Atoms consist mainly of three basic subatomic particles: protons, neutrons, and electrons.
The electrons are envisaged as forming a cloud of varying density around a denser atomic
nucleus containing almost all the mass of the atom. The outer electrons (high energy electrons) are the valence electrons, and it is mainly their behavior that determines the chemical reactivity of each atom.
Electrons obey the laws of quantum mechanics, and as a result the energies of electrons
are quantized. That is, an electron can have only certain allowed values of energies.
If an electron changes its energy, it must change to a new allowed energy level. During
an energy change, an electron emits or absorbs a photon of energy according to Planck’s
equation δE = hv where v is the frequency of the radiation. Each electron is associated with four quantum numbers: the principal quantum number n, the subsidiary quantum number l, the magnetic quantum number ml and the spin quantum number ms. According to Pauli’s exclusion principle, no two electrons in the same atom can have all four quantum numbers the same. Electrons also obey Heisenberg’s uncertainty principle, which states that it is impossible to determine the momentum and position of an electron simultaneously. Thus, the location of electrons in atoms must be considered in terms of electron density distributions.
There are two main types of atomic bonds: (1) strong primary bonds and (2) weak secondary bonds. Primary bonds can be subdivided into (1) ionic, (2) covalent, and (3) metallic bonds, and secondary bonds can be subdivided into (1) fluctuating dipoles and (2) permanent dipoles.Ionic bonds are formed by the transfer of one or more electrons from an electropositive
atom to an electronegative one. The ions are bonded together in a solid crystal by
electrostatic (coulombic) forces and are nondirectional. The size of the ions (geometric
factor) and electrical neutrality are the two main factors that determine the ion packing
arrangement. Covalent bonds are formed by the sharing of electrons in pairs by half-filled orbitals. The more the bonding orbitals overlap, the stronger the bond. Covalent bonds are directional. Metallic bonds are formed by metal atoms by a mutual sharing of valence electrons in the form of delocalized electron charge clouds. In general the fewer the valence electrons, the more delocalized they are and the more metallic the bonding. Metallic bonding only occurs among an aggregate of atoms and is nondirectional.
Secondary bonds are formed by the electrostatic attraction of electric dipoles within
atoms or molecules. Fluctuating dipoles bond atoms together due to an asymmetrical
distribution of electron charge within atoms. These bonding forces are important for the
liquefaction and solidification of noble gases. Permanent dipole bonds are important in the bonding of polar covalently bonded molecules such as water and hydrocarbons.
Mixed bonding commonly occurs between atoms and in molecules. For example,
metals such as titanium and iron have mixed metallic-covalent bonds; covalently bonded
compounds such as GaAs and ZnSe have a certain amount of ionic character; some intermetallic compounds such as NaZn13 have some ionic bonding mixed with metallic bonding. In general, bonding occurs between atoms or molecules because their energies are lowered by the bonding process.
