From Paramanu to Dalton
The notion that matter is built from small, indivisible building blocks is far older than chemistry as a measured science. NIOS records that the word atom comes from the Greek atomos, meaning "indivisible", and is generally credited to the philosopher Democritus (460–370 BC). The same module notes that in India, Acharya Kanada — born around 600 BC and author of the Vaiseshika Sutras — described eternal, indestructible, spherical particles called Paramanu, which could combine in pairs and triplets, roughly 2500 years before Dalton.
What separated Dalton from these philosophers was evidence. NCERT §1.6 frames the point precisely: the atomic idea "again started emerging as a result of several experimental studies which led to the laws" of chemical combination. By Dalton's time, Lavoisier (1789), Proust and others had measured masses with care, and those measurements demanded an explanation. NIOS puts Dalton's true contribution plainly: he arranged the older ideas "in proper order and give[s] evidence for the existence of atoms" by showing that the mass relationships of Lavoisier and Proust could be interpreted by postulating atoms of the various elements.
From philosophy to a testable theory.
The Postulates of Dalton's Theory
NCERT §1.6 lists the proposals of A New System of Chemical Philosophy as four numbered statements. The NIOS module, working from Dalton's 1803 lecture notes, splits the second of these into two — separating "atoms of one element are identical" from "atoms of different elements differ" — and adds the combination rule, giving five statements. The chemical content is identical; the difference is only in how the sentences are grouped. The table below states the four NCERT postulates and aligns the NIOS phrasing alongside.
| NCERT postulate (1808) | What it asserts | NIOS phrasing |
|---|---|---|
| 1. Matter consists of indivisible atoms. | The atom is the ultimate, unbreakable unit of all matter — element, compound or mixture. | "Matter consists of indivisible atoms." |
| 2. All atoms of a given element have identical properties, including identical mass; atoms of different elements differ in mass. | Identity within an element; difference between elements — mass is the defining label of an atom. | Split into two: "all atoms of a given element are identical in mass and all other properties" and "different elements have different kinds of atoms… different masses". |
| 3. Compounds are formed when atoms of different elements combine in a fixed ratio. | A compound is a fixed, small whole-number combination of unlike atoms. | "Formation of a compound… occurs through the combination of atoms of unlike elements in small whole number ratio." |
| 4. Chemical reactions involve reorganisation of atoms; atoms are neither created nor destroyed. | Reactions only rearrange existing atoms — none appear or vanish. | "Atoms are indestructible and retain their identity in chemical reactions." |
Two phrases in this table do all the heavy lifting later: "identical mass" (postulate 2) and "fixed ratio" / "small whole number ratio" (postulate 3). Together with the indestructibility of atoms (postulate 4), they are the bridge from a philosophical hunch to the three quantitative laws of chemical combination.
Reading Each Postulate
It is worth slowing down on what each statement actually claims, because NEET single-statement questions are usually built by altering one word inside one postulate.
Postulate 1 — atoms are indivisible
Dalton took the atom to be the smallest particle of an element that cannot be subdivided. This was the boldest claim and, as we will see, the one that did not survive. NIOS is candid about it: "Today, we know that atoms are not indivisible." For Dalton's chemistry, however, treating the atom as a single fixed lump was exactly what made the mass arithmetic work.
Postulate 2 — atoms of an element are identical in mass
Every atom of a given element carries one definite mass; atoms of different elements carry different definite masses. This is the postulate that lets mass behave like a conserved bookkeeping quantity. It is also the one most directly modified by isotopes — atoms of the same element with different masses, treated in the sibling note on atomic and molecular masses.
Postulates 3 and 4 — fixed ratios and conserved atoms
Postulate 3 says a given compound is always the same fixed combination of atoms, for example $\ce{H2O}$ being two hydrogen atoms locked to one oxygen atom. Postulate 4 says a reaction such as $\ce{2H2 + O2 -> 2H2O}$ does not destroy or manufacture any atom — it merely re-pairs them. NCERT summarises the consequence in §1.5: "a balanced chemical equation has the same number of atoms of each element on both sides," which is conservation of mass written atom-by-atom.
"Same number of atoms" vs "same number of molecules"
Dalton's fourth postulate guarantees the same number of atoms of each element on both sides of a balanced equation — not the same number of molecules. In $\ce{2H2 + O2 -> 2H2O}$, molecules go from 3 to 2, yet H atoms (4) and O atoms (2) are conserved. Equating molecule counts is a classic distractor.
Conserved by Dalton: atoms of each element, and therefore total mass. Not conserved: number of molecules, or number of moles of gas.
How the Theory Explains the Laws
The reason Dalton's theory mattered is that each postulate cashes out into a measurable law. NCERT lists five laws of chemical combination in §1.5; NIOS works through how the postulates produce three of them and where the theory stops. The mapping below is the single most examinable idea on this page.
| Law | Postulate that explains it | Reasoning (from source) |
|---|---|---|
| Conservation of mass (Lavoisier, 1789) | Postulate 4 — atoms reorganise, none created or destroyed | Each atom has a definite mass and is merely rearranged, so total mass before = total mass after. |
| Definite proportions (Proust) | Postulate 3 (+ 2) — fixed whole-number ratio of atoms | A compound holds the same atoms in the same ratio; since each atom has definite mass, the mass proportion is fixed regardless of source. |
| Multiple proportions (Dalton, 1803) | Postulate 3 — atoms combine in small whole-number ratios | Two elements can pair in different small ratios, so the masses of one element per fixed mass of the other are in small whole numbers. |
| Gay Lussac's law of gaseous volumes (1808) | Not explained | NCERT: the theory "could not explain the laws of gaseous volumes"; resolved later by Avogadro's atom–molecule distinction. |
Notice the symmetry NIOS draws out: the law of multiple proportions was not merely explained by the theory — it was deduced from it before being checked against data. NIOS calls this deduction "important in convincing chemists of the validity of the theory." A theory that predicts a new law and is then confirmed is doing exactly what a scientific theory should.
