Theory & Concepts

Master Chemistry (Atoms, Molecules, Acids/Bases, Periodic Table) for SSC CGL

Get comprehensive theory, expert shortcuts, and hand-picked practice questions for Chemistry (Atoms, Molecules, Acids/Bases, Periodic Table) specifically designed for the SSC CGL 2025-26 pattern.

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General Chemistry: Atoms, Acids/Bases & Periodic Table Trends

Chemistry in the SSC CGL general awareness syllabus revolves around atomic structures, periodic characteristics, and everyday chemical compounds. Standard questions target the discoverers of subatomic particles, shifts in electronegativity or atomic size across groups/periods, and household salts like baking soda and bleaching powder. This interactive study page synthesizes these subjects alongside 20 solved questions.

Learning path

Atomic Models & Isotopes/Isobars
pH Indicators & Daily Chemical Salts
Modern Periodic Law & Trends
20 Solved Practice Questions

1. Atomic Structure & Subatomic Particles

An atom is the basic unit of a chemical element, composed of a dense central nucleus surrounded by an electron cloud.

Discoveries of Subatomic Particles:
- Electron: Discovered by J.J. Thomson (1897) using cathode ray tube experiments. He proposed the "Plum Pudding" model.
- Proton: E. Goldstein observed canal rays (positive ions) in 1886. Ernest Rutherford is credited with naming and discovering the proton in his gold foil experiment (which also discovered the atomic nucleus).
- Neutron: Discovered by James Chadwick (1932) by bombarding beryllium with alpha particles. It has no charge.
Isotopes, Isobars & Isotones:
- Isotopes: Atoms of the same element with the same atomic number (protons) but different mass numbers (neutrons).
  Example: Protium (¹H), Deuterium (²H, heavy water base), and Tritium (³H, radioactive).
- Isobars: Atoms of different elements with the same mass number but different atomic numbers.
  Example: Argon (₁₈Ar⁴⁰) and Calcium (₂₀Ca⁴⁰).
- Isotones: Atoms of different elements containing the same number of neutrons (A - Z).
  Example: Silicon (₁₄Si³⁰) and Phosphorus (₁₅P³¹), both containing 16 neutrons.

2. Acids, Bases, pH & Daily Life Compounds

Acids and bases are defined under the Arrhenius concept: acids yield hydrogen ions (H⁺) in aqueous solution, while bases yield hydroxide ions (OH^-).

Interactive pH scale & Indicator chart

Explore the slider below to learn the pH levels of common domestic products and their corresponding acidity/alkalinity:

Drag Slider to test pH levelspH = 7

Pure Water

Perfect balance of H+ and OH- ions. Neutral.

NeutralLitmus test: No change

High-Yield Household Chemical Salts

CGL regularly tests the chemical names, formulas, and uses of daily life compounds:

Baking Soda (Sodium Bicarbonate - NaHCO₃): Used in baking (releases CO₂ gas to make cakes fluffy), fire extinguishers, and as an antacid.
Washing Soda (Sodium Carbonate Decahydrate - Na₂CO₃ · 10H₂O): Used in glass, soap, and paper industries, and for removing the permanent hardness of water.
Bleaching Powder (Calcium Oxychloride - CaOCl₂): Prepared by action of chlorine gas on dry slaked lime [Ca(OH)₂]. Used as a disinfectant for drinking water and in textile bleaching.
Plaster of Paris (Calcium Sulfate Hemihydrate - CaSO₄ · 1/2 H₂O): Obtained by heating Gypsum (CaSO₄ · 2H₂O) to 373 K. Used for plastering fractured bones, making toys, and casts.

3. The Modern Periodic Table & Trends

Early classification systems like Mendeleev's Periodic Table arranged elements based on atomic mass. In 1913, Henry Moseley showed that atomic number is a more fundamental property, leading to the Modern Periodic Table:

Structure: It contains 7 Periods (horizontal rows) and 18 Groups (vertical columns). Group 18 holds Noble Gases (inert); Group 17 holds Halogens; Group 1 holds Alkali Metals.
Periodic Trends: Master these trends (select the respective tab in the interactive tool above for complete explanations):
- Atomic Radius: Increases down a group (due to shell additions); decreases across a period (due to increased effective nuclear charge).
- Ionization Energy: Energy required to remove an electron. Decreases down a group; increases left-to-right.
- Electronegativity: Tendency to attract shared electrons. Decreases down a group; increases left-to-right. Fluorine has the highest electronegativity.

4. Practice Questions (20 Premium Solved Questions)

Question 01CGL Pattern

Who was awarded the Nobel Prize in Physics in 1906 for the discovery of the electron through experiments on cathode rays?

A) Ernest Rutherford

B) J.J. Thomson

C) John Dalton

D) James Chadwick

Correct answer: B) J.J. Thomson

Step-by-step Solution

Step 1: Identify the scientist credited with electron discovery.

Step 2: J.J. Thomson used the cathode ray tube in 1897 to identify negatively charged particles, naming them corpuscles (electrons).

Conclusion: Option B is correct.

Question 02CGL Pattern

What name is given to atoms of different chemical elements that share the same mass number but have different atomic numbers?

