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Atoms & Molecules

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This page covers the basic concepts of atoms and molecules, Dalton's atomic theory, writing chemical formulas from valency, and the mole concept, from the REET exam's point of view. REET asks direct questions from this topic on writing formulas and simple mole-mass calculations.

  • 2 questions in REET 2024
  • Class 6-8 level
  • 10 min read
Verified 17 September 2026 · Source: Avogadro Constant — Mole Concept and Value

How this topic is asked in REET

REET 2024 asked two questions on this topic — both were direct calculation/application questions, not mere definitions. The first was based on writing the formula of aluminium oxide by cross-multiplying valencies, and the second was a straightforward mole-concept calculation, finding the mass of 0.1 mole of N₂. This makes it clear that REET tests not just definitions from atoms and molecules, but practical calculations such as writing formulas from valency and converting between moles, mass and number of particles. So while preparing, rather than just memorising definitions, it pays off to practise writing a few formulas by hand and solving some mole calculations.

The Concept of an Atom and Dalton's Theory

The smallest particle of matter that can independently take part in a chemical reaction is called an atom, while a chemically bonded, independently existing particle made of two or more atoms is called a molecule. In 1808, John Dalton put forward the atomic theory, whose main points are: all matter is made of extremely small particles — atoms — that take part in chemical reactions; an atom can neither be created nor destroyed, nor broken down in a chemical reaction; all atoms of the same element are identical in mass and chemical properties, while atoms of different elements differ in mass and properties; atoms combine in simple whole-number ratios to form compounds; and the number and type of elements in a given compound is always fixed.

Some limitations of Dalton's theory came to light later — an atom is in fact made of sub-atomic particles like electrons, protons and neutrons, meaning it is not indivisible; the discovery of isotopes showed that atoms of the same element can also have different masses; and because of isobars, atoms of different elements can have the same mass too. Even so, Dalton's theory is considered the foundation of chemistry, and REET can ask about both its core points and its limitations.

Valency and Writing Formulas (Cross-Multiplication Method)

What valency is
The combining capacity of an element's atom with other atoms, determined by its valence electrons
Cross-multiplication rule
When forming a compound from two elements, the valency of one element becomes the subscript (number of atoms) of the other element — for example, Al (valency 3) and O (valency 2) combine to give Al₂O₃
Common valencies
H, Na, K, Cl = 1; O, Ca, Mg, S (usually) = 2; N, Al = 3; C, Si = 4 — some elements like Fe and Cu have variable valency (such as Fe²⁺ and Fe³⁺)

Atomic Symbols and Atomic Mass

Atomic symbol
Every element is shown with a symbol of one or two letters (such as H, O, Na, Ca) — this is an internationally accepted system, where the first letter is capital and the second is small, keeping elements like Ca (calcium) and Co (cobalt) clearly distinct
Atomic mass unit (amu/u)
1/12th of the mass of a carbon-12 (¹²C) atom is taken as 1 atomic mass unit (u), and the relative atomic masses of other elements are determined against it
Molecular mass
The sum of the atomic masses of all atoms present in a molecule — for example, the molecular mass of water (H₂O) = 2(1) + 16 = 18 u

Chemical Formulas of Some Common Compounds

CompoundFormula
WaterH₂O
Carbon dioxideCO₂
AmmoniaNH₃
Aluminium oxideAl₂O₃
Calcium carbonateCaCO₃
Sodium hydroxideNaOH

Mole Concept and Avogadro's Number

Mole
The amount of a substance that contains 6.022 × 10²³ basic particles (atoms/molecules/ions) — this is called 1 mole[Source 1]
Avogadro's number
6.022 × 10²³ per mole (the number of basic particles, whatever the substance)[Source 1]
Molar mass
The mass of 1 mole of a substance (in grams); the molar mass of the diatomic molecule N₂ = 2 × 14 = 28 g/mol
Formula
Number of moles = given mass ÷ molar mass; example: mass of 0.1 mole of N₂ = 0.1 × 28 = 2.8 g

How to Teach This in Class

Activity

Have students build models of water (H₂O) and carbon dioxide (CO₂) using pulses/peas or coloured beads (a ball-and-stick model), so they can see how many and which atoms are joined in a molecule.

