CHEMICAL BONDING

CHEMICAL BONDING Chemical bonding is defined as the union of two or more atoms through redistribution of electron in their outer shells either by the process of transfer of electrons from one atom to another or by the process of sharing of electronics amongst themselves so that all the atoms acquire the stable noble gas configuration of minimum energy. There are many different types of bonding of which electrovalent and covalent bonding are the most important ones. There are other types like co-ionic or co-ordinate covalence, metallic bond, hydrogen bonding and van-der-waal’s forces. Electrovalent (ionic bond) The main principle in ionic bond is called donor acceptor principle in which there is a complete transfer of electron from one specie to another i.e the atoms forming bond are usually present as ions. The donor atom is usually a metal of relatively larger atomic size. An atom of a metallic element loses electrons from its outermost shell, the number of electrons equal to its valency to attain the stable configuration of a noble gas. It then becomes positively charged ion known as cations. The non-metallic atom gain the electrons lost by the metallic atom and become negatively charged to also attain the noble gas configuration. It is called acceptor. The attraction between sodium ion (a metal) and chlorine ion(a non-metal ) is called electrostatic attraction. These electrostatic attractions between positively and negatively charged ions constitute the electrovalent or ionic bonding. Calcium chloride and calcium oxide, sodium oxide, magnesium chloride are other examples of electrovalent compounds. Characteristics of electrovalent compounds • Electrovalent compound have high melting and boiling point • They conduct electricity either in molten state or when in solution. • They are soluble in polar solvent such as water but not in non-polar solvents such as benzene ,toluene etc • Electrovalent bonding occur by complete transfer of electron from one specie to another (i.e from metal to non-metal) Covalent bond The main principle in covalent bond is that of sharing of electrons between atoms such that each of the atoms in the molecule has the electron arrangement of a noble gas. The two main atoms involved in covalent bond formation are of similar size and they are always small. Consider two chlorine atoms, each having the electron structure 2, 8, 7. In covalent bonding each atom contribute equal number of electrons which are equally shared by the participating atoms. Examples are in chlorine molecule, oxygen molecule, methane and other organic compounds The covalent bond can also be regarded as resulting from the overlap of electron clouds. The two clouds containing one electron each overlap to form a single cloud between the atoms The covalent bond is represented by a short line joining the symbols for the atoms. H – H methane. Characteristics of covalent compounds • Covalent bonds are found mainly in gases and volatile liquids. • They are soluble only in non-polar solvent such as benzene, toluene, ethers etc but not in polar solvent such as water. • They are nonconductor of electricity either in molten or when dissolved (non electrolyte) • They have low melting and boiling point. Co-ordinate covalency (co-ionic bond) These combine the principle of both covalent and ionic bonding; it is also referred to as dative covalent bonding or co ionic bonding. In coordinate covalent bonding participating species share electrons but the shared electrons are contributed only by one of the participating species unlike the ordinary covalent bonding where both species donate electron for sharing. The shared electrons are known as lone pair of electrons, thus for coordinate bonding to occur one of the participating atoms must have a lone pair of electrons. For example in the formation of ammonia, three hydrogen atoms share three pairs of electrons with a nitrogen atom by normal covalent bonding. Nitrogen atom has a lone pair of electron in its outermost shell. It shares this with a proton (hydrogen ion), from an acid to produce ammonium ion, NH4+ molecule carries over it positive charge to give the ammonium ion ,other example will include the formation of hydroxium ion Metallic bond Metallic bond is the bond that holds atoms together in a metal. Each metallic atom contributes its valence electrons to the electron cloud, thus becoming positively charged. The ions are held together in the lattice by the electron cloud. The larger the number of electrons in the outer shells of the metal, the stronger the bonding. This type of bonding is very strong in some metals eg. Iron which are difficult to cut but it is much weaker in some metals like sodium or potassium which can be cut with a knife. Hydrogen bonding Hydrogen bonding is a weak intermolecular bonding which occurs when hydrogen is covalently linked to highly electronegative element such as nitrogen, oxygen and fluorine which are in need of few electrons to attain octet configuration. They tend to pull the shared electron in covalent bond toward themselves resulting in the formation of a dipole where the hydrogen is partially positive and the oxygen , nitrogen or fluorine is partially negative, this create an electrostatic attraction between the two dipoles known as hydrogen bonding. Hydrogen bond is a weak bond but it has impact on the physical properties of compounds like hydrogen fluoride and water. The strongest hydrogen bond is in hydrogen fluoride HF. Other hydrogen bonds can easily break under the influence of heat into simple molecules but in hydrogen fluoride, the hydrogen bonds persist even in vapor state. Gaseous hydrogen fluoride consists mainly of a mixture of H2F2 and H3F3 molecules. Vander Waal’s Force This a weak intermolecular force existing between discrete molecules. Van der waal force was first described by J.D van der waal and it was named after him. This bond hold molecules together in liquid and gases. Contributed by: Habib Adamu Isa (B.sc Chemistry, M.sc Environmental chemistry, MCSN) Adeyemo College, Kaduna.