Target Health Blog

Atomic Theory

September 24, 2018


A bust of Democritus (or Democrites), who came up with the idea of indivisible atoms.Graphic credit: Public Domain,

It is fundamental to the understanding of science that science is an agreed upon process of trial and improvement and represents the best known at the time, not an unerring oracle of truth. Development of an idea and refinement through 1) ___ is clearly shown by examining the evolution of the theory of atomic structure, which began with the ancient thinkers. The earliest known proponent of anything resembling modern atomic theory was the ancient Greek thinker Democritus. He proposed the existence of indivisible atoms as a response to the arguments of Parmenides, and the paradoxes of Zeno. Parmenides argued against the possibility of movement, change, and plurality on the premise that something cannot come from 2) ___. Zeno attempted to prove Parmenides' point by a series of paradoxes based on difficulties with infinite divisibility. In response to these ideas, Democritus posited the existence of indestructible atoms that exist in a void. Their indestructibility provided a retort to Zeno, and the void allowed him to account for plurality, change, and movement. It remained for him to account for the properties of atoms, and how they related to our experiences of objects in the world. Democritus proposed that 3) ___ possessed few actual properties, with size, shape, and mass being the most important. All other properties, he argued, could be explained in terms of the three primary properties. A smooth substance, for instance, might be composed of primarily smooth atoms, while a rough substance is composed of sharp ones. Solid substances might be composed of atoms with numerous hooks, by which they connect to each other, while the atoms of liquid substances possess far fewer points of connection.

Democritus proposed 5 points to his theory of atoms. These are:

1. All matter is composed of atoms, which are bits of matter too small to be seen. These atoms CANNOT be further split into smaller portions.

2. There is a void, which is empty space between atoms.

3. Atoms are completely solid.

4. Atoms are homogeneous, with no internal structure.

5. Atoms are different in: their sizes, their shapes, and their weights.

The theory originated as a philosophical concept in ancient India and Greece. The word atom comes from the Ancient Greek word atomos, which means "indivisible". According to atomism, matter consisted of discrete particles. However, the theory was one of many explanations for matter and was not based on empirical data. In the fifth century B.C., Democritus proposed matter consisted of indestructible, indivisible units called atoms. The Roman poet Lucretius recorded the idea, so it survived through the 4) ___ Ages for later consideration.

Editor's note: As an undergraduate at Columbia University, it was in my history of science class, where I learned that as far back as 500 BCE, humans had conceived of the idea that, there existed units so tiny, as not to be visible by the unaided eye. The profound knowledge of the ancient Greeks and Romans has always amazed me, and still does to this day. Also, amazing is that this early theory of atoms survived the one thousand or more years of the Dark Ages, a time of unending battles, murder blood and revenge. A time of pestilence, dire poverty and nightmarish misery. A time where the church was threatened by any person or idea that it had no control over. And yet, in the midst of century after century of chaos, the atomic theory remained unscathed, passing from one mind to another, for hundreds of generations, as the church slowly did lose its strangle hold over human thought. The rise of evidentiary scientific thinking liberated creativity and laid down a fertile foundation out of which the brilliant seed of atomic theory could finally sprout and flourish, after lying dormant for so long.

Girolamo Fracastoro, an Italian physician, subscribed to the philosophy of atomism, in his essay on contagion De Contagione et Contagiosis Morbis, published in 1546: "I call fomites [from the Latin fomes, meaning "tinder"] such things as clothes, linen, etc., which although not themselves corrupt, can nevertheless foster the essential seeds of the (invisible) contagion and thus cause infection.". His theory remained influential for nearly three centuries, before being superseded by a fully developed germ theory. However, it took until the end of the 18th century for science to provide concrete evidence of the existence of atoms. Antoine Lavoisier formulated the law of conservation of mass in 1789, which states the mass of the products of a reaction is the same as the mass of reactants. Joseph Louis Proust proposed the law of definite proportions in 1799, which states the masses of elements in a compound always occur in the same proportion. These theories did not reference atoms, yet John Dalton built upon them to develop the law of multiple proportions, which states the ratio of masses of elements in a compound are small whole numbers. Dalton's law of multiple proportions drew from experimental 5) ___. He proposed each chemical element consists of a single type of atom that could not be destroyed by any chemical means. His oral presentation (1803) and publication (1805) marked the beginning of the scientific atomic theory. In 1811, Amedeo Avogadro corrected a problem with Dalton's theory when he proposed equal volumes of gases at equal temperature and pressure contain the same number of particles. Avogadro's law made it possible to accurately estimate the atomic masses of element and made clear there was a distinction between atoms and molecules. Another significant contribution to atomic theory was made in 1827 by botanist Robert Brown, who noticed dust particles floating in water seemed to move randomly for no known reason.

