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John Dalton and Atomic Theory
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Although the concept of the atom dates back to the ideas of Democritus, the first
modern description of it as the fundamental building block of chemical
structures was formulated by John Dalton, an English meteorologist and
chemist. Dalton studied and expanded upon the works of Antoine Lavoisier and
Lois Proust and developed the law of multiple proportions (https://www.boundless.com
/chemistry/definition/law-of-multiple-proportions/).
Proust had studied tin oxides and found that
their masses were either 88.1% tin and 11.9% oxygen or 78.7% tin and 21.3%
oxygen (these were tin(II) oxide and tin dioxide respectively). Dalton noted from
these percentages that 100g of tin will combine either with 13.5g or 27g of
oxygen; 13.5 and 27 form a ratio of 1:2. Dalton found an atomic theory of matter
could elegantly explain this common pattern in chemistry - in the case of Proust's
tin oxides, one tin atom will combine with either one or two oxygen atoms.
Dalton also believed atomic theory could explain why water absorbed different
gases in different proportions: for example, he found that water absorbed carbon
dioxide far better than it absorbed nitrogen. Dalton hypothesized this was due to
the differences in mass and complexity of the gases' respective particles. Indeed,
carbon dioxide molecules (CO2) are heavier and larger than nitrogen molecules
(N2).Dalton proposed that each chemical element is composed of atoms of a
single, unique type, and though they cannot be altered or destroyed by chemical
means, they can combine to form more complex structures (chemical
compounds). Since Dalton reached his conclusions by experimentation and
examination of the results in an empirical fashion, this marked the first truly
scientific theory of the atom.
Various atoms and molecules as depicted in John Dalton's A New System of
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Chemical Philosophy (1808) (Figure 1).
The main points of his theory are:
1. Everything is composed of atoms, which are the indivisible building blocks
of matter and cannot be destroyed.
2. All atoms of an element are identical.
3. The atoms of different elements differ in size and mass.
4. Compounds are produced through different whole-number combinations of
atoms.
5. A chemical reaction (https://www.boundless.com/chemistry/definition/chemical-reaction/) results
in the rearrangement of atoms in the reactant and product compounds.
Atomic theory has been revised over the years to incorporate the existence of
atomic isotopes and the interconversion of mass and energy. In addition, the
discovery of subatomic particles has shown that atoms can be divided into
smaller parts. However, Dalton's importance in the development of modern
atomic theory has been recognized by the designation of the atomic mass unit
(https://www.boundless.com/chemistry/definition/atomic-mass-unit/)
as a Dalton.
The Law of Multiple Proportions (https://www.boundless.com/chemistry/atoms-molecules-and-ions
/history-of-atomic-structure/the-law-of-multiple-proportions/)Cathode
Rays (https://www.boundless.com
/chemistry/atoms-molecules-and-ions/discoveries-leading-to-nuclear-atom-model/cathode-rays/)
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Key Term Glossary
atom:
the smallest possible amount of matter that still retains its identity as a
chemical element, now known to consist of a nucleus surrounded by electrons
atom:
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the smallest possible amount of matter which still retains its identity as a
chemical element, consisting of a nucleus surrounded by electrons
APPEARS IN THIS RELATED CONCEPT:
Overview of Atomic Structure (https://www.boundless.com/chemistry/atoms-moleculesand-ions/the-structure-of-the-atom/overview-of-atomic-structure/)
atomic mass:
the quantity of matter of a particle, sub-atomic particle, or molecule
APPEARS IN THIS RELATED CONCEPT:
atomic mass:
the quantity of matter in an atomic particle, sub-atomic particle, or molecule
APPEARS IN THIS RELATED CONCEPT:
Atomic Number and Mass Number (https://www.boundless.com/chemistry/atomsmolecules-and-ions/the-structure-of-the-atom/atomic-number-and-mass-number/)
atomic mass unit:
the standard unit that is used for indicating quantity on a molecular (or
smaller) scale
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John Dalton and Atomic Theory (https://www.boundless.com/chemistry/atoms-moleculesand-ions/history-of-atomic-structure/john-dalton-and-atomic-theory/)
carbon:
The chemical element (symbol C) with an atomic number of 6.
