4. Metallic bond.

A metal is composed of regularly arranged positive ions (nuclei and core electrons) with the valence electrons belonging to the crystal as a whole instead to any single atom. This is sometimes described as an array of positive ions in a "sea of electrons" or in the electron gas. An electrostatic attraction occurs between the lattice of positive ions and delocalized electrons. Metallic bond is nonsaturable and nondirectional, similar to ionic bond.

Electronic band structure model. When atoms of metallic element come together, their outermost atomic orbitals overlap to form bonding and antibonding molecular orbitals  in much the same sort of way that a covalent bond is formed. The difference, however, is that all identical outermost orbitals of all of the atoms overlap to give a vast number of molecular orbitals which extend over the whole piece of metal³.

A large number of molecular orbitals formed from identical atomic orbitals have very close energies; this range of energies is called an energy band. An energy band, occupied by electrons is called a valence band; an unoccupied one is called conduction band. A forbidden band (or a band gap) is a forbidden energy range which separates bands that may contain electrons. In a metal valence band is partially filled and not separated from conduction band. In a semiconductor or an insulator a valence band contains many orbitals, most of which are occupied, whereas conduction band (separated by a band gap) contains many orbitals most of which are empty.

The valence electrons in metals, unlike those in covalently bonded substances, are capable of wandering relatively freely from one atom to another throughout the entire crystal. Many of the characteristic properties of metals, such as metallic luster, high thermal and electrical conductivity are attributable to the delocalization of the valence electrons. The valence electrons are always free to move when an electrical field is applied.

Malleability and ductility of metals are related to the presence of the mobile valence electrons and nondirectional character of the metallic bond. When a metal is shaped or drawn, it does not fracture, because the ions in its crystal are quite easily displaced with respect to one another. The delocalized valence electrons prevent the ions from coming too close, therefore strong repulsive forces that can cause fracture of the crystal are not generated.

Until less then half of valent orbitals of an atom are filled, the more electrons can be involved in bonding, the stronger the attractions tend to be. So, melting points of metals differ significantly: mercury is a liquid at ambient conditions, while tungsten melts at 3410C.

4. Polarity of molecules and molecular structure.

Molecule polarity is an important property because many of the physical properties of substances depend on it. When a molecule consist only two atoms, it must be polar, if the bond is polar (the two atoms joined by the bond differ in electronegativity). However, if there are three or more atoms in a molecule, this molecule may be nonpolar, even when the bonds are polar. The overall polarity of a molecule (overall dipole moment) can be considered as to result from the sum of the individual bond dipoles, which add together like vectors. For example, the C-O bonds in carbon dioxide molecule are polar, because oxygen is more electronegative, than carbon (3.5 and 2.5, respectively). The individual bond dipoles can be represented by arrows, directed from positively charged atom to the negatively charged one (the arrow shows shift of the electron density). The bond dipoles in CO₂ molecule are oriented in opposite directions and they exactly cancel each other:

= =  

Similarly, bond dipoles of flat triangle molecules AX₃, tetrahedral AX₄, trigonal bipyramidal AX₅ and octahedral AX₆, cancel each other. Thus, if the atoms bonded to the central atom A are not the same, the individual bonds will differ in polarities and cancelation of the bond dipoles usually can’t occur.

In most cases, when there are lone pairs in the valence shell of the central atom, the bond dipoles do not completely cancel. For example, in bent molecules (Н₂О, OF₂ or SO₂) two bond dipoles do not cancel each other entirely. As a result, these molecules have a net dipole moment are polar. Another example is ammonia molecule. It has a trygonal pyramidal shape and the three bond dipoles do not cancel each other entirely. Therefore, the NH₃ molecule is polar. Thus, there are two types of nonpolar molecules that have lone pairs on the central atom: one is the linear AX₂E₃ structure and the other is the square planar AX₄E₂ structure.

Example 3.4. Predicting, where or not a molecule is polar.

Problem: is the PCl₅ molecule polar?

Solution: the first step is to determine the shape of the molecule using the VSEPR theory. Phosphorus is the central atom of the molecule; its valent shell configuration in the ground state is 3s²3p³3d⁰. The coordination number of phosphorus in PCl₅ molecule equals 5, the P atom forms 5 single bonds with Cl atoms (s-electrons of phosphorus should become unpaired). There are no lone pairs in the valence shell of the P atom. According to VSEPR theory, the PCl₅ molecule has a trigonal bipyramidal shape.

The P-Cl bonds in phosphorus pentachloride are polar because electronegativities of the atoms are different (2.1 and 2.8 for P and Cl, respectively). Nevertheless all the ligands are the same, there are two kinds of bonds in the PCl₅. Three equatorial bonds are not equivalent to two axial ones, perpendicular to this plane. If we consider these groups of bonds separately (there bonds forming a flat triangle and two bonds oriented in opposite directions), we’ll see, that bond dipoles within each group cancel each other entirely. Therefore, the molecule will be nonpolar.

Answer: the PCl₅ molecule is expected to be nonpolar.

Example 3.5. Predicting, where or not a molecule is polar.

Problem: is the IF₅ molecule polar?

Solution: the first step is to determine the shape of the molecule using the VSEPR theory.

Iodine is the central atom of the molecule; its valent shell configuration is 5s²5p⁵, additionally I atom has empty 5d orbitals. The coordination number of iodine in IF₅ molecule equals 5, all I-F bonds are single ones. There is one lone pair in the valence shell of the iodine atom, so, the molecule can be presented by a formula AX₅E. According to VSEPR theory, the AX₅E molecules have a tetragonal pyramidal shape.

The I-F bonds are polar, because electronegativities of the atoms are different (2.8 and 4.1 for I and F, respectively). Four bond dipoles located in one plane cancel each other, thus, remaining dipole has a lone pair on the opposite side and is not cancelled. So, the bond dipoles in IF₅ molecule don’t cancel each other entirely. Therefore, the molecule will be polar.

Answer: the IF₅ molecule is expected to be polar.