A Brief History of the Atom

Antiquity

• (400 B.C.) Democritus of Adbera (northern Greece) asserted that all material things are composed of extremely small irreducible particles called atoms. “Nothing exists except atoms and empty space. Everything else is opinion”. The atomic theory was roundly rejected by Aristotle, and, thus, by almost everybody else for the next two millennia.

17^th through 19^th Centuries

• (1627-1691) Robert Boyle (England) extended mathematics to chemistry and revived atomic theory.
• (1777) Antoine Lavoisier (France) demonstrated the conservation of matter (matter can be neither created nor destroyed) in a chemical reaction and defined the difference between an element and a compound.
• (1780) Charles Coulomb (France) described the force between two electric charges with a mathematical formula which looked very much like Newton’s law of gravity:
  where F is the force q₁ and q₂ are two charges and r is the distance between them. The electrical force is the chief force involved in atomic reactions. This force is attractive when charges q₁ and q₂ have opposite signs and repulsive when the charges have the same sign.
• (1803) John Dalton (England) formulated the modern version of the atomic theory. In his model all atoms in a given chemical element are exactly alike, while the atoms of different elements differ by atomic weight
• (1831-1879) James Clark Maxwell (England) showed that electricity and magnetism are two aspects of the same phenomena, and predicted that accelerating charges radiate waves traveling at the speed of light. These waves are known generically as electromagnetic waves of which visible light is one example.
• (1898) J.J. Thompson (England) discovered the electron, the component of the atom with negative charge. His model of the atom had the negatively charged electron evenly distributed throughout a sphere of positively charged material. This is known as the “plum pudding” model of the atom.

20^th Century

• (1900) Max Planck (Germany) introduced the quantum theory to explain the shape of the temperature versus color curve of a glowing solid. Briefly, he found that light cannot be converted into heat (energy) by any arbitrary amount, but only as discrete packets which he called quanta (known as photons today). For light of wavelength λ, the energy per quanta is given by:

where h is a constant which we now call planck’s constant.

• (1905) Albert Einstein (Germany, USA) published papers on special relativity which included the famous equation relating energy E to mass m:

Here, c is the speed of light. Thus, the mass of any particle has an equivalent energy and a photon, viewed by Planck as a packet of pure energy, has an equivalent mass.

• (1909) Ernest Rutherford (England) demonstrated that the atom is mostly empty space with a small positively charged nucleus containing most of the mass and low mass negatively charged particles (Thompson’s electrons) orbiting this nucleus. Rutherford could experimentally identify nuclear particles with positive charge that he called protons. Although he could explain the charge of atomic nuclei with the right number of protons, the mass of the nucleus for large atoms was always larger than the sum of its protons. Therefore he postulated the existence of a neutral particle with a mass nearly the same as the proton which, when added to the protons in the nucleus, would give the right mass. Rutherford called this hypothetical particle the neutron. Later (1930) Rutherford’s colleague James Chadwick was able to detect the neutron experimentally.
• (1913) Neils Bohr (Denmark) developed the first successful model of the atom. Since we still use Bohr’s model to explain many aspects of physical phenomena such as the appearance of spectra, it is worthwhile to spend some time describing it.
  
  

Bohr’s model of the atom builds on Rutherford’s basic conception. In detail, the nucleus contains a number of relatively high mass particles with positive charge called protons along (sometimes, not always) with some neutral particles of about the same mass called neutrons. A chemical element is defined and distinguished from all other chemical elements by the number of protons in its nucleus. Orbiting the nucleus, much like planets orbiting the sun, are the electrons. This is pretty much the picture that pops into most people’s heads when they think of atoms. They get this picture because that is how atoms are usually illustrated in everything from comic books to textbooks.

Now according to Maxwell, accelerating charges, such as electrons traveling in circular orbits, should radiate electromagnetic waves and, hence, energy. This loss of energy should make the electrons spiral down into the nucleus. To get around this problem, Bohr proposed that the electrons were confined to specific orbits that were quantized. As long as the electrons remained in one of the allowed orbits, no electromagnetic radiation will be released. Under ordinary conditions the electrons of most atoms are in the lowest orbit available; under such conditions the atom is said to be in the “ground state” and cannot radiate energy. To move an electron from the ground state to one of the higher orbits requires the input of energy exactly equal to the energy spacing between the two orbits. Once at the higher level, the electron can then fall back to a lower orbit, radiating a photon with an energy corresponding to the orbital spacing.

To summarize: To radiate energy, and atom must first be excited (electrons raised above the ground state). The excited atom then returns to the ground state by emitting energy in the form of electromagnetic radiation.

Bohr Hydrogen Atom

Hydrogen atom in ground state Hydrogen atom excited

Hydrogen atom emits photon

Bohr set about explaining the visible spectrum of the hydrogen atom, i.e., the Balmer series of lines familiar to just about everyone who has ever taken an astronomy lab. Bohr was able to show that this set of violet, blue and red lines originated from an electron falling from higher orbits down to the orbit immediately above the ground state. More precisely, if we designate each orbit with a number beginning with n=1 for the ground state, the Balmer series represents the transition of the electron from orbit n>2 to orbit n=2. The higher the originating orbit, the greater the energy of the photon emitted. For example, the red line, representing the longest wavelength (and, thus, the lowest energy photon), is produced by the electron falling from orbit n=3 to n=2. The next blue line comes from the electron in n=4 falling to n=2, and so forth.