A) Isotopes

B) Isobars

C) Isotones

D) Isomers

Correct answer: B) Isobars

Step-by-step Solution

Step 1: Define the terms. Isotopes have same atomic number, different mass number.

Step 2: Isobars have the same mass number (nucleon count) but different atomic numbers (protons).

Conclusion: Option B is correct.

Question 03CGL Pattern

Which subatomic particle was discovered by the English physicist James Chadwick in 1932 by bombarding beryllium with alpha particles?

A) Proton

B) Electron

C) Neutron

D) Positron

Correct answer: C) Neutron

Step-by-step Solution

Step 1: James Chadwick observed a new neutral radiation consisting of particles with a mass close to that of a proton.

Step 2: He identified these uncharged particles as neutrons.

Conclusion: Option C is correct.

Question 04CGL Pattern

The Modern Periodic Table is arranged based on the Modern Periodic Law formulated by Henry Moseley. Under this law, elements are organized by:

A) Increasing atomic mass

B) Increasing atomic number

C) Decreasing equivalent weight

D) Number of valence electrons only

Correct answer: B) Increasing atomic number

Step-by-step Solution

Step 1: Mendeleev's periodic table sorted elements by atomic mass.

Step 2: Henry Moseley discovered that the X-ray frequency emitted by elements is proportional to atomic number (nuclear charge).

Step 3: Hence, the Modern Periodic Table arranges elements by increasing atomic number.

Conclusion: Option B is correct.

Question 05CGL Pattern

What trend is observed in the atomic radius of elements as one moves from left to right across a period in the Modern Periodic Table?

A) It increases

B) It decreases

C) It remains constant

D) It first decreases then increases

Correct answer: B) It decreases

Step-by-step Solution

Step 1: As you go left to right, electrons are added to the same shell.

Step 2: Concurrently, protons are added to the nucleus, increasing effective nuclear charge.

Step 3: This stronger positive pull draws the electron shells closer, reducing atomic radius.

Conclusion: Option B is correct.

Question 06CGL Pattern

Which of the following chemical elements in the Modern Periodic Table possesses the highest electronegativity value on the Pauling scale?

A) Chlorine

B) Oxygen

C) Fluorine

D) Helium

Correct answer: C) Fluorine

Step-by-step Solution

Step 1: Electronegativity increases up and to the right in the periodic table.

Step 2: Halogens have high electronegativity. Fluorine is at the top of Group 17.

Step 3: It has the highest electronegativity of 4.0 on the Pauling scale.

Conclusion: Option C is correct.

Question 07CGL Pattern

What is the exact pH value of pure, neutral water at standard room temperature (25°C)?

A) 0

B) 5.6

C) 7.0

D) 8.2

Correct answer: C) 7.0

Step-by-step Solution

Step 1: The pH scale measures hydrogen ion concentration. Neutrality is defined as [H⁺] = 10⁻⁷ M.

Step 2: pH = -log[H⁺] = -log(10⁻⁷) = 7.

Conclusion: Option C is correct.

Question 08CGL Pattern

An aqueous solution of an acid will turn the color of litmus paper from:

A) Red to Blue

B) Blue to Red

C) Red to Colorless

D) Blue to Yellow

Correct answer: B) Blue to Red

Step-by-step Solution

Step 1: Litmus is a natural indicator. Acids are proton donors.

Step 2: Acidic solutions turn blue litmus paper red.

Step 3: Alkaline (basic) solutions turn red litmus paper blue.

Conclusion: Option B is correct.

Question 09CGL Pattern

What is the chemical name of the household compound commonly known as Baking Soda?

A) Sodium Carbonate

B) Sodium Bicarbonate

C) Calcium Carbonate

D) Sodium Chloride

Correct answer: B) Sodium Bicarbonate

Step-by-step Solution

Step 1: Baking soda formula is NaHCO3.

Step 2: The chemical name is Sodium Bicarbonate (or Sodium Hydrogen Carbonate).

Step 3: Sodium Carbonate (Na2CO3) is washing soda.

Conclusion: Option B is correct.

Question 10CGL Pattern

Which chemical compound, used in casting splints and making gypsum-based decorations, is chemically defined as Calcium Sulfate Hemihydrate?

A) Gypsum

B) Bleaching Powder

C) Plaster of Paris

D) Quicklime

Correct answer: C) Plaster of Paris

Step-by-step Solution

Step 1: Plaster of Paris has the formula CaSO4 · 1/2 H2O (hemihydrate).

Step 2: Gypsum is the dihydrate (CaSO4 · 2H2O). When heated, it dehydrates to PoP.

Conclusion: Option C is correct.

Question 11CGL Pattern

The pH of normal human blood is slightly alkaline, falling within which of the following standard ranges?

A) 6.0 - 6.5

B) 7.0 - 7.2

C) 7.35 - 7.45

D) 8.0 - 8.5

Correct answer: C) 7.35 - 7.45

Step-by-step Solution

Step 1: Human blood pH must be tightly regulated for metabolic function.

Step 2: It is slightly basic (alkaline), averaging around 7.4.

Conclusion: Option C is correct.

