Chemistry 2e · Atoms, Molecules, and Ions
Early Ideas in Atomic Theory
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In 30 seconds
The idea that matter is made of tiny, indivisible particles is over two thousand years old, but it took until the early 1800s to become a testable theory. Greek philosophers — especially Leucippus and his student Democritus — argued that matter could be chopped only so far, until reaching particles they called Atomos Greek for "uncuttable," Democritus' name for the smallest particle of matter Full entry → ("uncuttable"). Aristotle rejected the idea, and his four-element view (earth, air, fire, water) dominated Western thought for two millennia, since it required no experiments to support it. John Dalton finally combined ancient atomism with quantitative chemistry, explaining three experimental laws — conservation of mass, definite proportions, and multiple proportions.
Why this matters
Dalton's atomic theory is the foundation of the rest of this book: it explains why chemical equations must be balanced, why compounds have fixed composition, and why elements can combine in multiple whole-number ratios. The laws were accepted because careful mass measurements reproduced them again and again — and conservation of mass still lets you check lab work today. "Fixed ratios" are also why water is always 11.19% hydrogen by mass, rain, tap, or lab-made.
The college version
Core Concepts
Greek atomism: a good idea with no evidence
Democritus (c. 460–370 BCE) proposed that matter consists of indivisible particles (atomos) moving in empty space, and that substances differ because their atoms differ in size, shape, and arrangement. But the Greeks had no instruments and did not test ideas by measurement; Aristotle's four-element view won because it was intuitive and required no experimental support.
The law of conservation of mass
In the late 1700s, Antoine Lavoisier performed careful mass measurements on chemical reactions, often using sealed apparatus so nothing escaped as gas, and found that the total mass of products always equaled the total mass of reactants:
mreactants = mproducts
This law demolished the idea that matter could be created or destroyed, and it is why chemists balance equations: if atoms are conserved, so is mass.
The law of definite proportions
Joseph Proust showed (around 1800) that a given Compound A substance formed when atoms of two or more elements combine in fixed ratios Full entry → always contains the same elements in the same proportion by mass, however it is prepared — water is always 11.19% hydrogen by mass, whether it condenses from steam, falls as rain, or is synthesized in a lab. A substance that is 10% hydrogen is not water.
Dalton's atomic theory (1803–1808)
John Dalton's theory, published in A New System of Chemical Philosophy, had four central postulates:
- Matter is composed of extremely small particles called atoms.
- Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ.
- Atoms cannot be subdivided, created, or destroyed.
- Atoms of different elements combine in simple whole-number ratios to form compounds.
Dalton also assigned the first relative atomic weights by assuming simple formulas (for example, that water was HO); his values were often wrong, but the method — inferring composition from measured mass ratios — was revolutionary.
The law of multiple proportions
If atoms combine in whole-number ratios, then two elements forming more than one compound should do so in mass ratios that are simple whole-number multiples of each other. Carbon and oxygen illustrate this: 12.01 g of carbon combines with 16.00 g of oxygen in carbon monoxide, but with 32.00 g in carbon dioxide — a 1:2 ratio, which a continuous view of matter gives no reason to expect.
The limits of Dalton's theory
Modern chemistry requires three amendments: (1) atoms are divisible — they contain protons, neutrons, and electrons; (2) atoms of an element are not all identical — isotopes differ in mass; and (3) atoms can be created and destroyed in nuclear reactions, though not in ordinary chemical reactions. Dalton's core insight survives: reactions rearrange atoms; they do not transmute elements.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Democritus' atomos | Dalton's atom | Dalton's came with testable predictions and quantitative support; Democritus' was untested philosophy |
| Aristotle's four elements | Chemical elements | Earth/air/fire/water are qualities, not substances; elements cannot be interconverted by heating |
| Law of definite proportions | Law of multiple proportions | Definite: a single compound has fixed composition. Multiple: different compounds of the same elements differ by small whole-number ratios |
| Conserving mass in a reaction | Conserving atoms | Equivalent in ordinary reactions; nuclear reactions can change atoms, but the law applies only to chemical change |
| Dalton's atomic weights | Modern atomic masses | Dalton assumed simple formulas (e.g., water = HO), so his values were off; modern values come from mass spectrometry |
| Compounds | Mixtures | Compounds have fixed element ratios; mixtures can have any ratio — a key test question |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Thousands of years ago, a Greek thinker named Democritus said that if you keep cutting anything in half forever, you eventually reach a piece too small to cut — he called it an "atom," meaning "uncuttable." Nobody tested his idea for a very long time. About 200 years ago, John Dalton used careful measurements to show that when chemicals react, the total weight never changes, and that elements join in fixed, simple number ratios — like 1 carbon with 2 oxygens. It's like discovering every cookie recipe uses a whole number of chocolate chips — that only makes sense if cookies are built from countable pieces.
