Metallic Crystals And Bonds
Approximately 80 percent of all the elements are metals. The properties of individual metals vary to a great extent. Because of this they serve many useful purposes. Gold, silver, platinum, and iridium are used in valuable jewelry. Copper and aluminum are used in thousands of miles of wire which bring electricity to every part of the country, and iron is used as the structural backbone of our buildings and bridges. Although some of these metals have been known and used for thousands of years, it has been only in the last 100 years that scientists have developed theories which help explain many of their properties.
Metallic Bonds Most observable properties of substances can be interpreted in terms of three atomic characteristics: outerlevel electron configurations, ionization energies, and atomic radii.
In turn, these properties are related to the nature of the bond between atoms of the aggregate. A satisfactory model of metallic bonding must account for observed metallic properties. Let us first look for features common to all metallic atoms. Then we shall determine the manner in which these characteristics can be used to develop a model of metallic bonding which will explain the observed properties of metals. Examination of the electron configurations and the ionization energies of atoms of substance with metallic properties reveals two common features: (1) a number of vacant orbitals and (2) low ionization energies.
Consider the element sodium, whose atoms meet these requirements. A sodium atom has only one electron in its outer energy level and a low ionization energy, indicating that the 3s electron is rather loosely bound. X-ray diffraction shows that each sodium atom in the solid crystal is surrounded by eight other sodium atoms. Each has one electron and several vacant orbitals in the third energy level.
There are not enough valence electrons for each sodium atom to form a covalent bond with all of its neighbors. Each atom would need eight electrons to form covalent bonds with each of its eight neighbors. Because this is not the case, the single, loosely held electron from each atom remains delocalized in the region between the atoms. The space about each of the atoms is subject to the same positive nuclear charge. The space between the atoms, therefore, represents a region of uniformly low potential energy for the negative electrons. A given electron in the crystal can move easily through this region of low potential energy which extends throughout the crystal. In other words, the outer, loosely bound electrons of atoms in a metallic crystal are not localized but are free to move throughout the crystal.
In a metallic crystal, each atom contributes its outer electrons to the common pool of electrons which extends uniformly throughout the crystal. The contribution of an electron by each atom leaves a positive ion, or positive atomic kernel, occupying a lattice point of the crystal. The electrostatic attraction between the mobile electrons and the atomic kernels gives rise to what is called the metallic bond. The structure of a metallic crystal and the nature of the metallic bond is shown in Fig. 9-26 and is summarized below.
1. Metallic crystals consist of a three-dimensional, closely-packed latticework of atomic kernels surrounded by a sea of delocalized, mobile valence electrons.
2. The electrons can move throughout the crystal rather like gas molecules confined in a container. The electrons are held within the metal by the attraction of the positive atomic kernels.
3. The atomic kernels are held together by the electrostatic at- , traction of the electrons which move between them. It is this force of attraction which results in a metallic bond between the atoms of a metal. |
