2.1 Atoms and Molecules
Everything is built
from atoms
& molecules.
Matter — everything around us — is made of very small particles. Understanding how these particles are structured and combine explains how substances form and react.
Atoms & Molecules — the basics
The smallest unit
The smallest fundamental unit of an element that still shows the properties of that element. Atoms are the smallest units of an element that take part in chemical reactions.
Most atoms don’t exist freely in nature — noble gases are the exception.
Cl → one chlorine atomExists independently
The smallest particle of an element or compound that exists independently, carrying all the properties of that element or compound.
CO₂ → compound | Cl₂ → element🧱 Atoms are the building blocks. Molecules are groups of atoms joined together.
Homoatomic molecules — same element
Molecules made of atoms of the same element are classified by how many atoms they contain.
| Type | Atoms | Examples |
|---|---|---|
| Monoatomic | 1 | He, Ne, Ar (noble gases) |
| Diatomic | 2 | H₂, O₂, N₂, Cl₂, F₂, Br₂, I₂ |
| Triatomic | 3 | O₃ (ozone) |
| Tetra-atomic | 4 | P₄ (phosphorus) |
| Octa-atomic | 8 | S₈ (sulfur) |
mono = 1 · di = 2 · tri = 3 · tetra = 4 · octa = 8
Heteroatomic molecules — different elements
⭐ Remember
Molecule of an element — atoms of the same element. Example: O₂
Molecule of a compound — atoms of different elements. Example: H₂O
Dalton’s Atomic Theory
Long before microscopes existed, the Greek thinker Democritus proposed that matter is made of very small, indivisible parts he called “atoms” — but he had no scientific proof. It wasn’t until 1808, when John Dalton gave the first scientific atomic theory, grounded in the experimental evidence of his time.
↗ How did the ancient Greeks predict the atom?
All matter is made up of very small particles called atoms. These particles cannot be broken down further — they are indivisible.
Atoms cannot be created, destroyed, or changed into atoms of a different element.
All atoms of the same element are alike — they share the same size, mass, and properties.
Atoms of different elements differ in shape, size, and mass.
Atoms join in simple whole-number ratios to form “compound atoms” — what we now call molecules.
Modern experiments show these postulates aren’t precisely true in their original form — but modern chemistry was built on this very foundation.
Limitations & the Modern View
Dalton’s theory was a landmark beginning — but later discoveries showed some of his ideas needed correction.
An atom can be subdivided — it’s a complex structure of electrons, protons, and neutrons.
Atoms of the same element may not be identical. Isotopes share the same atomic number but have different atomic masses.
Dalton proposed simple whole-number ratios. Modern chemistry shows ratios can be far more complex.
Molecules — not “compound atoms” — are the smallest particles with independent existence. A molecule of an element has identical atoms; a molecule of a compound has dissimilar atoms.
Atoms can be altered. Some radioactive elements change into atoms of another element by emitting radiation.
Summary
| Dalton’s idea | Modern science |
|---|---|
| Atom cannot be divided | Atom contains smaller particles |
| Atoms of the same element are alike | Isotopes have different masses |
| Atoms combine in simple ratios | Some ratios are larger / less familiar |
| “Compound atom” | Molecule |
| Atoms cannot change into another element | Nuclear reactions can change elements |
Important Questions
Test Yourself 🧪
Try this quick quiz to check your understanding of atoms and molecules.
Key Vocabulary Helper महत्त्वपूर्ण शब्दकोश
| English | नेपाली अर्थ | Simple explanation |
|---|---|---|
| Atom | परमाणु | Smallest unit of an element |
| Molecule | अणु | Smallest particle that can exist independently |
| Element | तत्त्व | A pure substance made of only ONE kind of atom (e.g., pure gold, oxygen) |
| Compound | यौगिक | A substance made of TWO or more different atoms (e.g., water: H₂O) |
| Postulate | अवधारणा / बुँदा | Main statement or rule proposed in a scientific theory |
| Limitation | सीमा / कमी-कमजोरी | Points where an old theory fails or needs correction |