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2.2: The Atomic Theory

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    The development of the atomic theory owes much to the work of two men: Antoine Lavoisier, who did not himself think of matter in terms of atoms but whose work laid organization groundwork for thinking about elements, and John Dalton, to whom the atomic theory is attributed. Much of Lavoisier’s work as a chemist was devoted to the study of combustion. He became convinced that when a substance is burned in air, it combines with some component of the air. Eventually he realized that this component was the dephlogisticated air which had been discovered by Joseph Priestly (1733 to 1804) a few years earlier. Lavoisier renamed this substance oxygen. In an important series of experiments he showed that when mercury is heated in oxygen at a moderate temperature, a red substance, calx of mercury, is obtained. (A calx is the ash left when a substance burns in air.) At a higher temperature this calx decomposes into mercury and oxygen. Lavoisier’s careful experiments also revealed that the combined masses of mercury and oxygen were exactly equal to the mass of calx of mercury. That is, there was no change in mass upon formation or decomposition of the calx. Lavoisier hypothesized that this should be true of all chemical changes, and further experiments showed that he was right. This principle is now called the law of conservation of mass.

    As Lavoisier continued his experiments with oxygen, he noticed something else. Although oxygen combined with many other substances, it never behaved as though it were itself a combination of other substances. Lavoisier was able to decompose the red calx into mercury and oxygen, but he could find no way to break down oxygen into two or more new substances. Because of this he suggested that oxygen must be an element—an ultimately simple substance which could not be decomposed by chemical changes.

    Lavoisier did not originate the idea that certain substances (elements) were fundamental and all others could be derived from them. This had first been proposed in Greece during the fifth century B.C. by Empedocles, who speculated that all matter consisted of combinations of earth, air, fire, and water. These ideas were further developed and taught by Aristotle and remained influential for 2000 years.

    Lavoisier did produce the first table of the elements which contained a large number of substances that modern chemists would agree should be classifies as elements. He published it with the knowledge that further research might succeed decomposing some of the substances listed, thus showing them not to be elements. One of his objectives was to prod his contemporaries into just that kind of research. Sure enough the “earth substances” listed at the bottom were eventually shown to be combinations of certain metals with oxygen. It is also interesting to note that not even Lavoisier could entirely escape from Aristotle’s influence. The second element in his list is Aristotle’s “fire,” which Lavoisier called “caloric,” and which we now call “heat.” Both heat and light, the first two items in the table, are now regarded as forms of energy rather than of matter.

    Although his table of elements was incomplete, and even incorrect in some instances, Lavoisier’s work represented a major step forward. By classifying certain substances as elements, he stimulated much additional chemical research and brought order and structure to the subject where none had existed before. His contemporaries accepted his ideas very readily, and he became known as the father of chemistry.

    John Dalton (1766 to 1844) was a generation younger than Lavoisier and different from him in almost every respect. Dalton came from a working class family and only attended elementary school. Apart from this, he was entirely self-taught. Even after he became famous, he never aspired beyond a modest bachelor’s existence in which he supported himself by teaching mathematics to private pupils. Dalton made many contributions to science, and he seems not to have realized that his atomic theory was the most important of them. In his “New System of Chemical Philosophy” published in 1808, only the last seven pages out of a total of 168 are devoted to it!

    The postulates of the atomic theory are given below. The first is no advance on the ancient Greek philosopher Democritus who had theorized almost 2000 years earlier that matter consists of very small particles.

    The Postulates of Dalton's Atomic Theory

    1. All matter is composed of a very large number of very small particles called atoms.
    2. For a given element, all atoms are identical in all respects. In particular all atoms of the same element have the same constant mass, while atoms of different elements have different masses.
    3. The atoms are the units of chemical changes. Chemical reactions involve the combination, separation, or rearrangement of atoms, but atoms are neither created, destroyed, divided into parts, or converted into atoms of any other kind.
    4. Atoms combine to form molecules in fixed ratios of small whole numbers.