Teaching aid

Put up a chart of common elements' symbols and valencies, and explain the cross-multiplication method step by step using coloured arrows to show how a formula is built.

Common misconception

Many students think atom and molecule are the same thing, or assume that all molecules are made of just two atoms — use examples (O₂ vs. O₃, H₂O vs. NH₃) to make it clear that the number of atoms in a molecule can vary.

Assessment

Give students the valencies of a few elements and ask them to write the formulas of 3-4 compounds on their own, and have them solve a simple mole-mass calculation (such as the mass of 2 moles of O₂), so you can check whether students can apply both formula-writing and the mole concept practically, and let them check each other's calculations in a group for a chance to correct mistakes.

Memory trick / exam tip

To avoid mistakes in formula writing, always check the valencies are in their simplest ratio after cross-multiplying — for example, crossing Ca (2) and O (2) gives Ca₂O₂, but in its simplest form this is CaO. In mole questions, first find the molar mass (number of atoms × atomic mass), and only then apply the mole-mass formula — this is the most common mistake with diatomic molecules like N₂.

Questions Asked in REET 2024 (With Solutions)

Question 122

Chemical formula of aluminium oxide is

  1. AAlO₂
  2. BAlO₃
  3. CAl₂O₃
  4. DAl₃O₂

Explanation

एलुमिनियम की संयोजकता 3 (Al³⁺) और ऑक्सीजन की संयोजकता 2 (O²⁻) है। संयोजकताओं का क्रॉस-गुणन (criss-cross) करने पर सूत्र Al₂O₃ प्राप्त होता है, जिसमें कुल धनावेश (+6) और ऋणावेश (−6) संतुलित रहते हैं।

Question 144

Mass of 0.1 mole N₂ is

  1. A1.4 g
  2. B0.14 g
  3. C2.8 g
  4. D0.014 g

Explanation

N₂ का मोलर द्रव्यमान = 2 × 14 = 28 g/mol। द्रव्यमान = मोल × मोलर द्रव्यमान = 0.1 × 28 = 2.8 g। नाइट्रोजन परमाणु का द्रव्यमान (14) लेने पर 1.4 g आता है, जो गलत है क्योंकि नाइट्रोजन द्विपरमाणुक अणु है।

Practice Questions

Question 1

What will be the formula of the compound formed by calcium (valency 2) and chlorine (valency 1)?

  1. ACaCl
  2. BCa₂Cl₂
  3. CCa₂Cl
  4. DCaCl₂

Explanation

By cross-multiplication, calcium's valency (2) becomes chlorine's subscript and chlorine's valency (1) becomes calcium's subscript, which gives the formula CaCl₂.

Question 2

What will be the mass of 0.5 mole of O₂ gas? (atomic mass of oxygen = 16)

  1. A8 g
  2. B16 g
  3. C32 g
  4. D4 g

Explanation

The molar mass of O₂ = 2 × 16 = 32 g/mol. Mass = moles × molar mass = 0.5 × 32 = 16 g.

Question 3

According to Dalton's atomic theory, what is true about all atoms of the same element?

  1. Aउनका द्रव्यमान व रासायनिक गुण समान होते हैं
  2. Bउनका द्रव्यमान सदैव भिन्न होता है
  3. Cवे रासायनिक अभिक्रिया में भाग नहीं लेते
  4. Dवे हमेशा अणु के रूप में पाए जाते हैं

Explanation

According to Dalton's atomic theory, all atoms of the same element are identical in mass and chemical properties, while atoms of different elements differ in mass and properties. (The discovery of isotopes later refined this point somewhat, but this remains the core idea.)

Question 4

What is the correct formula of the compound formed by sodium (Na, valency 1) and oxygen (O, valency 2)?

  1. ANaO
  2. BNaO₂
  3. CNa₂O
  4. DNa₂O₂

Explanation

By cross-multiplication, sodium's valency (1) becomes oxygen's subscript and oxygen's valency (2) becomes sodium's subscript, which gives the formula Na₂O.

Question 5

The number of basic particles (atoms/molecules) present in 1 mole of any substance is:

  1. A6.022 × 10²²
  2. B1.66 × 10⁻²⁴
  3. C3.011 × 10²³
  4. D6.022 × 10²³

Explanation

By Avogadro's number, 1 mole of any substance contains 6.022 × 10²³ basic particles (atoms, molecules or ions).

