What Are Covalent Compounds?

The combination of two atoms, whether covalent or ionic, depends on the difference between the electronegativity of the two atoms. The combination of metal and nonmetal with large electronegativity difference is mainly ionic bond; the combination of two nonmetals with small electronegativity difference is mainly covalent bond. A covalent bond is a chemical bond formed by the common electron pair between atoms of two elements with the same electronegativity or little difference. Compounds formed by covalent bonds are called covalent compounds. In order to clarify this type of chemical bond problem, as early as 1916, the American chemist Lewis proposed the covalent bond theory of shared electron pairs between atoms. According to this theory, each atom in the molecule strives to achieve the electronic structure of the corresponding rare gas atom by sharing one or several pairs of electrons.

In 1916, the American chemist GNLewis proposed the earliest covalent bond theory, which believed that each atom in the molecule should have a stable electronic layer structure of rare gas atoms. This structure is obtained by sharing one or several pairs of electrons between atoms. Realization, that is, the "octet rule". This type of chemical bond formed between atoms in a molecule by a common electron pair is called a covalent bond. Lewis's covalent bond theory successfully explained the formation of some simple covalent molecules and laid the foundation for the development of valence bond theory. It is worth noting that the Lewis theory is not yet complete, and it cannot explain the reason and substance of electron pairing and the geometric configuration of molecules. It is impossible to explain compounds that do not conform to the "octahedral rule", such as boron trifluoride (6 electrons), phosphorus pentachloride (10 electrons), sulfur hexafluoride (12 electrons). In 1927, German chemists WHHeitler and F. London applied the theory of quantum mechanics to molecular structures, and explained the nature of covalent bonds. Later, LC Pauling and others developed this achievement and established modern valence bond theory, hybrid orbital theory, valence layer electron pair mutual exclusion theory, and molecular orbital theory. ^[1]

The valence bond theory is a generalization of the results of quantum mechanics approximate processing of hydrogen molecules, also called the electron pairing method, or VB method for short. This method is different from Lewis's covalent bond theory, which is based on quantum mechanics. The basic points of the price key theory are as follows:

(1) When the single electrons with opposite spins in the atom are close to each other, the single electrons can be paired to form a stable chemical bond (single, double or triple bond).

(2) If there is no single electron or single electron in the atom, but

Compounds that form molecules with shared electron pairs are called covalent compounds. Features:

(1) A molecule composed of atoms of the same element

With the development of chemistry and physics technology, people have been able to measure the geometric configuration of many molecules, but many of the measurement results cannot be explained by valence bond theory. In order to solve this kind of contradiction, L. Pauling proposed the hybrid orbit theory on the basis of the electron pairing theory in 1931, and further developed the valence bond theory.

(1) In the process of bonding, due to the interaction between atoms, several different types of atomic orbits (ie wave functions) in the same atom with similar energy can be linearly combined to redistribute energy and determine the spatial direction, and the number of components Equal new atomic orbits. The process of this recombination of orbits is called hybridization, and the new orbits formed after hybridization are called hybrid orbitals.

(2) The number of hybrid orbits is equal to the number of atomic orbitals participating in hybridization.

(3) When the hybrid orbitals form a bond, the principle of maximum overlap of atomic orbitals must be satisfied. The angle of the hybrid orbit for this

Both valence bond theory and hybrid orbital theory can explain the directionality of covalent bonds, especially the hybrid orbital theory successfully explains the relationship between the hybridization and spatial configuration of some covalent molecules, but some molecules use these two theories to predict Sometimes it is difficult to determine. In 1940, NVSidgwick and others in the United States successively proposed the theory of valence layer electron pair mutual exclusion, referred to as the VSEPR method, which is applicable to AB _n -type molecules or ions formed between the main group elements.

(1) In covalent molecules, the geometric configuration of the atoms or atomic groups arranged around the central atom is mainly determined by the mutual repulsion of the electron pairs in the valence layer of the central atom. The geometric configuration of the molecule always takes the smallest mutual repulsion of the electron pair That structure. Valence layer electron pairs include bonded electron pairs and lone pair electrons.

(2) For a covalent molecule, the geometric configuration of the molecule is mainly determined by the number and type of valence layer electron pairs of the central atom.

(3) The magnitude of mutual exclusion of valence layer electron pairs depends on the angle between the electron pairs and the bonding of the electron pairs. The general rule is: the smaller the angle between the electron pairs, the greater the repulsive force; the order of the electrostatic repulsion between the valence layer electron pairs is: Pair, bond electron pair-bond electron pair; The order of the repulsive force from large to small is: triple bond, double bond, single bond.

(4) If a double or triple bond exists in a covalent molecule, the theory of valence layer electron pair mutual exclusion is still applicable, and the double or triple bond can be treated as an electron. ^[1]