    The second postulate, however, shows the mark of an original genius; here Dalton links the idea of atom to the idea of element. Lavoisier’s criterion for an element had been essentially a macroscopic, experimental one. If a substance could not be decomposed chemically, then it was probably an element. By contrast, Dalton defines an element in theoretical, sub-microscopic terms. An element is an element because all its atoms are the same. Different elements have different atoms. There are just as many different kinds of elements as there are different kinds of atoms.

    Now look back a moment to the physical states of mercury, where sub-microscopic pictures of solid, liquid, and gaseous mercury were given. Applying Dalton’s second postulate to this figure, you can immediately conclude that mercury is an element, because only one kind of atom appears. Although mercury atoms are drawn as spheres in the figure, it would be more common today to represent them using chemical symbols. The chemical symbol for an element (or an atom of that element) is a one- or two-letter abbreviation of its name. Usually, but not always, the first two letters are used. To complicate matters further, chemical symbols are sometimes derived from a language other than English. For example the symbol for Hg for mercury comes from the first and seventh letters of the element’s Latin name, hydrargyrum.

    Table \(\PageIndex{1}\): Names, Chemical Symbols, and Atomic Weights of the Element
    Name Symbol Atomic Number Atomic Weight Name Symbol Atomic Number Atomic Weight
    Actinium2 Ac 89 (227) Molybdenum Mo 42 95.96(2)
    Aluminum Al 13 26.981 5386(8) Neodymium Nd 60 144.242(3)
    Americium2 Am 95 (243) Neon Ne 10 20.1797(6)
    Antimony Sb 51 121.760(1) Neptunium2 Np 93 (237)
    Argon Ar 18 39.948(1) Nickel Ni 28 58.6934(4)
    Arsenic As 33 74.92160(2) Niobium Nb 41 92.90638(2)
    Astatine2 At 85 (210) Nitrogen N 7 14.0067(2)
    Barium Ba 56 137.327(7) Nobelium2 No 102 (259)
    Berkelium2 Bk 97 (247) Osmium Os 76 190.23(3)
    Beryllium Be 4 9.012182(3) Oxygen O 8 15.9994(3)
    Bismuth Bi 83 208.98040(1) Palladium Pd 46 106.42(1)
    Bohrium2 Bh 107 (272) Phosphorus P 15 30.973762(2)
    Boron B 5 10.811(7) Platinum Pt 78 195.084(9)
    Bromine Br 35 79.904(1) Plutonium2 Pu 94 (244)
    Cadmium Cd 48 112.411(8) Polonium2 Po 84 (209)
    Calcium Ca 20 40.078(4) Potassium K 19 39.0983(1)
    Californium2 Cf 98 (251) Praseodymium Pr 59 140.90765(2)
    Carbon C 6 12.0107(8) Promethium2 Pm 61 (145)
    Cerium Ce 58 140.116(1) Protactinium2 Pa 91 231.03588(2)
    Cesium Cs 55 132.9054519(2) Radium2 Ra 88 (226)
    Chlorine Cl 17 35.453(2) Radon2 Rn 86 (222)
    Chromium Cr 24 51.9961(6) Rhenium Re 75 186.207(1)
    Cobalt Co 27 58.933195(5) Rhodium Rh 45 102.90550(2)
    Copper Cu 29 63.546(3) Roentgenium2 Rg 111 (280)
    Curium2 Cm 96 (247) Rubidium Rb 37 85.4678(3)
    Darmstadtium2 Ds 110 (281) Ruthenium Ru 44 101.07(2)
    Dubnium2 Db 105 (268) Rutherfordium2 Rf 104 (267)
    Dysprosium Dy 66 162.500(1) Samarium Sm 62 150.36(2)
    Einsteinium2 Es 99 (252) Scandium Sc 21 44.955912(6)
    Erbium Er 68 167.259(3) Seaborgium2 Sg 106 (271)
    Europium Eu 63 151.964(1) Selenium Se 34 78.96(3)
    Fermium2 Fm 100 (257) Silicon Si 14 28.0855(3)
    Fluorine F 9 18.9984032(5) Silver Ag 47 107.8682(2)
    Francium2 Fr 87 (223) Sodium Na 11 22.98976928(2)
    Gadolinium Gd 64 157.25(3) Strontium Sr 38 87.62(1)
    Gallium Ga 31 69.723(1) Sulfur S 16 32.065(5)
    Germanium Ge 32 72.64(1) Tantalum Ta 73 180.94788(2)
    Gold Au 79 196.966569(4) Technetium2 Tc 43 (98)
    Hafnium Hf 72 178.49(2) Tellurium Te 52 127.60(3)
    Hassium2 Hs 108 (277) Terbium Tb 65 158.92535(2)
    Helium He 2 4.002602(2) Thallium Tl 81 204.3833(2)
    Holmium Ho 67 164.93032(2) Thorium2 Th 90 232.03806(2)
    Hydrogen H 1 1.00794(7) Thulium Tm 69 168.93421(2)
    Indium In 49 114.818(3) Tin Sn 50 118.710(7)
    Iodine I 53 126.90447(3) Titanium Ti 22 47.867(1)
    Iridium Ir 49 192.217(3) Tungsten W 74 183.84(1)
    Iron Fe 26 55.845(2) Uranium2 U 92 238.02891(3)
    Krypton Kr 36 83.798(2) Vanadium V 23 50.9415(1)
    Lanthanum La 57 138.90547(7) Xenon Xe 54 131.293(6)
    Lawrencium2 Lr 103 (262) Ytterbium Yb 70 173.054(5)
    Lead Pb 82 207.2(1) Yttrium Y 39 88.90585(2)
    Lithium Li 3 [6.941(2)]1 Zinc Zn 30 65.38(2)
    Lutetium Lu 71 174.9668(1) Zirconium Zr 40 91.224(2)
    Magnesium Mg 12 24.3050(6) -2,3,4   112 (285)
    Manganese Mn 25 54.938045(5) -2,3   113 (284)
    Meitnerium2 Mt 109 (276) - 2,3   114 (287)
    Mendelevium2 Md 101 (258) -2,3   115 (288)
    Mercury Hg 80 200.59(2) -2,3   116 (293)
            -2,3   118 (294)