Now before we can obtain the Balmer spectrum from a hydrogen atom, two criteria must be satisfied: (1) there must be an electron available and (2) it must be in an orbit greater than n=2. Criterion (1) will not be satisfied if the atom has been stripped of its electron. An atom in this condition is referred to as ionized and it occurs at elevated temperatures. On the other hand, criterion (2) will not be satisfied if the hydrogen atom is in the ground state, i.e., its electron has not been excited into a higher orbit. From this we can see why neither hot, blue O type stars nor cool, red M type stars exhibit strong hydrogen lines. Type O stars are so hot that most of the hydrogen atoms in their atmospheres have been ionized, and are, hence, unavailable to form spectra. On the other hand type M stars are too cool to excite very many of the hydrogen atoms above the ground state. Thus, for opposite reasons, neither type O or type M stars have strong hydrogen lines in their spectra.

• (1924) Louis de Broglie (France) hypothesized that the electrons in Bohr’s model were confined to discrete orbits because they had the properties of standing waves. He proposed that any particle with a momentum p (p = mv) has an equivalent wavelength λ given by

. Here m is the mass of the particle and v is its velocity. Calculations based on the assumption that matter at the atomic level can be viewed as waves agreed so well with experiments that it became a cornerstone of quantum mechanics. The theory is known today as the Principle of Complementarity :

Waves and particles represent complementary aspects of the same phenomenon.

In short, wave phenomena such as light can also have the properties of particles, and particle phenomena such as the constituents of atoms can also have the properties of waves.

• (1925) Cecilia Payne (England, USA), using the new model of the atom, showed that the sun and stars are composed almost entirely of hydrogen and helium, with only trace amounts of more familiar, heavier, elements. She came to this conclusion by studying the spectral data which had been accumulated at Harvard Observatory over the past quarter century. As was pointed out in the discussion of the Bohr atom, both hot stars (O and B) and cool stars (K and M) do not exhibit strong hydrogen lines for opposite reasons. In the first instance (hot stars), most of the hydrogen atoms are ionized and, thus, have no electrons available to be raised to a higher orbit. In the second instance (cool stars), the stars do not produce many photons of the energy required to raise electrons above the ground state. However, hydrogen lines are not entirely absent; faint hydrogen lines are seen in both groups of stars. This is so because at a given instant a few hydrogen atoms in hot stars do have electrons, and even the coolest stars produce some energy in the range necessary to excite the hydrogen atom. The percentage of non-ionized atoms in hot stars and the percentage of excitation photons in the energy output of cool stars can be calculated statistically. When Cecilia Payne made these calculations, she came to the conclusion that the fact that any hydrogen lines at all are visible in these stars implies that the number of hydrogen atoms present must be enormous – over 90% of the total number of atoms and over 70 % of the stellar mass. Similar reasoning led to the conclusion that most of the remaining mass was made up of helium. The same statistical approach was applied to the spectra of the middle range stars (A, F, and G) and a similar composition was found for these stars as well. For all stars, only a tiny fraction of the stellar mass, typically no more than 2%, was comprised of the heavier elements such as oxygen and silicon, the most common elements on Earth. When the hydrogen emission hydrogen spectra found in gaseous nebulae was factored in, the message seemed clear: The universe is mostly hydrogen and helium, the two simplest elements.

• (1925) Sir Arthur Eddington (England) produced the first model of stellar structure based on nuclear physics. The energy source of the sun and stars had been a mystery for centuries. It is easy to determine the total energy output of the sun. Basically, you measure how much heat is transferred to a square meter of water (or other calibrated material) in a given amount of time, then multiply this number times the surface area of a sphere centered on the sun with a radius of 1 A.U. This yields a total energy output of about 4 x 10²⁶ joules/sec (watts). From geological considerations this amount of energy has been produced without interruption for about 5 billion years. No energy source known before the 20^th century could have produced energy at this rate for this length of time.
  
  

Scientist studying the nucleus in the early twentieth century noticed that the atomic weight of the helium nucleus was slightly less than the sum of the protons and neutrons that comprised it. The implication was that when protons and neutrons were added together to make helium, energy was produced equal to the mass loss in accordance with Einstein’s E=mc² equation. However, in order for two or more protons to come together, they had to overcome the couloumb barrier, the electromagnetic repulsion between like charges. This requires the protons to be tremendously energetic, which in turn requires that they be in a very high temperature environment.

Eddington showed that the core of the sun was an environment with the necessary temperature. He did this by reasoning that the sun was neither getting smaller or larger. For a fluid substance such a condition is known as hydrostatic equilibrium. The force trying to collapse the sun is gravity. Since the sun’s dimensions are not changing, the force of gravity must be counter balanced by force acting in the opposite direction. There is a simple equation that relates the pressure (force per unit area), P, of a gas to its density, D, and its temperature, T: P =DT. Knowing the mass and volume of the sun and the temperature at its surface, Eddington was able to calculate the temperature required at any point in the sun’s interior to produce the outward pressure necessary to counter balance the inward gravitational pressure. He found that at its core, the sun’s temperature would have to be around 10 million K. When Eddington first published his results it was felt that this was not hot enough. However, further understanding of the behavior of matter at the quantum level showed that the temperature was sufficient. Today we recognize that the conversion of hydrogen (one proton) into helium (two protons + two neutrons) plus energy at the core of the sun is the basic process that makes the sun shine. This process is known generically as thermonuclear fusion.