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chemical reaction:
A process involving the breaking or making of interatomic bonds, in which one
or more substances are changed into others.
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APPEARS IN THIS RELATED CONCEPT:
Physical and Chemical Changes to Matter (https://www.boundless.com/chemistry
/introduction-to-chemistry/physical-and-chemical-properties-of-matter/physical-and-chemical-changesto-matter/)
chemistry:
The branch of natural science that deals with the composition and constitution
of substances and the changes that they undergo as a consequence of
alterations in the constitution of their molecules.
APPEARS IN THIS RELATED CONCEPT:
element:
Any one of the simplest chemical substances that cannot be decomposed in a
chemical reaction or by any chemical means, and are made up of atoms all
having the same number of protons.
empirical:
Pertaining to, derived from, or testable by observations made using the
physical senses or using instruments which extend the senses.
APPEARS IN THIS RELATED CONCEPT:
energy:
The capacity to do work.
APPEARS IN THIS RELATED CONCEPT:
energy:
a quantity that denotes the ability to do work and is measured in a unit
dimensioned in mass × distance²/time² (ML²/T²) or the equivalent
APPEARS IN THIS RELATED CONCEPT:
law of multiple proportions:
a law stating that if two elements form more than one compound between
them, then the ratios of the masses of the second element which combine with
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a fixed mass of the first element will be ratios of small whole numbers
APPEARS IN THIS RELATED CONCEPT:
The Law of Multiple Proportions (https://www.boundless.com/chemistry/atoms-moleculesand-ions/history-of-atomic-structure/the-law-of-multiple-proportions/)
mass:
The quantity of matter that a body contains, irrespective of its bulk or volume.
It is one of four fundamental properties of matter. It is measured in kilograms
in the SI system of measurement.
APPEARS IN THIS RELATED CONCEPT:
Physical and Chemical Properties of Matter (https://www.boundless.com/chemistry
/introduction-to-chemistry/physical-and-chemical-properties-of-matter/physical-and-chemicalproperties-of-matter/)
matter:
The basic structural component of the universe. Matter usually has mass and
volume.
APPEARS IN THIS RELATED CONCEPT:
Physical and Chemical Properties of Matter (https://www.boundless.com/chemistry
/introduction-to-chemistry/physical-and-chemical-properties-of-matter/physical-and-chemicalproperties-of-matter/)
multiple:
one of a set of the same thing; a duplicate
APPEARS IN THIS RELATED CONCEPT:
nitrogen:
A chemical element (symbol N) with an atomic number of 7 and atomic weight
of 14.0067.
APPEARS IN THIS RELATED CONCEPT:
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nitrogen:
a chemical element (symbol N) with an atomic number of 7 and atomic weight
of 14.0067
APPEARS IN THIS RELATED CONCEPT:
oxide:
a binary chemical compound of oxygen with another chemical element
oxygen:
a chemical element (symbol O) with an atomic number of 8 and relative atomic
mass of 15.9994
APPEARS IN THIS RELATED CONCEPT:
product:
a chemical substance formed as a result of a chemical reaction
product:
A chemical substance formed as a result of a chemical reaction.
APPEARS IN THIS RELATED CONCEPT:
ratio:
The relative magnitudes of two quantities (usually expressed as a quotient).
reactant:
Any of the participants present at the start of a chemical reaction. Also a
molecule before it undergoes a chemical change.
system:
the part of the universe being studied, arbitrarily defined to any size desired
APPEARS IN THIS RELATED CONCEPT:
Comparison of Enthalpy to Internal Energy (https://www.boundless.com/chemistry
/thermochemistry/enthalpy/comparison-of-enthalpy-to-internal-energy/)
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