Question 6

What is the total number of atoms in a molecule of methane (CH₄)?

  1. A2
  2. B4
  3. C5
  4. D6

Explanation

A molecule of methane, CH₄, has 1 carbon atom and 4 hydrogen atoms, so the total number of atoms = 1 + 4 = 5.

Question 7

One limitation of Dalton's atomic theory is that it fails to explain:

  1. Aपरमाणु सरल अनुपात में संयोग करते हैं
  2. Bसमस्थानिकों (isotopes) का अस्तित्व
  3. Cयौगिक में तत्वों का अनुपात स्थिर होता है
  4. Dपदार्थ परमाणुओं से बना है

Explanation

Dalton assumed that all atoms of an element have identical mass, but the discovery of isotopes showed that atoms of the same element can have different masses too — this is a major limitation of Dalton's theory.

Question 8

In carbon dioxide, why does the formula remain CO2 after applying the criss-cross method with carbon (valency 4) and oxygen (valency 2)?

  1. Aक्योंकि C₂O₄ को सरलतम अनुपात में CO₂ लिखा जाता है
  2. Bक्योंकि कार्बन की संयोजकता वास्तव में 2 है
  3. Cक्योंकि ऑक्सीजन की संयोजकता वास्तव में 4 है
  4. Dक्योंकि CO₂ में क्रॉस-गुणन लागू नहीं होता

Explanation

Cross-multiplication directly gives C₂O₄, but a formula is always written in its simplest whole-number ratio, so simplifying the 2:4 ratio to 1:2 gives the final formula CO₂.

Question 9

The most suitable classroom activity to help class 8 students understand that a molecule can contain more than one type of atom is:

  1. Aकेवल परिभाषा श्यामपट्ट पर लिखना
  2. Bरंगीन गोलियों/मॉडल से H₂O व CO₂ जैसे अणु बनवाना
  3. Cपाठ्यपुस्तक की परिभाषा रटवाना
  4. Dकेवल तत्वों की सूची याद कराना

Explanation

At the class 6-8 level, building molecules like H₂O and CO₂ with concrete material (coloured beads/models) lets students see for themselves how many and which atoms are joined in each molecule, making the abstract idea click naturally.

Question 10

What is the total number of hydrogen atoms in 2 moles of water (H2O)? (Avogadro number = 6.022 × 10^23)

  1. A6.022 × 10²³
  2. B1.2044 × 10²⁴
  3. C2.4088 × 10²⁴
  4. D3.011 × 10²³

Explanation

In 2 moles of H₂O, the total number of molecules = 2 × 6.022×10²³ = 1.2044×10²⁴. Each H₂O molecule has 2 hydrogen atoms, so the number of hydrogen atoms = 2 × 1.2044×10²⁴ = 2.4088×10²⁴.

Frequently Asked Questions

What is the difference between an atom and a molecule?+

An atom is the smallest particle of matter that can take part in a chemical reaction on its own, while a molecule is an independently existing unit formed by the chemical bonding of two or more atoms, such as O₂ (a molecule of two oxygen atoms).

What is the value of Avogadro's number?+

Avogadro's number is 6.022 × 10²³ per mole — it tells us the number of basic particles (atoms/molecules) present in 1 mole of any substance.

What is 1 mole?+

The amount of a substance that contains basic particles equal to Avogadro's number (6.022 × 10²³) is called 1 mole.

What is the main point of Dalton's atomic theory?+

According to Dalton, all matter is made of indivisible atoms, all atoms of an element are identical, and atoms combine in simple whole-number ratios to form compounds. The later discovery of sub-atomic particles and isotopes revealed some limitations of this theory.

How important is this topic for REET, and where do students go wrong most often in formula writing?+

This is the foundation of chemistry, so all later topics — elements-compounds-mixtures, acids-bases-salts — build on it, and REET 2024 asked 2 direct questions from it. In the cross-multiplication method, most students forget to reduce the subscripts to their simplest ratio — for example, crossing Ca and O first gives Ca₂O₂, which must be simplified to CaO. Also remember that the atomic mass unit (u) is defined as 1/12th the mass of a carbon-12 atom.

Read Next

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Elements, Compounds and Mixtures

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Official Sources

  1. Avogadro Constant — Mole Concept and Value

Change Log

  • 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked