crystal lattice structures
- Generally soluble in water
Covalent Bonds
Types of Covalent Bonds
- Single bond: 1 shared pair (e.g., H-H)
- Double bond: 2 shared pairs (e.g., O=O)
- Triple bond: 3 shared pairs (e.g., N≡N)
- Polar covalent: Unequal sharing (e.g., H-O)
- Nonpolar covalent: Equal sharing (e.g., H-H)
ΔEN = 0.1-1.7 → Polar Covalent
ΔEN > 1.7 → Ionic
Lewis Dot Structures
- Count total valence electrons
- Draw skeleton (central atom surrounded by others)
- Place bonding pairs between atoms
- Distribute remaining electrons as lone pairs
- Check: Does each atom have an octet? (Except H = 2)
⚡ Chemical Reactions
Types of Chemical Reactions
| Reaction Type | General Form | 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 → NaNO₃ + AgCl |
| Combustion | CₓHᵧ + O₂ → CO₂ + H₂O | CH₄ + 2O₂ → CO₂ + 2H₂O |
Balancing Chemical Equations
- Write the unbalanced equation
- Count atoms of each element on both sides
- Add coefficients (NEVER change subscripts)
- Balance one element at a time
- Start with metals, then nonmetals, save O and H for last
- Verify: same atoms on both sides
Stoichiometry
n = m / M
Moles = Volume (L) × Molarity
n = V × M
Acids and Bases
| Property | Acids | Bases |
|---|---|---|
| Produces | H⁺ ions | OH⁻ ions |
| pH | Below 7 | Above 7 |
| Taste | Sour | Bitter |
| Litmus | Turns blue → red | Turns red → blue |
| Examples | HCl, H₂SO₄, HNO₃ | NaOH, KOH, NH₃ |
✏️ Practice Problems
Answer: 17 protons, 18 neutrons, 17 electrons
Answer: 4Fe + 3O₂ → 2Fe₂O₃
Answer: 2 moles of H₂O
Answer: [H⁺] = 1 × 10⁻⁴ M, [OH⁻] = 1 × 10⁻¹⁰ M
💡 Tips & Tricks
NO₃⁻ (nitrate), SO₄²⁻ (sulfate), PO₄³⁻ (phosphate), CO₃²⁻ (carbonate), OH⁻ (hydroxide), NH₄⁺ (ammonium)
Always include units in calculations. Cancel units to check your work!
Save hydrogen and oxygen for last—they're usually in multiple compounds.
H₂, O₂, F₂, Br₂, I₂, N₂, Cl₂
⚠️ Common Mistakes to Avoid
NEVER change subscripts! Only add coefficients in front of formulas. Changing subscripts changes the substance itself.
Seven elements exist as diatomic molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂. Remember them!
Atomic number (Z) = protons. Mass number (A) = protons + neutrons. Don't confuse them!
In multiplication/division: answer has same number of sig figs as the least precise value.
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