Complete the following :
(a) When the nuclei of two different reacting atoms are of ......... mass, then a bond so formed is called ...................... covalent bond (equal, unequal, polar, non-polar).
(b) In case of non-polar covalent bond, the covalent bond is formed in the .......... of atoms and shared electrons are ........... distributed (corner, middle, equally, unequally)
(c) The ions in ................ compounds are held very strongly due to strong ............ forces (electrovalent, covalent, electromagnetic, electrostatic).