The bond formed by sharing of valence electrons of atoms having opposite charges, which forms a neutral compound, is called ionic bond. This type of bond mainly happens between metal and nonmetal atoms with opposite charges. Salts like sodium chloride, potassium chloride, calcium chloride and others illustrate most commonly cited example for this bond formation. These compounds have combination of metals (sodium, potassium and calcium) and nonmetal like chlorine.
It is observed that most metal have extra electron in their valence orbital. By losing this one electron they can achieve nearest noble gas electronic configuration as per octate rule. For example, in case of sodium (atomic number: 11) atomic distribution as 2, 8, 1. By losing one electron it can achieve Neon gas configuration (2, 8). Similarly, for chlorine (atomic number: 17) atomic distribution is 2, 8, 7. By gaining one electron it can achieve Argon gas configuration (2, 8, 8). Hence, in case of formation of NaCl there occurs an exchange of electrons between two atoms, which leads to formation of ionic bond between two atoms.
Electronegativity of an atom is a measure of attracting pair of electron. Linus Pauling developed a scale of electronegativity to determine the nature of atom and type of bond formation. The Pauling scale ranges from 0.7 to 3.98.In a periodic table if we go right and up we can see atoms with increase in electronegativity (shown in table 1). Moreover, it can be observed that both covalent and ionic bonds are type of polar bond. Hence, both share some common characteristics. Hence, it is taken into consideration that electronegativity difference > 1.7 between two atoms are found to show ionic bond characteristics (1).
Table 1: Electronegativity difference and predicted bond type
Table 2: Electronegativity Pauling scale
The bond is formed is by sharing the power of an atom in a molecule to attract electrons to itself, which forms a noble gas configuration for each atom is called Covalent Bond. This type of bond always is formed with two nonmetals having similar electro negativities. Most commonly known examples of these kinds of bonds are ozone gas, water, hydrogen chloride, carbon dioxide, methane, ammonia etc. These compounds are generally combination of non-metals like hydrogen, chlorine, oxygen and carbon. It is observed that these non-metal leads to stabilization when they share electrons from outer molecular orbits with other in such way to create a gas configuration for each atom. Hydrogen gas forms the simplest covalent bond, as it is a diatomic hydrogen molecule.
Covalent bond is type of polar bond. As shown in table 1 if the electronegativity difference between two atoms is <0.4 then the predicted form of bond is covalent. This happens because both atoms have similar affinity towards electron and non of them have tendency to donate it. Hence, they share their electron to achieve octet configuration and become stable. The ionization energies of the atom are too large and the electronic affinity of the atom is too small for ionic bond to occur. For example carbon has 4 electrons and it needs four electrons to from four bonds in form of single, double or triple bond combinations. This bond includes interactions between sigma and pi orbitals. Compared to ionic bond covalent bond has low polar nature. Covalent bonds have definite shapes whereas incase of ionic bonds shapes are indefinite. Further, the melting and boiling point of covalent bond is lower compared to ionic bond. Products of the covalent bonding at room temperature are solid, liquid or gas whereas the ionic bond products are room temperature exists in form of solids. In some cases a covalent bond is formed when the electron is shared by same atom to form coordinate covalent bond as shown in reaction equation 1a.
Here the ammonia molecules from a coordinate bond with a hydrogen ion by sharing its lone pair of electrons. Similarly, developing coordinate covalent bond between ammonia and borontrifluoride also forms NH3Bf3.