Worked example
Example 1 — Conservation of mass. A student reacts 12.0 g of magnesium with oxygen and collects 19.9 g of magnesium oxide. What mass of oxygen reacted?
Conservation of mass: product mass = sum of reactant masses:
mMgO = mMg + mO2
Solve for the oxygen by subtracting the magnesium mass:
mO2 = mMgO - mMg = 19.9 g - 12.0 g = 7.9 g
So 7.9 g of oxygen combined with the magnesium (an open crucible would let some material escape — a classic student error).
Example 2 — Definite proportions. A chemist synthesizes 25.0 g of water by burning hydrogen in oxygen. How many grams of hydrogen and oxygen does it contain? (Water is 11.19% hydrogen by mass.)
The percent is a conversion factor:
mH = 25.0 g H2O × 11.19 g H100.0 g H2O = 2.80 g H
The remainder is oxygen:
mO = 25.0 g - 2.80 g = 22.2 g O
Check: 2.80 : 22.2 ≈ 1 : 7.9, close to the 1:8 H:O mass ratio the law predicts for water.
Example 3 — Multiple proportions. Nitrogen and oxygen form several compounds: in one, 14.01 g of nitrogen combines with 16.00 g of oxygen (NO); in another, the same 14.01 g combines with 32.00 g (NO₂). Show that these data obey the law of multiple proportions.
Compare the oxygen masses combining with a fixed 14.01 g of nitrogen:
32.00 g O16.00 g O = 21
The ratio is 2:1 — small whole numbers, exactly as atomic theory demands: one oxygen per nitrogen in NO, two in NO₂.
Key takeaways
- Democritus coined the concept of the atom (atomos = uncuttable); Aristotle's four-element theory won because it was untested, not because it was better.
- Law of conservation of mass (Lavoisier): product mass = reactant mass.
- Law of definite proportions (Proust): a compound always has the same elements in the same mass ratio — water is always 11.19% H by mass.
- Dalton's postulates: atoms exist; atoms of an element are identical; atoms are indivisible; atoms combine in simple whole-number ratios to form compounds.
- Law of multiple proportions: when two elements form multiple compounds, the mass ratios of one element combining with a fixed mass of the other are small whole numbers (CO and CO₂: 16 g vs 32 g O per 12.01 g C → 1:2).
- Modern amendments: atoms are divisible; isotopes make "identical atoms" approximate; nuclear reactions can create and destroy atoms.
- Exam trap: Dalton's theory explains the three laws — know which postulate explains which.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
State the Law of conservation of mass Total mass of reactants equals total mass of products in a reaction Full entry → and give the equation that summarizes it.
Show answer
In any chemical reaction, total product mass equals total reactant mass: mreactants = mproducts. Lavoisier proved it with sealed-apparatus measurements.
Why is it significant that water is 11.19% hydrogen by mass whether it comes from rain or from a lab synthesis?
Show answer
It confirms the law of definite proportions: composition is fixed by atomic structure, not source. Any water is built from the same 2:1 H:O atom ratio.
List Dalton's four postulates.
Show answer
(1) Matter is made of atoms; (2) atoms of an element are identical in mass and properties, and differ from other elements' atoms; (3) atoms cannot be subdivided, created, or destroyed; (4) atoms combine in simple whole-number ratios to form compounds.
Using CO and CO₂, explain how the Law of multiple proportions Elements forming multiple compounds do so in small whole-number mass ratios Full entry → provides evidence for atoms.
Show answer
With a fixed 12.01 g of carbon, oxygen combines as 16.00 g (CO) or 32.00 g (CO₂) — a 1:2 ratio. If matter were continuous, any ratio should be possible; clean integers only make sense if compounds form from whole atoms.
Which of Dalton's postulates needed revision after isotopes were discovered, and how?
Show answer
Postulate 2 — "atoms of a given element are identical." Isotopes are atoms of the same element with different masses, so it holds only as a chemical approximation (identical chemical behavior), not literally.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Atom
- The smallest particle of an element retaining its chemical identity
- Atomos
- Greek for "uncuttable," Democritus' name for the smallest particle of matter
- Law of conservation of mass
- Total mass of reactants equals total mass of products in a reaction
- Law of definite proportions
- A given compound always contains the same elements in the same proportions by mass
- Law of multiple proportions
- Elements forming multiple compounds do so in small whole-number mass ratios
- Compound
- A substance formed when atoms of two or more elements combine in fixed ratios
- Atomic weight
- The relative mass of an atom (Dalton's term; today, relative atomic mass)
Sources & references
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