The ionic bond is a result of an electron transfer from one atom to another. Consider the example of sodium (Na) bonding with chlorine (Cl) to produce sodium chloride (NaCl), also known as table salt. Na has one valence electron (an electron in the outermost orbital shell that can take part in bonding) while Cl has seven valence electrons. As a result, the transfer of one electron from Na to Cl is favored because both atoms will achieve a more stable electron configuration (full outer orbital shells of eight electrons). Due to the transfer of its electron, Na is considered a cation, with a net positive charge. Meanwhile, Cl now has a net negative charge and is considered an anion. The electrostatic or Coulombic attraction between oppositely charged ions is what is called the ionic bond. Ionic bonds form between atoms that vastly differ in their electronegativity values. A difference of 1.7 in electronegativity values generally suggests that if a bond forms, it will be ionic. Ionic bonds most frequently form between metals and non-metals.
The covalent bond is formed when adjacent atoms share valence electrons. Generally, sharing electrons in such a fashion allows each atom involved to achieve a more stable electron configuration. Consider two chlorine atoms each with 7 electrons in each respective outer shell. The atoms will share one of their outer electrons with the other, such that each individual atom effectively has a complete outer orbital. Covalent bonds form between atoms that have similar electronegativity values. This type of bond is common to organic compounds, where the atoms composing the compounds are non-metals.
Polar Covalent Bond
A polar covalent bond is bond that has a mix of ionic character and covalent character. It is important to understand that all ionic compounds (compounds formed by ionic bonds) have some measure of electron sharing (covalent bonding) even though an ionic bond is not considered to be a type of covalent bond. A large difference in the electronegativity of two atoms indicates a greater ionic character and is considered a purely ionic bond; whereas a very small, negligible difference is considered a purely covalent bond. A polar covalent bond exists when the electronegativity difference is somewhere in between, generally more towards the covalent side (small electronegativity difference). When a polar covalent bond is formed, the result is an unequal sharing of electrons between atoms. An example of a molecule with a polar covalent bond is hydrogen fluoride. In this molecule, hydrogen has a partial positive charge while fluorine has a partial negative charge.
Similar to the covalent bond, the metallic bond involves electron sharing. However, in a metallic bond, valence electrons are delocalized meaning that the electrons are mobile and can therefore be associated with any of the plentiful adjacent atoms. In this sense, the electrons form an electron cloud around the atoms, which is the basis for classic metallic properties such as: high electrical conductivity, ductility and luster.