Core ConceptRead this first — the foundation of the topic
Atoms and molecules form the basic building blocks of all matter. An atom is the smallest unit of an element that retains its chemical properties. A molecule is formed when two or more atoms join together through chemical bonds. Understanding these concepts is crucial for RRB Group D as questions appear frequently in every paper. Core Properties: Every atom has three main particles - protons (positive charge), neutrons (no charge), and electrons (negative charge). The nucleus contains protons and neutrons, while electrons orbit around it. Atomic number equals the number of protons. Mass number equals protons plus neutrons.
Formula BlockMemorise — at least one formula appears in every paper
Block:
- Atomic Number (Z) = Number of protons
- Mass Number (A) = Number of protons + Number of neutrons
- Number of neutrons = Mass Number - Atomic Number
- Number of molecules = Given mass / Molecular mass
- Avogadro's number = 6.022 × 10²³
Exam PatternsWhat examiners ask — read before attempting PYQs
RRB Group D typically asks direct questions about atomic structure, molecular formulas, and Avogadro's number. Common question types include finding atomic/mass numbers, calculating molecules in given mass, and identifying correct molecular formulas.
ShortcutsUse these to save 30–60 seconds per question
#1 - Quick Neutron Calculation
For any element, subtract atomic number from mass number
Example
Carbon-14 has mass number 14 and atomic number 6, so neutrons = 14-6 = 8.
Shortcut Trick #2 - Molecule Counting: To find number of molecules, use the formula: Number of molecules = (Given mass × Avogadro's number) / Molecular mass. Remember Avogadro's number as 6 × 10²³ for quick calculations.
Worked ExampleSolve this step-by-step before moving on
1
Step 1
Identify given data - Chlorine-35 means mass number = 35
2
Step 2
Find atomic number of Chlorine = 17 (standard value)
3
Step 3
Apply formula - Number of neutrons = Mass number - Atomic number
4
Step 4
Calculate - Neutrons = 35 - 17 = 18
Answer: Chlorine-35 has 18 neutrons.
Worked Example 2: Calculate number of molecules in 18g of water (H₂O).
1
Step 1
Find molecular mass of H₂O = (2×1) + (1×16) = 18g/mol
2
Step 2
Apply formula - Number of molecules = Given mass / Molecular mass
Memory HookRemember this — never confuse the two again
for Common Elements: Remember atomic numbers using 'H-He-Li-Be' pattern: Hydrogen(1), Helium(2), Lithium(3), Beryllium(4). For heavier elements, Carbon(6), Nitrogen(7), Oxygen(8), and Sodium(11) appear frequently.
Most
Exam TrapsCommon mistakes students make — avoid these
Students often confuse atomic mass with mass number. Atomic mass is the average mass considering all isotopes, while mass number is the total protons plus neutrons in a specific isotope. In exams, always use mass number for calculations unless specifically asked for atomic mass.
Another frequent error is forgetting to multiply by Avogadro's number when converting moles to molecules.
Always remember: moles give you the amount, but molecules require multiplication by 6.022 × 10²³.
Key Points to Remember
Atom is the smallest unit of an element that cannot be divided further
Molecule forms when two or more atoms combine through chemical bonds
Atomic number = Number of protons in an atom
Mass number = Number of protons + Number of neutrons
Number of neutrons = Mass number - Atomic number
Avogadro's number = 6.022 × 10²³ (remember as 6 × 10²³ for quick calculation)
Number of molecules = Given mass / Molecular mass × Avogadro's number
Electrons orbit around the nucleus in specific energy levels
Isotopes are atoms of same element with different mass numbers
One mole of any substance contains 6.022 × 10²³ particles
Exam-Specific Tips
Avogadro's number is exactly 6.022 × 10²³
Hydrogen has atomic number 1 and mass number 1
Carbon has atomic number 6 and most common mass number 12
Oxygen has atomic number 8 and mass number 16
Sodium has atomic number 11 and mass number 23
Chlorine has atomic number 17 and mass number 35
Water molecule (H₂O) has molecular mass 18 g/mol
Carbon dioxide (CO₂) has molecular mass 44 g/mol
Practice MCQs
Atoms & Molecules — Practice Questions
4graded MCQs · easy to hard · full solution & trap analysis
An atom of an element has 8 protons and 8 neutrons. What is its mass number?
Practice 2medium
An atom of element X has 17 protons and 18 neutrons. Another atom of the same element has 17 protons and 20 neutrons. What is the relationship between these two atoms?
Practice 3medium
Which of the following pairs of elements are ISOBARS?
Practice 4hard
An atom of element X has 17 protons and 18 neutrons. Another atom of the same element has 17 protons and 20 neutrons. Which of the following statements correctly describes the relationship between these two atoms?
60-Second Revision — Atoms & Molecules
Remember: Atomic number = Protons, Mass number = Protons + Neutrons
Formula: Number of neutrons = Mass number - Atomic number
Trick: Avogadro's number ≈ 6 × 10²³ for quick calculations
Trap: Don't confuse atomic mass with mass number in calculations