The 19th century chemists began using the term “atomic“ in connection with the growing number of irreducible chemical elements. While seemingly apropos, around the turn of the 20th century, through various experiments with electromagnetism and radioactivity, physicists discovered that the so-called “uncuttable atom“ was actually a conglomerate of various subatomic particles (chiefly, electrons, 6) ___ and neutrons) which can exist separately from each other. In fact, in certain extreme environments, such as neutron stars, extreme temperature and pressure prevents atoms from existing at all. Since atoms were found to be divisible, physicists later invented the term “elementary particles“ to describe the “uncuttable“, though not indestructible, parts of an atom. The field of science which studies subatomic particles is particle 7) ___, and it is in this field that physicists hope to discover the true fundamental nature of matter. The idea that matter is made up of discrete units is a very old one, appearing in many ancient cultures. However, these ideas were founded in philosophical and theological reasoning rather than 8) ___ and experimentation. Because of this, they could not convince everybody, so atomism was but one of a number of competing theories on the nature of matter. It was not until the 19th century that the idea was embraced and refined by scientists, as the blossoming science of chemistry produced discoveries that could easily be explained using the concept of atoms.

Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory. The first was the law of conservation of mass, formulated by Antoine Lavoisier in 1789, which states that the total mass in a chemical reaction remains constant (that is, the reactants have the same mass as the products). The second was the law of definite proportions. First proven by the French chemist Joseph Louis Proust in 1799, this law states that if a compound is broken down into its constituent elements, then the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance. John Dalton studied and expanded upon this previous work and developed the law of multiple proportions: if two elements can be combined to form a number of possible compounds, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers. For example, Proust had studied tin oxides and found that there is one type of tin oxide that is 88.1% tin and 11.9% oxygen and another type that is 78.7% tin and 21.3% oxygen (these are 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 that 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 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). This marked the first truly scientific theory of the atom, since Dalton reached his conclusions by experimentation and examination of the results in an empirical fashion. In 1803 Dalton orally presented his first list of relative atomic weights for a number of substances. This paper was published in 1805, but he did not discuss, exactly how he obtained these figures. The method was first revealed in 1807 by his acquaintance Thomas Thomson, in the third edition of Thomson's textbook, A System of Chemistry. Finally, Dalton published a full account in his own textbook, A New System of Chemical Philosophy, 1808 and 1810.

Dalton estimated the atomic weights according to the mass ratios in which they combined, with the hydrogen atom taken as unity. However, Dalton did not conceive that with some elements atoms exist in molecules?e.g. pure oxygen exists as O2. He also mistakenly believed that the simplest compound between any two elements is always one atom of each (so he thought water was HO, not H2O). This, in addition to the crudity of his equipment, flawed his results. For instance, in 1803 he believed that oxygen atoms were 5.5 times heavier than hydrogen atoms, because in water he measured 5.5 grams of oxygen for every 1 gram of hydrogen and believed the formula for water was HO. Adopting better data, in 1806 he concluded that the atomic weight of oxygen must actually be 7 rather than 5.5, and he retained this weight for the rest of his life. Others at this time had already concluded that the oxygen atom must weigh 8 relative to hydrogen equals 1, if one assumes Dalton's formula for the water molecule (HO), or 16 if one assumes the modern water formula (H2O). The flaw in Dalton's theory was corrected in principle in 1811 by Amedeo Avogadro. Avogadro had proposed that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules (in other words, the mass of a gas's particles does not affect the volume that it occupies). Avogadro's law allowed him to deduce the diatomic nature of numerous gases by studying the volumes at which they reacted. For instance: since two liters of hydrogen will react with just one liter of oxygen to produce two liters of water vapor (at constant pressure and temperature), it meant a single oxygen molecule splits in two in order to form two particles of water. Thus, Avogadro was able to offer more accurate estimates of the atomic mass of oxygen and various other elements, and made a clear distinction between molecules and atoms. In 1827, the British botanist Robert Brown observed that dust particles inside pollen grains floating in water constantly jiggled about for no apparent reason.

Albert Einstein became famous for the theory of relativity, which laid the basis for the release of atomic energy. In 1905 Albert Einstein formulates Special Theory of Relativity. Einstein calculates how the movement of molecules in a liquid can cause the Brownian motion. He theorized that this Brownian motion was caused by the water molecules continuously knocking the grains about, and developed a hypothetical mathematical model to describe it. This model was validated experimentally in 1908 by French physicist Jean Perrin, thus providing additional validation for particle theory (and by extension atomic theory). Albert Einstein (1879 - 1955) was a German-born Jewish theoretical physicist who developed the theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics). His work is also known for its influence on the philosophy of science. He is best known to the general public for his mass-energy equivalence formula E = mc2, which has been dubbed “the world's most famous equation“. He received the 1921 Nobel Prize in Physics “for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect“, a pivotal step in the development of quantum theory.

