Some Theoretical Atomic Models
Some Theoretical Atomic Models
The earliest concept of atomic structure dates back to Greece in the fifth century BCE, when Leucippus and Democritus postulated that tiny particles of matter, which they called atomos, were indivisible.
Over the centuries, many other concepts were proposed to explain the nature of matter— many of them extensions of the Greek concept of an ultimately indivisible and indestructible elementary bit of matter. But it was not until J. J. Thomson proposed his model of the atom, consisting of a sphere with an agglomeration of particles with negative electric charges some- how positioned randomly inside a very small ball of matter, that the modern structure of the atom began to take shape.
The Rutherford model of the atom is a significant improvement over the Thomson model. Baron Ernest Rutherford incorporated the background and understanding of many scientists as he developed experiments designed to show that the atom has a central and small, but relatively heavy, nucleus. His experiments verified that this positively charged dense nucleus has negatively charged electrons surrounding it at a very great distance compared to the size of the nucleus. This concept resembles the planets revolving around the sun, including with regard to the laws of motion and energy.
The Bohr model of the atom took shape in 1913. Niels Bohr (1885–1962), a Danish physicist, started with the classic Rutherford model and applied a new theory of quantum mechanics to develop a new model that is still in use, but with many enhancements. His assumptions are based on several aspects of quantum theory. One assumption is that light is emitted in tiny bunches (packets) of energy call photons (quanta of light energy).
The Bohr model continues: First, the orbital or quantum theory of matter assumes that the electron is not a particle, as
we normally think of particles. Orbital theory considers the electron as a three-dimensional wave that can exist at several energy levels (orbitals), but not at the same time.
Second, the electrons are in constant motion around the nucleus, even though it is not easy to determine the position of a particular electron in its shell at any particular moment. Third, the electrons are revolving at different distances from their nuclei; these pathways are called shells, orbitals, or energy levels. Fourth, there are more than a few shells, orbitals, or energy levels in most atoms. Fifth, the electrons can continue to move in a specific shell without emitting or absorbing
energy. Sixth, an electron at a specific energy level will remain in its shell until it either loses energy and “jumps” to an outer shell or gains energy and proceeds to a higher energy level (shell). Seventh, if the electron is excited by external energy, it can “jump” to a different shell or higher energy level.
Eighth, when the electron returns to its former shell or lower energy level, it will emit the energy (photon), which represents energy the electron acquired to raise it to the higher energy level. Bohr’s idea led to the comparison that likened the structure of the atom to the structure of an onion. The outer layers of skin on an onion are the “shells” where the electrons exist.
14 | The History and Use of Our Earth’s Chemical Elements At the center of the onion is a dense, tiny BB-like shot for
a nucleus. Unlike the onion, however, the area between the nucleus and the electron’s shells is merely space. There is no distinct boundary for the shells. The electrons assigned to a particular shell are in constant motion, so the shell does not seem to have a sharp definition. It is something like a “fuzzy ball,” with no distinct edges.
A similar model proposed at that time was related to “raisins in a plum pudding,” with the electrons resembling raisins spread throughout the pudding. The fuzzy-ball or raisin-in-plum-pudding model of the atom has no sharp boundaries, but does have definite limits as to the number
Figure2.2:Artist’sdepictionofthe and energy levels for the electrons residing within each shell. “fuzzyball”atomwhoseelectronshad
The fuzzy ball depicts the energy concept of the “electron nosharpboundariesbutexhibitedlim-
cloud,” which considers the electrons as energy levels around itstothenumberofenergylevels.
the nucleus. The concept states that the atom is spherical, and the electrons are all over the atom at any one time. The
electron cloud concept statistically depicts the probable distribution of electrons as they exist at any particular distance from the nucleus at any one particular time. The fuzziness is the image one might see while viewing the atom over an extended period, such as a time exposure with a camera.
The number of electrons in each shell is partially dependent on how far the shell is from the nucleus. In addition, electrons assigned to a specific shell stay in position until forced out by an input or loss of energy. Even more important, a particular shell or energy level “dis- likes” having any electrons missing. If a shell does not have its complete quota of electrons, the atom will “bond” with other atoms by “taking in electrons” or “giving up electrons” or “sharing electrons” in order to maintain a complete outer shell, thus forming molecules. This is the essence of how atoms of elements form molecules of compounds, and it is the essence of chemical reactions.
There are seven possible shells or energy levels for electrons surrounding the nucleus at a rela- tively great distance. The lightest atoms have only one shell, which is the innermost shell closest to the nucleus. Other atoms have multiple shells, and the largest and heaviest atoms have all seven shells of electrons. All the electrons in a particular shell have the same energy. The electrons
at the greatest distance from the nucleus are ones with the weakest attraction to the nucle- us; thus, they are usually the first electrons to be involved in a chemical reaction. These outer electrons then become the valence electrons.
As previously men- Figure2.3:Representationoforbitalshapes.
tioned, orbital theory
Atomic Structure | 15
Figure2.4:Tableofsevenenergylevels,shellsKthroughQ,maximumnumberof electronsineachshell,andthesuborbitalsequencesineachshell.
is based on quantum mechanics, which states that the position of electrons cannot be pre- cisely determined in their orbits, but rather they are somewhat similar to an electron cloud. However, their positions can be predicted by laws of mathematical probability. Orbitals take the shape of indefinite spheres and elliptical-shaped doughnuts. Each orbital is assigned a let- ter: s, p, d, or f. The energy levels, shells, orbitals, and their electron numbers are presented along with figures of each individual element.
Following is a list of the seven major shells, including the four orbital energy levels, with the letters assigned for their position extending from the nucleus. The number of electrons required to fill each shell and orbital is shown. This is also the maximum number of possible electrons in the shells of the known elements.
Up to this point, we have been describing single atoms and their electrons. Chemical reac- tions occur when electrons from the outer shells of atoms of two or more different elements interact. Nuclear reactions involve interactions of particles in the nucleus (mainly protons and neutrons) of atoms, not the atoms’ electrons. This distinction is fundamental. The former is atomic chemistry (or electron chemistry), and the latter is nuclear chemistry (or nuclear physics).
Some confusion exists. For instance, in normal chemical reactions, the electrons of different element’s atoms interact to form molecules of new chemical compounds. These compounds have different characteristics from their original elements. For example, the explosion of gunpowder is a chemical reaction involving the interactions of electrons of different elements that release a great deal of energy and gases when their atoms combine to form new molecular compounds. In contrast, the atom bomb or an atomic energy power plant derives its energy from the nucleus, not the electrons of different atoms combining. Therefore, it is inappropri- ate to refer to the “bomb” as an atomic bomb, because the electrons of the atom are not the source of the energy. The fission (splitting) of nuclei or the fusion (combining) of nuclei is the source of the energy that causes nuclear explosions. A detonation of a so-called atom bomb or the production of energy in a nuclear power plant is not a chemical reaction but a nuclear reaction. To nonscientists there is little distinction between a chemical weapon and a nuclear weapon—both are deadly, but the distinction is fundamental to chemistry and physics.
16 | The History and Use of Our Earth’s Chemical Elements