Electrons
Electrons are fundamental subatomic particles that play a crucial role in the structure of atoms and the behavior of chemical elements. They possess a negative charge and are significantly less massive than protons, which are positively charged particles located in the atom's dense nucleus. The modern understanding of atomic structure, informed by quantum mechanics, depicts electrons existing in a cloud around the nucleus and organized into distinct energy levels known as electron shells. The outermost shell, or valence shell, is particularly important as it contains the valence electrons that dictate an atom's chemical reactivity.
Electrons can be involved in various types of chemical bonding, including covalent bonds, where they are shared between atoms, and ionic bonds, where they are transferred from one atom to another, resulting in charged ions. This transfer or sharing of electrons is fundamental to chemical reactions, such as oxidation and reduction processes. Understanding the role of electrons is essential for grasping the principles of chemistry, as they determine how elements interact with one another. Overall, electrons are central to both the theoretical underpinnings of atomic structure and the practical aspects of chemical behavior.
Electrons
FIELDS OF STUDY: Inorganic Chemistry; Geochemistry; Metallurgy
ABSTRACT
The basic structure of atoms and the role of electrons in that structure are defined, and the development of the modern theory of atomic structure is explained. Atomic structure is fundamental to all fields of chemistry, and especially to fields that rely on the intrinsic properties of individual atoms.
The Identification of Electrons
Various models of atomic structure arose through the nineteenth century, the most useful being the planetary model and the plum pudding model. In the first, atoms were envisioned to be composed of a nucleus with electrical charges whirling about it like the planets around the sun. In the latter model, developed by British physicist J. J. Thomson (1856–1940), the atom was envisioned to be a round blob with bits of electrical charge embedded throughout. In conducting research on the nature of cathode rays (beams observed when electricity was run through a glass vacuum tube with a cathode, or terminal, at the end) in 1897, Thomson) determined that the behavior of the cathode rays suited the mathematics of streams of charged particles rather than electromagnetic radiation. Logically, they had come from within the metal that made up the cathode, yet their loss had no significant effect on the mass of the cathode. To these particles he assigned a negative charge. Alongside the emission of cathode rays was the emission of so-called canal rays. When German physicist Wilhelm Wien (1864–1928) examined these in a similar way in 1898, he found that they also suited streams of charged particles rather than electromagnetic radiation. However, the canal ray particles were much more massive and possessed the opposite electrical charge from cathode rays, so he therefore assigned them a positive electrical charge. This verified that atoms had an internal structure, but it did not resolve the issue of which model was correct. Further research by British physicists Ernest Rutherford (1871–1937) and James Chadwick (1891–1974) eventually provided the definitive evidence of fundamental particles for the planetary model that has been refined into the modern theory of atomic structure.
Subatomic Sizes
According to the modern theory, atoms are composed of an extremely small, dense nucleus made up of positively charged protons and neutral neutrons, surrounded by a very large, diffuse cloud of electrons. The magnitude of the negative charge on an electron is exactly equal to the positive charge of a proton, but the masses of the two particles are very different. Measurement of the charge-to-mass ratio of electrons demonstrates that the electron has a mass of about 9.1 x 10−31 kilograms, or about 9.1 x 10−28 grams. The proton, however, is almost two thousand times more massive than the electron, and the nucleus of any atom makes up about 99.98 percent of the atom’s total mass.
Electrons in Atoms
Quantum mechanics, supported by experimental evidence, defines the energies that electrons are allowed to possess in an atom. These energies define spaces about the nucleus called "electron shells," each of which may contain only a certain maximum number of electrons. The outermost or highest energy electron shell in an atom is termed the valence shell, while the electrons within that shell are known as "valence electrons." Atoms are at a minimum energy state when the outermost electron shell contains its full complement of electrons, whether as a neutral atom or as an ion. This drives atoms to give up electrons from an incomplete valence shell to an electronegative atom and to accept electrons from a more electropositive atom in order to have a completely filled outermost electron shell. Within each shell are orbitals, specific regions that can each be occupied by no more than two electrons.
Electrons in Chemical Reactions
The electrons located within the valence shell of an atom determine how that atom interacts with others and thus the chemical behaviors of each element. When two atoms come into contact with each other, they may form a covalent bond by sharing valence electrons. In other cases, one atom may give up valence electrons, which then occupy the valence shell of the other atom, thus causing the previously neutral atoms to become a positive ion and a negative ion, respectively. The transfer of valence electrons is responsible for chemical reactions such as oxidation and reduction.
PRINCIPAL TERMS
- electronegative: describes an atom that tends to accept and retain electrons to form a negatively charged ion.
- fundamental particle: one of the smaller, indivisible particles that make up a larger, composite particle; commonly used to refer to electrons, protons, and neutrons, although these are themselves composed of various actual fundamental particles, such as quarks, leptons, and certain types of bosons.
- ion: an atom, molecule, or neutral radical that has either lost or gained electrons and is therefore electrically charged.
- orbital: a specific region of space about the nucleus of an atom in which electrons of a given energy level are most likely to be found.
- valence shell: the outermost energy level occupied by electrons in an atom.
Bibliography
Gribbin, John. Science: A History, 1543–2001. London: Lane, 2002. Print.
Johnson, Rebecca L. Atomic Structure. Minneapolis: Lerner, 2008. Print.
Miessler, Gary L., Paul J. Fischer, and Donald A. Tarr. Inorganic Chemistry. 5th ed. Boston: Pearson, 2014. Print.
Morrison, Robert Thornton, and Robert Neilson Boyd. Organic Chemistry. 6th ed. Englewood Cliffs: Prentice, 1992. Print.
Wehr, M. Russell, James A. Richards Jr., and Thomas W. Adair III. Physics of the Atom. 4th ed. Reading: Addison-Wesley, 1984. Print.
Winter, Mark J. The Orbitron: A Gallery of Atomic Orbitals and Molecular Orbitals on the WWW. U of Sheffield, 2002. Web. 31 Mar. 2014.