Covalent compounds are formed through the sharing of electron pairs. Due to the lack of free ions or delocalized electrons, they are poor conductors of electricity. Furthermore, the weak intermolecular forces between molecules result in relatively low melting and boiling points compared to ionic compounds.

During hydration, water molecules act as ligands, donating a lone pair of electrons from the oxygen atom into the empty orbitals of the metal cation. This electron-pair donation forms a coordinate covalent bond, also known as a dative bond.

Ionic compounds are generally insoluble in non-polar or low-polarity solvents like alcohol. However, they do not conduct electricity in the solid state because the ions are locked in a rigid lattice. They only conduct when molten or dissolved in water, where ions are free to move.

Calcium has a valency of +2, and the phosphate ion (PO4) has a charge of -3. To achieve electrical neutrality, three calcium ions (3 x +2 = +6) must balance two phosphate ions (2 x -3 = -6), resulting in the formula Ca3(PO4)2.

Ionic bond formation is a multi-step process. Low ionization energy of the metal is required to form the cation, high electron affinity of the non-metal is needed to form the anion, and high lattice energy is necessary to stabilize the resulting crystal structure. Therefore, all these energetic factors are critical for the formation and stability of an ionic bond.

Valency is defined by the number of chemical bonds an atom forms. In ammonia (NH3), the nitrogen atom is bonded to three hydrogen atoms, each via a single covalent bond, thus its valency is 3.

In the formula Na2X, sodium (Na) has a +1 charge. Since there are two sodium atoms, the anion X must have a -2 charge to balance the total charge. Aluminium (Al) typically forms a +3 ion (Al3+). To form a neutral compound with an X2- anion, the charges must be balanced: 2 atoms of Al (+6 total) and 3 atoms of X (-6 total), resulting in Al2X3.

In standard covalent bonding, the maximum number of bonds between two atoms is three, forming a triple bond (one sigma bond and two pi bonds). While quadruple bonds exist in certain transition metal complexes due to d-orbital overlap, in the context of general chemistry and main group elements, the triple bond is the recognized maximum for stable molecular structures.

In ammonia (NH3), the nitrogen atom forms three covalent bonds with three hydrogen atoms to achieve a stable octet configuration. Valency is defined as the combining capacity of an element, which in this case corresponds to the three electrons shared by nitrogen.

Covalent bonding occurs when atoms share valence electrons to attain a stable electronic configuration, typically an octet, in their outermost shell. This sharing allows atoms to achieve the stability of a noble gas configuration, which is the primary driving force for the formation of covalent molecules.