6.2 Dalton's Atomic Theory

Dalton's Atomic Theory revolutionized chemistry by proposing that all matter is made of indivisible atoms. This idea explained key chemical laws and laid the groundwork for understanding elements, compounds, and reactions at a fundamental level.

The theory provided a framework for quantitative chemistry and stimulated decades of further research. While it had real limitations, Dalton's work set the stage for more advanced atomic models and deepened our understanding of matter's structure.

Dalton's Atomic Theory

Key Postulates and Impact on Chemistry

Dalton put forward several core claims that, taken together, transformed how chemists thought about matter:

• All matter is composed of indivisible particles called atoms, the building blocks of elements.
  • This marked a shift from earlier theories that viewed matter as continuous rather than particulate. Before Dalton, many chemists still thought of substances as infinitely divisible.
• Atoms of the same element are identical in their properties, while atoms of different elements differ in mass and behavior.
  • This explained why hydrogen and oxygen, for instance, behave so differently in reactions and have distinct physical properties.
• Chemical reactions involve the rearrangement, combination, or separation of atoms, but atoms themselves remain unchanged during these processes.
  • So when hydrogen and oxygen combine to form water, the atoms aren't destroyed or transformed into something new. They're just reorganized.
• Compounds form when atoms of different elements combine in simple whole-number ratios.
  • This postulate laid the foundation for chemical formulas (like $H_2O$ for water) and for stoichiometry, the math of chemical reactions.

These postulates provided a coherent explanation for several empirical laws that chemists had already observed but couldn't fully account for:

• The law of conservation of mass
• The law of definite proportions
• The law of multiple proportions

By grounding these laws in a concrete model of matter, Dalton helped establish chemistry as a truly quantitative science. Acceptance of his theory also stimulated further experimentation, eventually leading to more sophisticated atomic models like Thomson's plum pudding model and Rutherford's nuclear model.

Implications for Chemical Laws

Dalton's postulates didn't just describe atoms in the abstract. They directly explained why certain chemical laws hold true.

• Conservation of mass: If atoms are neither created nor destroyed in reactions, then the total mass before and after a reaction must stay the same. The mass of each atom is constant and characteristic of its element, so the mass of a compound is simply the sum of its constituent atoms' masses.
• Definite proportions: If atoms combine in fixed whole-number ratios, then a given compound will always have the same composition by mass. Water ($H_2O$) always contains hydrogen and oxygen in a 2:1 atomic ratio, no matter where the sample comes from.
• Multiple proportions: When two elements form more than one compound, the ratios of the masses of one element that combine with a fixed mass of the other are small whole numbers. Carbon and oxygen, for example, form both $CO$ and $CO_2$. For a fixed mass of carbon, the mass of oxygen in $CO_2$ is exactly twice that in $CO$. Dalton's whole-number ratio postulate predicts this directly.

Dalton's Theory & Chemical Laws

Law of Conservation of Mass

This law states that the total mass of reactants in a chemical reaction equals the total mass of the products. Nothing is gained or lost.

Dalton's theory explains this directly: since atoms are neither created nor destroyed during a reaction, only rearranged, the total mass can't change. Each atom keeps its characteristic mass throughout.

Consider the reaction:

$2H_2 + O_2 \rightarrow 2H_2O$

You start with four hydrogen atoms and two oxygen atoms. You end with four hydrogen atoms and two oxygen atoms, now arranged as two water molecules. The atoms are the same before and after, so the mass is the same.

Law of Definite Proportions

Also called the law of constant composition, this states that a chemical compound always contains the same elements in the same proportions by mass, regardless of its source or how it was prepared. Antoine Lavoisier and Joseph Proust were key figures in establishing this law experimentally before Dalton provided the theoretical explanation.

Dalton's theory accounts for this because atoms combine in fixed whole-number ratios. The relative numbers and types of atoms in a compound don't vary, so the composition by mass stays constant.

• Water ($H_2O$) always contains hydrogen and oxygen atoms in a 2:1 ratio, whether the water comes from a glacier or a lab synthesis.
• Sodium chloride ($NaCl$) always contains sodium and chlorine atoms in a 1:1 ratio, whether it's mined from the earth or produced in a reaction.

Law of Multiple Proportions

This law, which Dalton himself formulated, states that when two elements combine to form more than one compound, the different masses of one element that combine with a fixed mass of the other element are in a ratio of small whole numbers.

This follows naturally from the atomic theory. If atoms are discrete units that combine in whole-number ratios, then moving from one compound to another just means changing how many atoms of each type join together.

• Carbon monoxide ($CO$) has one carbon atom bonded to one oxygen atom. Carbon dioxide ($CO_2$) has one carbon atom bonded to two oxygen atoms. For the same mass of carbon, the mass of oxygen in $CO_2$ is exactly double that in $CO$, a clean 2:1 ratio.
• Nitrogen and oxygen form several compounds ($NO$, $NO_2$, $N_2O$), and the oxygen masses per fixed mass of nitrogen fall into simple whole-number ratios.

This law was powerful evidence for the atomic theory, since it's hard to explain without assuming matter comes in discrete, countable units.

Limitations of Dalton's Theory

Dalton's atomic theory was a huge step forward, but it couldn't explain everything. As experimental techniques improved over the following century, several gaps became clear.

Existence of Isotopes

Dalton assumed all atoms of a given element are identical. But isotopes are atoms of the same element with different masses because they have different numbers of neutrons. Carbon-12 and Carbon-14, for example, are both carbon, yet they differ in mass. This limitation was resolved only after the discovery of protons and neutrons in the early twentieth century.

Divisibility of Atoms

Dalton treated atoms as truly indivisible. Later experiments revealed that atoms are composed of smaller subatomic particles: electrons, protons, and neutrons. Thomson's cathode ray experiments (1897) showed that negatively charged electrons could be pulled out of atoms, proving atoms have internal structure. This discovery led to new models:

• Thomson's plum pudding model (1904)
• Rutherford's nuclear model (1911)

Chemical Bonding and Molecular Structure

The theory said nothing about how atoms are arranged within molecules or why certain atoms bond together. Dalton could tell you that $H_2$ or $CH_4$ existed, but not what held those atoms together. Later theories filled this gap:

• Lewis's theory of chemical bonding (electron sharing and transfer)
• Pauling's valence bond theory

Allotropes

Dalton's framework couldn't explain why the same element can exist in different physical forms with very different properties. Graphite and diamond are both pure carbon, yet one is soft and opaque while the other is the hardest natural material and transparent. The explanation lies in how atoms are bonded and arranged in their crystal structures, something Dalton's theory didn't address.

Subatomic Particles and Electric Charges

Dalton didn't consider the possibility that atoms contain electrically charged components. The work of Thomson (who discovered the electron), Millikan (who measured the electron's charge), and Rutherford (who identified the proton and the nucleus) revealed that atoms are structured arrangements of charged particles. This understanding eventually led to the electron cloud model and quantum mechanical descriptions of atoms.

Periodic Trends

Dalton's theory offered no explanation for why elements show repeating patterns in their properties. The periodic trend in atomic radius, for example (decreasing from left to right across a period), depends on increasing nuclear charge and the arrangement of electrons in shells. These trends were later explained through the work of Mendeleev (who organized elements by atomic weight and chemical behavior) and Moseley (who reordered the periodic table by atomic number), as well as through the development of electron orbital theory.