THE ATOM AND ATOMIC STRUCTURE

• Electron
• Proton
• Neutron
• Isotopes

By the nineteenth century chemists had recognized that the wide variety of materials that they studied were mixtures of basic, elementary substances. The composition of these elements was a matter of speculation until the early twentieth century, when the existence of atoms became widely accepted thanks to the work of Albert Einstein. An atom is a piece of matter that cannot be broken down into smaller units by chemical means, giving the smallest possible unit of any given element. For this reason, they are often considered to be the building blocks of nature. Yet scientists realized that these atoms are themselves are made up of even smaller particles called electrons, neutrons, and protons. And at an even smaller scale, scientists now know that neutrons and protons are composed of tiny fundamental particles called quarks. Current models of the structure of the atom consist of a small and relatively heavy nucleus surrounded by a cloud of electrons. The nucleus is made up of protons and neutrons and makes a tiny fraction of the overall size of the atom. The nucleus is only about 10-12 cm across, while the atom itself is about 10-8 cm. This means that if the nucleus was the same size as Earth, the radius of the atom would stretch all the way to the sun. Protons and neutrons have similar masses, and are about 1,800 times heavier than electrons. Even though the nucleus is so small compared with the rest of the atom, almost all of the mass of the atom is located there. Scientists define the atomic mass to be the total number of protons and neutrons. The atomic number, which defines a given element, consists solely of the number of protons in the nucleus. This means that atoms of the same element must have the same number of protons, but may have different numbers of neutrons, and consequently different atomic weights. These are known as isotopes. For example, all atoms of uranium have 92 protons, but one type of uranium, the isotope uranium-235 has 143 neutrons (92 + 143 = 235) while uranium-238 has 146 neutrons (92 + 146 = 238).

Only ninety-two elements occur in nature, from the lightest, hydrogen, having one proton, to the heaviest, uranium, with ninety-two protons. Yet, scientists can produce even heavier elements in the laboratory with the use of nuclear reactors. The element plutonium, which has ninety-four electrons, is one such element. Scientists have produced at least twenty-three elements in the laboratory, including berkelium (named for the University of California, Berkeley); bohrium (named for Niels Bohr); einsteinium (named for Albert Einstein); fermium (after Enrico Fermi); lawrencium (after Ernest O. Lawrence); meitnerium (after Lise Meitner); and seaborgium (after Glenn T. Seaborg).

For many physicists and chemists, the study of atomic structure was an exciting area of research by the turn of the 20th century. To satisfactorily model the structure of atoms, scientists needed to ensure that their models would be stable and that they would be able to account for experimental results. With the discovery of the electron in 1897, J.J. Thomson began developing an early and influential model for atomic structure. Thomson imagined an atom composed of only electrons, and while his model had some promising features, it was not clear to observers how it could be used to describe certain known experimental phenomena. Also published in 1904, the same year and in the same journal as Thomson's model, was Japanese physicist Hantaro Nagaoka's "Saturnian" model of atomic structure. This approach imagined a positively charged nucleus circled by rings of electrons, but it too suffered from theoretical shortcomings. With the 1911 publication of Ernest Rutherford's model, ideas about atomic structure began to meaningfully resemble the model accepted today. Through scattering experiments Rutherford argued that an atom was composed of a hard, heavy, and electrically charged nucleus surrounded by oppositely charged particles.

Importantly, Rutherford's model would be picked up and reworked with newly developed quantum mechanical tools by Niels Bohr in what would come to be called the Bohr Atom. Niels Bohr published his hugely influential 1913 paper "On the Constitution of Atoms and Molecules," in three parts in Philosophical Magazine. In this work Bohr combined Rutherford's atomic model of a massive nucleus with a quantum mechanical theory of energy. Bohr postulated that the atom consisted of a hard nucleus, surrounded by electrons orbiting in discrete energy states. Bohr's very influential model proved to be very useful in the description of known experimental results and earned him the Nobel Prize in physics in 1922.

To continue with a quick overview of the Science of the Manhattan Project, jump ahead to the description of Nuclear Physics. To learn more about the atom, choose a web page from the menu below:

• Electron
• Proton
• Neutron
• Isotopes

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