Question 12CGL Pattern

How does the synthetic indicator Phenolphthalein behave when added to a basic (alkaline) solution?

A) It remains colorless

B) It turns bright yellow

C) It turns pink/magenta

D) It turns blue

Correct answer: C) It turns pink/magenta

Step-by-step Solution

Step 1: Phenolphthalein is a color-changing pH indicator.

Step 2: It is colorless in acidic and neutral solutions (pH < 8.2).

Step 3: It turns pink or deep magenta in basic solutions (pH > 10).

Conclusion: Option C is correct.

Question 13CGL Pattern

Argon (18Ar40) and Calcium (20Ca40) are classic examples of which type of atomic relation?

A) Isotopes

B) Isobars

C) Isotones

D) Isotopes of different elements

Correct answer: B) Isobars

Step-by-step Solution

Step 1: Note the symbols: both have the mass number 40 at the top.

Step 2: They have different atomic numbers (18 and 20).

Step 3: Atoms with same mass number but different atomic numbers are Isobars.

Conclusion: Option B is correct.

Question 14CGL Pattern

Which organic acid is injected into the skin during an ant bite or bee sting, causing irritation and swelling?

A) Acetic Acid

B) Citric Acid

C) Formic Acid

D) Tartaric Acid

Correct answer: C) Formic Acid

Step-by-step Solution

Step 1: Ants and bees produce methanoic acid in their venom sacs.

Step 2: Methanoic acid is commonly called Formic Acid (derived from the Latin word for ant, formica).

Conclusion: Option C is correct.

Question 15CGL Pattern

What is the correct chemical formula for Bleaching Powder, which is used extensively to sanitize drinking water?

A) Ca(OH)2

B) CaOCl2

C) CaCO3

D) CaCl2

Correct answer: B) CaOCl2

Step-by-step Solution

Step 1: Bleaching powder is Calcium Oxychloride.

Step 2: Its formula is CaOCl2, produced by passing chlorine gas through dry slaked lime [Ca(OH)2].

Conclusion: Option B is correct.

Question 16CGL Pattern

Which specific group in the Modern Periodic Table comprises the highly reactive non-metallic elements known as Halogens?

A) Group 1

B) Group 2

C) Group 17

D) Group 18

Correct answer: C) Group 17

Step-by-step Solution

Step 1: Group 18 contains noble gases. Group 1 has alkali metals.

Step 2: Group 17 contains Fluorine, Chlorine, Bromine, Iodine, Astatine, which are the Halogens.

Conclusion: Option C is correct.

Question 17CGL Pattern

As one moves down a vertical group in the Modern Periodic Table, what occurs to the Ionization Energy of elements?

A) It increases

B) It decreases

C) It remains constant

D) It first increases then decreases

Correct answer: B) It decreases

Step-by-step Solution

Step 1: Down a group, atomic size increases and shielding of inner shells increases.

Step 2: The outermost electrons are further from nuclear positive charge, reducing the electrostatic pull.

Step 3: Less energy is needed to remove this outer electron, meaning ionization energy decreases.

Conclusion: Option B is correct.

Question 18CGL Pattern

What is the chemical name and formula of common table salt used daily in cooking?

A) Sodium Carbonate (Na2CO3)

B) Sodium Chloride (NaCl)

C) Sodium Hydroxide (NaOH)

D) Calcium Chloride (CaCl2)

Correct answer: B) Sodium Chloride (NaCl)

Step-by-step Solution

Step 1: Table salt is NaCl, Sodium Chloride.

Conclusion: Option B is correct.

Question 19CGL Pattern

Which of the following gas species is liberated when an active metal (like zinc or iron) reacts with dilute sulfuric acid?

A) Oxygen gas

B) Carbon dioxide gas

C) Hydrogen gas

D) Chlorine gas

Correct answer: C) Hydrogen gas

Step-by-step Solution

Step 1: Metal + Acid → Salt + Hydrogen Gas.

Step 2: For example, Zn + H2SO4 → ZnSO4 + H2.

Conclusion: Option C is correct.

Question 20CGL Pattern

In Ernest Rutherford's historic alpha-particle scattering gold foil experiment of 1911, the observations led directly to the discovery of:

A) The electron

B) The atomic nucleus

C) The neutron

D) Quantum energy orbits

Correct answer: B) The atomic nucleus

Step-by-step Solution

Step 1: Rutherford bombarded a thin gold foil with positive alpha particles.

Step 2: Most passed through, but some rebounded at large angles.

Step 3: This led to the conclusion that a dense, positively charged mass exists in the center of the atom, which he called the nucleus.

Conclusion: Option B is correct.

Strategy errors to avoid

Baking Soda vs. Washing Soda Formulas

Do not mix up NaHCO₃ (baking soda) with Na₂CO₃ (washing soda). A simple way to remember: 'Bi'-carbonate contains hydrogen, which you can think of as 'by' cooking at home (baking).

Moseley vs. Mendeleev Periodic Basis

Always check if the question refers to the Modern Periodic Table (Moseley, based on atomic number) or the old Periodic Table (Mendeleev, based on atomic mass). Picking mass for modern table is a common mark-loser.

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