1. Atom & Molecule
- Atom: smallest particle of an element that takes part in a chemical reaction.
- Molecule: group of two or more atoms chemically combined.
- O, H, Na – atoms
- O₂, H₂, H₂O, CO₂ – molecules
हिंदी: Atom = सबसे छोटी इकाई (element की)
Molecule = दो या अधिक atoms का समूह।
2. Structure of Atom (Basic NDA View)
Atom has:
- Nucleus – contains protons (+) and neutrons (0).
- Electrons (−) revolve around nucleus in shells/orbits.
- Proton charge = +1
- Electron charge = −1
- Neutron – no charge
3. Element, Compound & Mixture
Element: substance made of only one kind of atom.
Examples: H, O, N, Cu, Fe, Au, Ag.
Compound: substance formed by chemical combination of two or more elements in fixed proportion. Examples: H₂O, CO₂, NaCl, NH₃.
Mixture: two or more substances physically mixed (no fixed proportion). Examples: air, salt-sand, sugar-water.
Compound: substance formed by chemical combination of two or more elements in fixed proportion. Examples: H₂O, CO₂, NaCl, NH₃.
Mixture: two or more substances physically mixed (no fixed proportion). Examples: air, salt-sand, sugar-water.
हिंदी:
Element = एक ही प्रकार के atom
Compound = अलग–अलग elements रासायनिक रूप से जुड़कर एक नया पदार्थ बनाते हैं
Mixture = बस मिल गए, chemical change नहीं हुआ।
4. Valency (Simple Definition)
Valency: combining capacity of an element.
Examples (common valencies):
- H = 1
- O = 2
- N = 3
- C = 4
- Na = 1, Cl = 1
5. Important Gases & Their Uses (UPSC Favourite)
- Oxygen (O₂): supports combustion & respiration; used in hospitals & industry.
- Hydrogen (H₂): light gas; used in fuel, welding, manufacture of ammonia.
- Nitrogen (N₂): major component of air (~78%); used in fertilizers, inert atmosphere.
- Carbon dioxide (CO₂): used in soft drinks, fire extinguishers; plant photosynthesis.
📘 Practice Questions (NDA Style)
1. Smallest particle of an element that takes part in chemical reactions is:
a) Molecule
b) Atom
c) Ion
d) Proton
2. Protons and neutrons are present in:
a) Orbit
b) Shell
c) Nucleus
d) Electron cloud
3. Which of the following is a compound?
a) O₂
b) N₂
c) CO₂
d) He
4. Air is an example of:
a) Element
b) Compound
c) Mixture
d) Metal
5. Major component of air is:
a) Oxygen
b) Nitrogen
c) Carbon dioxide
d) Hydrogen
a) Molecule
b) Atom
c) Ion
d) Proton
2. Protons and neutrons are present in:
a) Orbit
b) Shell
c) Nucleus
d) Electron cloud
3. Which of the following is a compound?
a) O₂
b) N₂
c) CO₂
d) He
4. Air is an example of:
a) Element
b) Compound
c) Mixture
d) Metal
5. Major component of air is:
a) Oxygen
b) Nitrogen
c) Carbon dioxide
d) Hydrogen
Answers:
1 – b
2 – c
3 – c
4 – c
5 – b
1 – b
2 – c
3 – c
4 – c
5 – b
Quick Recap | सार
Now you should be able to:
- Recall the most important ideas of this topic.
- Solve typical NDA-style questions from this chapter.
- Spot and avoid common traps used in competitive exams.
Self-check: 3–4 random questions उठा कर देखिए – अगर आसानी से हो रहे हैं, तो chapter strong है।
Common Exam Mistakes | आम गलतियाँ
- Concept ठीक से clear न करना और सीधे रटने की कोशिश करना।
- Question पूरी तरह पढ़े बिना answer mark कर देना।
- Figures, dates, या formulas को last step में verify न करना।
NDA में smart strategy यह है कि आसान वाले सवाल पहले, doubtful बाद में – और silly mistakes से बचना सबसे ज़रूरी है।