Formation of Compounds - Part II

2. Covalent Bonds
There are many atoms where the electronic configuration of the last orbits or shells is not as simple as Na or Cl. For example, in an Oxygen (O) atom the electronic configuration is: 2 electrons in the K shell and 6 electrons in the L shell. It would rather borrow two electrons from some other atom to complete its last shell than give up its six electrons. Hydrogen (H) atom has only one electron and needs to borrow only one extra electron to complete its first shell. Two atoms of H and one atom of O fulfill each other’s needs by sharing electrons in their outermost orbits and form H₂O or a water molecule. In an H₂O molecule, the electrons are not totally given up, as in the case of Na⁺-Cl^-, but are shared by each of the neighboring atoms. 

Atomic bond formed by sharing of electrons is called a covalent bond. Compounds that are formed due to covalent bonding of atoms are called covalent compounds. Covalent bonds are generally formed between non-metals. Hydrogen gas (H₂), Nitrogen gas (N₂), Oxygen gas  (O₂), Hydrochloric acid (HCl) are some of the examples of covalent bonded compounds. Most of the carbon bonds in organic materials like cells, sugar etc. are examples of covalent bonds. If the neighboring atoms share a pair of electrons, then the covalent bond is a single covalent bond. If on the other hand they share two or three pairs of electrons then the covalent bond is double or triple covalent bond respectively. Covalent bonds, where larger than three pairs of electrons are shared, has not been observed.

In terms of anion and cation electron-dot structure seen earlier, we can write the formation of covalent bonds as follows :

Covalent bonds are difficult to break as compared to the ionic bonds. Even the lengths of covalent bonds are shorter than the ionic bonds. The length of the bond implies the distance between the atoms forming the respective bonds. Since the covalent bonds are directional in space, the molecules formed have definite shapes. Due to the absence of free ions, covalent compounds are bad conductors of heat and electricity. The attractive forces between the molecules of a covalent compound are weak, hence they show low melting points. Covalent compounds are mostly insoluble in water.  

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