This page covers Agniveer Navy MR Atoms & Molecules with complete concept notes, 12 graded practice MCQs, key points and exam-specific tips. Free to study.
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
12graded MCQs · easy to hard · full solution & trap analysis
What is the molecular mass of CO₂ (carbon dioxide)? [Atomic masses: C = 12, O = 16]
Practice 2easy
What is the atomic mass unit (amu) approximately equal to?
Practice 3easy
Which of the following correctly defines a molecule?
Practice 4easy
If an element has an atomic number of 8, how many electrons does a neutral atom of this element have?
Practice 5medium
Which of the following represents a molecule composed of atoms of different elements?
Practice 6medium
What is the mass number of an atom that contains 8 protons and 8 neutrons?
Practice 7medium
The atomic mass unit (amu) is defined as one-twelfth of the mass of which element?
Practice 8medium
If an atom has an atomic number of 6 and a mass number of 14, how many neutrons does it have?
Practice 9medium
What is the molar mass of a substance if one mole contains 6.02 × 10²³ molecules with a total mass of 32 grams?
Practice 10hard
The relative atomic mass of an element is the average mass of its atoms compared to 1/12th of the mass of a carbon-12 atom. If an element has a relative atomic mass of 32, how many times heavier is one atom of this element than 1/12th of a carbon-12 atom?
Practice 11hard
A compound contains 75% carbon and 25% oxygen by mass. What is the empirical formula if the atomic masses are C = 12 and O = 16?
Practice 12hard
What is the total number of atoms present in 2 molecules of carbon dioxide (CO₂)?
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