Fisica

Chemical Bonding
Valence Electrons With a Lewis dot symbol of an element we represent its outer shell electrons as dots around the letter symbol for the element. This is a useful way of representing atoms because it is usually only the outer shell electrons which are involved in the formation of chemical bonds. The outermost electrons of an atom are called valence electrons. Note that the numberof outer shell electrons is the same for all the atoms within a given group on the periodic table. Therefore, the Lewis dot symbols for beryllium and magnesium, both being Group II elements, are similar:

Likewise the Lewis dot symbols for oxygen and sulfur, both Group VI, are similar:

Note that for the representative elements, elements with highest energy orbitals being s or p (groups IA -VIIIA), the number of valence electrons is equal to the group number. The Chemical Bond A chemical bond is a strong force of attraction which holds atoms together in molecules. The most common type of chemical bond is the covalent bond, which is formed by the sharing of valence electrons between atoms. Another type is the ionic bond, which is formed by electron transfer. Most chemical bonding isgoverned by the octet rule, which states that an atom tends to gain, lose or share electrons until there are eight electrons in its valence shell. The octet is a very stable electron configuration, sometimes referred to as the noble gas configuration. In elemental form the noble gases, except helium, possess an octet and thus rarely enter into chemical bonds:

In fo rming water, an oxygen atomachieves noble gas configuration by entering into two covalent bonds with two hydrogen atoms (note that hydrogen attains the configuration of helium):

Sodium and chloride each attain noble gas configuration in an ionic compound by way of electron transfer:

© J.S Wetzel, 1993

The Covalent Bond To gain a deeper understanding of the covalent bond, or shared electron bond, let's look at asimple example, the hydrogen molecule:

The two nuclei of the hydrogen molecule are strongly held together, about 0.74 apart, by their covalent bond. This bond consists of the electrostatic attraction for the nuclei by the shared electrons (balancing the mutual repulsion of the protons). The bond energy (the energy required to split H2 into H and H) is 432 kJ mole-1. The bond is created by overlapof the two 1s orbitals. As pictured at right the electrons occupy the space around both nuclei with their motion largely concentrated in the space between the nuclei. The electron waves resonate between the two nuclei. Before bonding, there were two 1s atomic orbitals, and after bonding there will be two molecular orbitals.

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The bond exists when the electron pair is located in the oneof lower energy of these two molecular orbitals, a symmetric, bonding orbital. The other molecular orbital as shown in the molecular orbital diagram of H2 at right, is of higher energy, and is antisymmetric and antibonding. This orbital exists along with the bonding orbital, Antibonding but the space occupied by the antibonding orbital is beyond the internuclear space (not between the nuclei). Ifboth electrons 1s 1s were to occupy such an orbital, the hydrogen nuclei would both repel one another and be attracted to the electrons. Imagine a Bonding covalent bond between two helium atoms, in which H H H2 case the antibonding orbital would be filled in addition to the bonding orbital, and you can see why two helium atoms show only a weak attraction. Note that like atomic orbitals, molecularorbitals obey the Pauli exclusion principle (giving a maximum of two electrons per orbital), the aufbau principle (which states that lowest energy orbitals fill first), and Hund's rule (which governs the filling of orbitals of equal energy). Molecular Orbital Theory and the Oxygen Molecule Like hydrogen, pure oxygen most often exists as a diatomic molecule. However, the elec-

Above we have...