Atoms, Molecules, and Compounds
🌍 Everything around us is made of elements like sodium, copper, hydrogen, and oxygen. Each element contains only one type of atom. But here’s the cool part:
- Some elements (like sodium) exist as individual atoms
- Others (like hydrogen, oxygen) form molecules:
Hydrogen: \( \text{H} + \text{H} \rightarrow \text{H}_2 \)
Oxygen: \( \text{O} + \text{O} \rightarrow \text{O}_2 \)
✨ Compounds form when different atoms combine in fixed ratios. For example:
- Water (H₂O): 2 hydrogen + 1 oxygen
- Carbon dioxide (CO₂): 1 carbon + 2 oxygen
🔥 Fun fact: Hydrogen burns, oxygen supports fire, but together they make water – which puts out fires!
Physical vs. Chemical Properties
🔬 Every substance has unique characteristics:
| Physical Properties | Chemical Properties |
|---|---|
| Can be observed without changing the substance | Require a chemical change to observe |
| Examples: Color, melting point, density, odor | Examples: Acidity, combustibility, reactivity |
💡 Chemists predict how substances behave by studying these properties!
Measuring Physical Properties
📏 All measurements need a number + unit (like 6 m for length). Scientists use the SI system (International System of Units) because it’s universal and decimal-based.
SI Base Units (The Magnificent Seven!)
| Quantity | Unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
⚡ Cool fact: 1 mole = \( 6.022 \times 10^{23} \) particles (atoms/molecules)!
Handy SI Prefixes
📊 For really big or small measurements:
- milli- (m) = \( 10^{-3} \) → 1 mm = 0.001 m
- kilo- (k) = \( 10^{3} \) → 1 kg = 1000 g
- micro- (μ) = \( 10^{-6} \) → 1 μm = 0.000001 m
Key Measurements in Chemistry
Mass vs. Weight
⚖️ Mass = Amount of matter (constant everywhere)
Weight = Force of gravity on mass (changes on moon!)
💡 Lab tool: Analytical balance (measures mass)
Volume
🧪 Space occupied by a substance
SI unit: m³ → But chemists use smaller units:
\( 1 \text{L} = 1000 \text{mL} = 1000 \text{cm}^3 = 1 \text{dm}^3 \)
🔍 Lab tools: Graduated cylinder, burette, pipette
Density
📦 Mass per unit volume → How tightly packed particles are!
Formula: \( \text{Density} = \frac{\text{Mass}}{\text{Volume}} \)
Units: SI → kg/m³, Chemistry → g/cm³
Temperature
🌡️ Three scales:
| Celsius (°C) | Fahrenheit (°F) | Kelvin (K) |
|---|---|---|
| Water freezes: 0°C | Water freezes: 32°F | Water freezes: 273 K |
| Water boils: 100°C | Water boils: 212°F | Water boils: 373 K |
Conversion magic:
\( °F = \frac{9}{5}(°C) + 32 \)
\( K = °C + 273.15 \)
🚫 Kelvin never goes negative!
NEET Spotlight 🔍
Top 3 must-know concepts:
- Mole concept: 1 mole = \( 6.022 \times 10^{23} \) particles (Avogadro’s number)
- Density calculations: \( \rho = \frac{m}{V} \) with units (g/cm³ common)
- Temperature conversions: Kelvin ↔ Celsius (\( K = °C + 273.15 \))
