⭐ Sabse Important Points · एक नज़र में
- Atomic Number (Z): Number of protons; determines element identity.
- Ionic Bond: Formed by complete transfer of electrons; high melting point, conduct electricity in molten/aqueous state.
- Mass Number (A): Protons + neutrons; isotopes differ in A.
- Covalent Bond: Formed by sharing electrons; generally low melting point, poor conductivity.
- Periodic Trends: Across a period, atomic radius decreases and ionization energy increases; down a group, radius increases and ionization energy decreases.
- Metallic Bond: Delocalized electrons; good conductivity, malleability, ductility.
Introduction · परिचय
Chemistry is a core component of NDA General Knowledge, covering fundamental concepts of matter, atomic structure, chemical reactions, and everyday applications. This short revision note focuses on high-yield topics: atomic structure and the periodic table, and chemical bonding, which frequently appear in the exam.
Atomic Structure and the Periodic Table
Atoms consist of protons, neutrons, and electrons. The atomic number (Z) equals the number of protons, defining the element. Isotopes have the same Z but different mass numbers (A) due to varying neutrons. The periodic table arranges elements by increasing Z, revealing periodic trends in properties like atomic radius, ionization energy, and electronegativity.
- Atom
- Nucleus (protons + neutrons)
- Electrons in shells
- Valence electrons determine group
- Periodic trends
📐 Formula / Rule Box · सूत्र याद रखो
Mass Number
A = Z + n
Number of neutrons
n = A - Z
| Property | Across a Period (left to right) | Down a Group (top to bottom) |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Ionization energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
⭐ Key Points · याद रखने वाली बातें
- Atomic Number (Z): Number of protons; determines element identity.
- Mass Number (A): Protons + neutrons; isotopes differ in A.
- Periodic Trends: Across a period, atomic radius decreases and ionization energy increases; down a group, radius increases and ionization energy decreases.
- Valence Electrons: Electrons in the outermost shell; determine chemical properties and group number (for main-group elements).
✍️ Solved Example 1 · हल किया हुआ उदाहरण
❓ Question: An element has atomic number 17 and mass number 35. Determine the number of protons, neutrons, and electrons.
✅ Solution: Protons = 17, Neutrons = 18, Electrons = 17
💡 Explanation: Protons = Z = 17. Neutrons = A - Z = 35 - 17 = 18. In a neutral atom, electrons = protons = 17.
Chemical Bonding
Atoms bond to achieve stable electron configurations (noble gas configuration). Ionic bonds form by transfer of electrons between metals and non-metals, creating ions. Covalent bonds form by sharing electrons between non-metals. Metallic bonds involve a sea of delocalized electrons. Bond type influences properties like melting point, conductivity, and solubility.
📐 Formula / Rule Box · सूत्र याद रखो
Octet Rule
Atoms tend to gain, lose, or share electrons to have 8 electrons in the valence shell (except H, He).
| Property | Ionic Compounds | Covalent Compounds |
|---|---|---|
| Melting/Boiling Point | High | Low |
| Electrical Conductivity | Conduct in molten/aqueous | Poor conductors |
| Solubility | Soluble in water | Generally soluble in organic solvents |
⭐ Key Points · याद रखने वाली बातें
- Ionic Bond: Formed by complete transfer of electrons; high melting point, conduct electricity in molten/aqueous state.
- Covalent Bond: Formed by sharing electrons; generally low melting point, poor conductivity.
- Metallic Bond: Delocalized electrons; good conductivity, malleability, ductility.
✍️ Solved Example 1 · हल किया हुआ उदाहरण
❓ Question: Explain the bond formation in sodium chloride (NaCl).
✅ Solution: Ionic bond: Na loses one electron to form Na+, Cl gains one electron to form Cl-.
💡 Explanation: Sodium (Z=11) has 1 valence electron, chlorine (Z=17) has 7. Na loses its valence electron to achieve Ne configuration (2,8), becoming Na+. Cl gains an electron to achieve Ar configuration (2,8,8), becoming Cl-. Electrostatic attraction between Na+ and Cl- forms the ionic bond.
Practice Questions · खुद हल करके देखो
पहले खुद try karo, phir “Answer dekho” dabakar check karo. 👇
Q1Which of the following has the highest ionization energy? (a) Na (b) Mg (c) Al (d) Si
👀 Answer dekho
✅ Answer: (d) Si
💡 Explanation: Ionization energy increases across a period from left to right. Si is furthest right among the given elements, so it has the highest ionization energy.
Q2The bond in a molecule of chlorine (Cl2) is: (a) Ionic (b) Covalent (c) Metallic (d) Coordinate
👀 Answer dekho
✅ Answer: (b) Covalent
💡 Explanation: Chlorine is a non-metal; two chlorine atoms share a pair of electrons to form a covalent bond.
Q3An atom of an element has 19 protons and 20 neutrons. Its mass number is: (a) 19 (b) 20 (c) 39 (d) 58
👀 Answer dekho
✅ Answer: (c) 39
💡 Explanation: Mass number A = protons + neutrons = 19 + 20 = 39.
⚠️ Common Mistakes · ये गलतियाँ बिल्कुल मत करना
- Confusing atomic number with mass number: Atomic number is protons only; mass number includes neutrons.
- Assuming all covalent compounds are poor conductors: Some covalent compounds (like acids) ionize in water and conduct electricity.
- Forgetting that ionic compounds do not conduct electricity in solid state: Ions are fixed in lattice; conduction requires mobile ions.
⚡ Quick Revision · Exam से पहले 2 मिनट में दोहराओ
- Atomic Number (Z): Number of protons; defines element.
- Mass Number (A): Protons + neutrons; isotopes have same Z, different A.
- Periodic Trends: Across period: radius decreases, ionization energy increases; down group: radius increases, ionization energy decreases.
- Ionic Bond: Transfer of electrons; high melting point, conduct when molten/aqueous.
- Covalent Bond: Sharing of electrons; low melting point, poor conductivity.