    The chemical symbols for all the currently known elements are listed above in the table, which also includes atomic weights. These symbols are the basic vocabulary of chemistry because the atoms they represent make up all matter. You will see symbols for the more important elements over and over again, and the sooner you know what element they stand for, the easier it will be for you to learn chemistry. These more important element have been indicated in the above table by colored shading around their names.

    Dalton’s fourth postulate states that atoms may combine to form molecules. An example of this is provided by bromine, the only element other than mercury which is a liquid at ordinary room temperature (20°C). Macroscopically, bromine consists of dark-colored crystals below –7.2°C and a reddish brown gas above 58.8°C. The liquid is dark red-brown and has a pungent odor similar to the chlorine used in swimming pools. It can cause severe burns on human skin and should not be handled without the protection of rubber gloves.

    (a) in the gaseous state (b) as a liquid (c) in solid form

    Figure \(\PageIndex{1}\): Sub-microscopic view of the diatomic molecules of the element bromine (a) in the gaseous state (above 58°C); (b) in liquid form (between -7.2 and 58.8°C); and (c) in solid form (below -7.2°C). 

    The sub-microscopic view of bromine in the following figure is in agreement with its designation as an element—only one kind of atom is present. Except at very high temperatures, though, bromine atoms always double up. Whether in solid, liquid, or gas, they go around in pairs. Such a tightly held combination of two or more atoms is called a molecule.


    Figure \(\PageIndex{1}\): Macroscopic view of the diatomic molecules of the element bromine (left) in the gaseous state (above 58°C); (middle) and in liquid form (between -7.2 and 58.8°C); (right) bromine atoms are present in the solid compound Silver Bromide (AgBr).


    The composition of a molecule is indicated by a chemical formula. A subscript to the right of the symbol for each element tells how many atoms of that element are in the molecule. For example, the atomic weights table gives the chemical symbol Br for bromine, but each molecule contains two bromine atoms, and so the chemical formula is Br2. According to Dalton’s fourth postulate, atoms combine in the ratio of small whole numbers, and so the subscripts in a formula should be small whole numbers.