Module 3 · Agniveer Science

Chemistry Basics

Atomic structure, chemical bonding, acids-bases, periodic table.

Atomic Structure · Bonding · Acids-Bases · Periodic Table

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Learning Objectives

Understand the fundamental concepts of Chemistry Basics
Apply key formulas and techniques to solve problems
Practice with exam-level questions to build speed and accuracy

Key Concepts

What is Chemistry?

Chemistry is the study of matter — its composition, properties, structure, and the changes it undergoes. In Agniveer and defence exams, chemistry questions cover atomic structure, chemical bonding, acids-bases-salts, periodic table, metals and non-metals, and everyday chemistry. Questions are at the high school level and focus on basic concepts and applications.

Key Topics in Chemistry

1. Atomic Structure: Atom = protons (positive, in nucleus) + neutrons (neutral, in nucleus) + electrons (negative, orbiting). Atomic number (Z) = number of protons. Mass number (A) = protons + neutrons. Isotopes (same Z, different A), Isobars (same A, different Z), Isotones (same number of neutrons). Bohr's model: electrons in fixed orbits (shells K, L, M, N).

2. Chemical Bonding: Ionic bond (transfer of electrons — metal + non-metal, e.g., NaCl). Covalent bond (sharing of electrons — non-metal + non-metal, e.g., H₂O, CO₂). Metallic bond (electron sea model). Valency — combining capacity of an element.

3. Acids, Bases, and Salts: Acids (pH < 7) — sour, turn blue litmus red, examples: HCl, H₂SO₄, CH₃COOH. Bases (pH > 7) — bitter, turn red litmus blue, examples: NaOH, Ca(OH)₂, NH₄OH. Neutralization: Acid + Base → Salt + Water. pH scale (0-14), universal indicator.

4. Periodic Table: Mendeleev's periodic table (based on atomic mass). Modern periodic table (based on atomic number). 18 groups (vertical) and 7 periods (horizontal). Groups: Alkali metals (Group 1), Alkaline earth metals (Group 2), Halogens (Group 17), Noble gases (Group 18). Periodic trends: atomic size (decreases across period, increases down group), metallic character, electronegativity.

5. Metals and Non-Metals: Metals (lustrous, malleable, ductile, good conductors of heat/electricity). Non-metals (dull, brittle, poor conductors). Reaction of metals with oxygen, water, acids, and displacement reactions (reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au).

6. Everyday Chemistry: Baking soda (NaHCO₃), washing soda (Na₂CO₃·10H₂O), bleaching powder (CaOCl₂), vinegar (CH₃COOH), soap (sodium salts of fatty acids), detergents, plastics, fertilizers (urea, NPK), CFCs and ozone depletion.

Solved Examples

Example 1 (Atomic Structure)

What is the atomic number of an element with 12 protons and 12 neutrons?

a) 12 b) 24 c) 6 d) 18

Solution: a) 12. Atomic number = number of protons = 12. Mass number = 12 + 12 = 24 (not asked here).

Example 2 (pH Scale)

Which of the following has the highest pH value?

a) Lemon juice b) Vinegar c) Soap solution d) Milk

Solution: c) Soap solution. Lemon juice (~2), vinegar (~3), milk (~6.5) are acidic/weakly acidic. Soap solution (~9-10) is basic, having the highest pH.

Example 3 (Reactvity Series)

Which metal is the most reactive in the reactivity series?

a) Iron b) Zinc c) Sodium d) Gold

Solution: c) Sodium is the most reactive among these. The reactivity series order: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au. Gold is the least reactive.

Shortcut Techniques

Remember the first 20 elements of the periodic table in order: H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca.
Valency shortcuts: Group 1 = +1, Group 2 = +2, Group 13 = +3, Group 14 = ±4, Group 15 = -3, Group 16 = -2, Group 17 = -1, Group 18 = 0.
For acids: Hydrochloric (HCl), Sulphuric (H₂SO₄), Nitric (HNO₃) are strong acids. Acetic (CH₃COOH), Carbonic (H₂CO₃) are weak acids.
For bases: NaOH, KOH are strong bases; Ca(OH)₂, NH₄OH are weak bases.

Pro Tip

In Agniveer exams, chemistry questions are often from NCERT Class 9-10 syllabus. Focus on the practical applications — what is baking soda used for, why do we use antacids, how does soap clean — rather than deep theoretical concepts.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip

Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip

Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip

Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip

Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Practice Questions

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