Atoms combining in a fixed ratio: one carbon to two oxygens in $\ce{CO2}$.
The three laws Dalton explains are stated and measured in Laws of Chemical Combinations — read it alongside this note.
Worked Example: CO and CO₂
The law of multiple proportions is the law NEET is most likely to test numerically, and NIOS supplies the cleanest carbon–oxygen data. The task is always the same: fix the mass of one element, read off the masses of the other in two compounds, and check that they form a small whole-number ratio.
Carbon and oxygen form carbon monoxide $\ce{CO}$ and carbon dioxide $\ce{CO2}$. Per 1.0000 g of carbon, $\ce{CO}$ contains 1.3321 g of oxygen and $\ce{CO2}$ contains 2.6642 g of oxygen. Show that this obeys the law of multiple proportions.
Fix the carbon mass at 1.0000 g for both compounds — already done in the data.
Form the oxygen ratio:
$$\frac{m_{\text{O in }\ce{CO2}}}{m_{\text{O in }\ce{CO}}} = \frac{2.6642}{1.3321} = 2$$
The masses of oxygen combining with a fixed mass of carbon are in the ratio 1 : 2 — small whole numbers. The theory's explanation, in NIOS's words, is that "carbon dioxide contains twice as many oxygen atoms for a given number of carbon atoms as does carbon monoxide," exactly the $\ce{CO}$ versus $\ce{CO2}$ atom counts.
NCERT gives a second, parallel illustration with hydrogen and oxygen: water ($\ce{H2O}$) supplies 16 g of oxygen per 2 g of hydrogen, while hydrogen peroxide ($\ce{H2O2}$) supplies 32 g. The oxygen masses, 16 g and 32 g, again form the ratio 1 : 2. Either pair of compounds is fair game in an exam.
Fix the right element before taking the ratio
The law compares the masses of one element against a fixed mass of the other. If the raw data does not hold one element constant, normalise first. Taking a ratio of un-normalised masses is the commonest error and will not give the clean whole numbers the question expects.
Recipe: (1) choose an element to fix; (2) scale both compounds to the same mass of it; (3) ratio the other element's masses; (4) reduce to lowest whole numbers.
Limitations and Modern Modifications
NCERT is careful to record where Dalton's theory fails, and these limitations are themselves examinable. Two are stated directly in §1.6: the theory "could not explain the laws of gaseous volumes," and it "could not provide the reason for combining of atoms," a gap filled later by other scientists. The volume problem was resolved by Avogadro's 1811 distinction between atoms and molecules, which Dalton himself rejected — he believed atoms of the same kind could not combine, so a molecule like $\ce{O2}$ could not exist in his picture.
NIOS adds the deepest modification of all. The first postulate — that the atom is indivisible — is now known to be false: "atoms are not indivisible; they can be broken down into still smaller particles, although they lose their chemical identity in this process." The internal structure of the atom is the subject of the Structure of Atom chapter. The table below collects what modern chemistry keeps and what it has overturned.
| Dalton's claim | Modern status | Source / replacement |
|---|---|---|
| The atom is indivisible. | Overturned — atoms split into sub-atomic particles (losing chemical identity). | NIOS §1.4.2; see Structure of Atom. |
| All atoms of an element have identical mass. | Modified — isotopes are atoms of one element with different masses. | Atomic & molecular masses (sibling note). |
| Atoms combine in fixed small whole-number ratios. | Retained for most compounds; the bedrock of stoichiometry. | NCERT §1.6; mole concept. |
| Atoms are neither created nor destroyed in reactions. | Retained for chemical reactions (conservation of mass holds). | NCERT §1.5.1, balanced equations. |
| Explains gaseous-volume relationships. | Failed — could not explain Gay Lussac's law. | Resolved by Avogadro's law. |
Despite these revisions, NIOS insists that "the atom still remains a building block of matter," and NCERT's chapter summary still treats Dalton's theory as the conclusion "that atoms are building blocks of matter." For NEET stoichiometry, the two surviving postulates — fixed ratios and conservation — are precisely the ones every mole and mass calculation relies on.
Dalton's atomic theory in one screen
- Published 1808 in A New System of Chemical Philosophy; the atomic statements date to Dalton's 1803 work. Idea predates him (Democritus; Kanada's Paramanu).
- Four NCERT postulates: indivisible atoms; identical mass within an element; fixed-ratio compounds; reactions only reorganise atoms.
- Postulate 4 → conservation of mass. Postulate 3 (with 2) → definite proportions. Postulate 3 → multiple proportions (which Dalton deduced).
- Worked ratio: oxygen per fixed carbon in $\ce{CO}$ vs $\ce{CO2}$ = 1.3321 : 2.6642 = 1 : 2.
- Failures: cannot explain Gay Lussac's gaseous-volume law, and gives no reason why atoms combine. Indivisibility and identical mass later overturned by sub-atomic particles and isotopes.