Ernest Rutherford, discovered that most of the mass and positive charge of an atom is concentrated in a very small fraction of its volume, which he assumed to be at the very center. In the Geiger-Marsden experiment, Hans Geiger and Ernest Marsden (colleagues of Rutherford working at his behest) shot alpha particles at thin sheets of metal and measured their deflection through the use of a fluorescent screen. Given the very small mass of the electrons, the high momentum of the alpha particles, and the low concentration of the positive charge of the plum pudding model, the experimenters expected all the alpha particles to pass through the metal foil without significant deflection. To their astonishment, a small fraction of the alpha particles experienced heavy deflection. Rutherford concluded that the positive charge of the atom must be concentrated in a very tiny volume to produce an electric field sufficiently intense to deflect the alpha particles so strongly. This led Rutherford to propose a planetary model in which a cloud of electrons surrounded a small, compact nucleus of positive charge. Only such a concentration of charge could produce the electric field strong enough to cause the heavy deflection. The planetary model of the atom had two significant shortcomings. The first is that, unlike planets orbiting a sun, electrons are charged particles. An accelerating electric charge is known to emit electromagnetic waves according to the Larmor formula in classical electromagnetism. An orbiting charge should steadily lose energy and spiral toward the nucleus, colliding with it in a small fraction of a second. The second problem was that the planetary model could not explain the highly peaked emission and absorption spectra of atoms that were observed. Up to this point, atoms were believed to be the smallest units of matter. In 1897, J.J. Thomson discovered the electron, which carried a negative charge. Rutherford was on the right track, but his model could not explain the emission and absorption spectra of atoms nor why the electrons didn't crash into the nucleus.

In 1913, Niels Bohr proposed the Bohr model, which states electron only orbit the nucleus at specific distances from the nucleus. According to his model, electrons couldn't spiral into the nucleus, but could make quantum leaps between energy levels. Bohr's model explained the spectral lines of hydrogen, but didn't extend to the behavior of atoms with multiple electrons. Several discoveries expanded the understanding of atoms. In 1913, Frederick Soddy described isotopes, which were forms of an atom of one element that contained different numbers of neutrons. 9) ___ were discovered in 1932. Louis de Broglie proposed a wave-like behavior of moving particles, which Erwin Schrodinger described using Schrodinger's equation (1926). This, in turn, led to the Heisenberg uncertainty principle (1927), which states it's not possible to simultaneously know both the position and momentum of an electron.

Quantum mechanics led to an atomic theory in which atoms consist of smaller particles. The electron can potentially be found anywhere in the atom, but is found with greatest probability in an atomic orbital or energy level. The circular orbits of Rutherford's model, modern atomic theory describes orbitals that may be spherical, dumb bell shaped, etc. For atoms with a high number of electrons, relativistic effects come into play, since the particles are moving at speeds that are a fraction of the speed of light. Modern scientists have found smaller particles that make up the protons, neutrons, electrons, although the atom remains the smallest unit of matter that cannot be divided using any chemical means. Quantum theory revolutionized physics at the beginning of the 20th century, when Max Planck and Albert 10) ___ postulated that light energy is emitted or absorbed in discrete amounts known as quanta (singular, quantum). In 1913, Niels Bohr incorporated this idea into his Bohr model of the atom, in which an electron could only orbit the nucleus in particular circular orbits with fixed angular momentum and energy, its distance from the nucleus (i.e., their radii) being proportional to its energy. Under this model an electron could not spiral into the nucleus because it could not lose energy in a continuous manner; instead, it could only make instantaneous "quantum leaps" between the fixed energy levels. When this occurred, light was emitted or absorbed at a frequency proportional to the change in energy (hence the absorption and emission of light in discrete spectra). Bohr's model was not perfect. It could only predict the spectral lines of hydrogen; it couldn't predict those of multielectron atoms. Worse still, as spectrographic technology improved, additional spectral lines in hydrogen were observed which Bohr's model couldn't explain. In 1916, Arnold Sommerfeld added elliptical orbits to the Bohr model to explain the extra emission lines, but this made the model very difficult to use, and it still couldn't explain more complex atoms.

Sources: Helmenstine, Anne Marie, Ph.D. "History of Atomic Theory." ThoughtCo, Oct. 3, 2017,;; Wikipedia

ANSWERS: 1) testing; 2) nothing; 3) atoms; 4) Dark; 5) data; 6) protons; 7) physics; 8) evidence; 9) Neutrons; 10) Einstein

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