ATOMIC STRUCTURE

Each atom consists of a very small nucleus composed of protons and neutrons, which is encircled by moving electrons.  Both electrons and protons are electrically charged, the charge magnitude being 1.60 x 10¹⁹ C, which is negative in sign for electrons and positive for protons; neutrons are electrically neutral. Masses for these subatomic particles are infinitesimally small; protons and neutrons have approximately the same mass, 1.67 x 10²⁷  kg, which is significantly larger than that of an electron, 9.11 x 10³¹ kg. Each chemical element is characterized by the number of protons in the nucleus, or the atomic number (Z).

The atomic mass (A) of a specific atom may be expressed as the sum of the masses of protons and neutrons within the nucleus. Although the number of protons is the same for all atoms of a given element, the number of neutrons (N) may be variable. Thus atoms of some elements have two or more different atomic masses, which are called isotopes. The atomic weight of an element corresponds to the weighted average of the atomic masses of the atom’s naturally occurring isotopes.

The atomic mass unit (amu) may be used for computations of atomic weight.

ATOMIC MODELS

One early outgrowth of quantum mechanics was the simplified Bohr atomic model, in which electrons are assumed to revolve around the atomic nucleus in discrete orbitals, and the position of any particular electron is more or less well defined in terms of its orbital.

 

FIGURE 1 Schematic representation of the Bohr atom.

ELECTRON CONFIGURATIONS

The preceding discussion has dealt primarily with electron states—values of energy that are permitted for electrons. To determine the manner in which these states are filled with electrons, we use the Pauli exclusion principle, another quantum mechanical concept. This principle stipulates that each electron state can hold no more than two electrons, which must have opposite spins. Thus, s, p, d, and f sub shells may each accommodate, respectively, a total of 2, 6, 10, and 14 electrons.

In addition, some atoms have what are termed ‘‘stable electron configurations’’; that is, the states within the outermost or valence electron shell are completely filled. Normally this corresponds to the occupation of just the s and p states for

the outermost shell by a total of eight electrons, as in neon, argon, and krypton. These elements (Ne, Ar, Kr, and He) are the inert, or noble, gases, which are virtually unreactive chemically.

THE PERIODIC TABLE

All the elements have been classified according to electron configuration in the periodic table. Here, the elements are situated, with increasing atomic number, in seven horizontal rows called periods. The arrangement is such that all elements that are arrayed in a given column or group have similar valence electron structures, as well as chemical and physical properties. These properties change gradually and systematically, moving horizontally across each period.

The Group VIIA elements (F, Cl, Br, I, and At) are sometimes termed the halogens. The alkali and the alkaline earth metals (Li, Na, K, Be, Mg, Ca, etc.) are labeled as Groups IA and IIA, having, respectively, one and two electrons in excess of stable structures.

FIGURE 2 The periodic table of the elements. The numbers in parentheses are the atomic weights of the most stable or common isotopes.

VAN DER WAALS BONDING

Van der Waals bonding exists between virtually all atoms or molecules, but its presence may be obscured if any of the three primary bonding types is present. Van der Waals bonding is evidenced for the inert gases, which have stable electron structures, and, in addition, between molecules in molecular structures that are covalently bonded. Secondary bonding forces arise from atomic or molecular dipoles. In essence, an electric dipole exists whenever there is some separation of positive and negative portions of an atom or molecule.

FIGURE 3  Schematic illustration of van der Waals bonding between two dipoles.

Hydrogen bonding, a special type of secondary bonding, is found to exist between some molecules that have hydrogen as one of the constituents.