In the formula A2O3, oxygen has a valency of 2. The total negative charge from three oxygen atoms is 3 × 2 = 6. To maintain electrical neutrality, the two atoms of element A must contribute a total positive charge of +6. Dividing this by the two atoms of A gives a valency of +3 for element A.

Ionic compounds consist of a lattice of positive and negative ions held together by strong electrostatic forces. These strong forces require significant energy to overcome, resulting in high melting and boiling points. They are typically hard, brittle, and soluble in polar solvents like water, rather than non-polar solvents like kerosene.

In an oxygen molecule (O2), each oxygen atom has six valence electrons. To achieve a stable octet configuration, the two atoms share two pairs of electrons, resulting in the formation of a double covalent bond between them.

Electrovalent compounds are formed by the electrostatic attraction between oppositely charged ions. These ions are either individual charged atoms (like Na+ or Cl-) or polyatomic groups of atoms that carry a net charge (like SO4 2-). These charged species are the building blocks of the ionic crystal lattice.

A covalent compound is formed by the sharing of electron pairs between atoms, typically non-metals. Carbon tetrachloride (CCl4) consists of carbon and chlorine, both non-metals, which share electrons to form covalent bonds. In contrast, calcium chloride, magnesium fluoride, and sodium chloride are ionic compounds formed by the electrostatic attraction between metal cations and non-metal anions.

In the compound Cl2O7, oxygen typically exhibits a valency of 2. Since there are seven oxygen atoms, the total valency contributed is 14. Dividing this by the two chlorine atoms gives a valency of 7 for each chlorine atom, demonstrating its ability to expand its valence shell.

Oxygen typically has a valency of 2 because it requires two electrons to complete its valence shell. The match 'Oxygen: 1' is incorrect as it does not represent the standard combining capacity of oxygen in most stable compounds.

Ionic bonds form through the complete transfer of electrons from a metal to a non-metal, resulting in electrostatic attraction. NaCl, KCl, MgF2, and CaCl2 are all composed of metal cations (Na+, K+, Mg2+, Ca2+) and non-metal anions (Cl-, F-), characterizing them as ionic compounds. Other options contain covalent molecules formed by non-metal atoms sharing electrons.

When atoms bond, they move from a state of higher potential energy to a state of lower potential energy. This decrease in potential energy is a fundamental requirement for the formation of a stable chemical bond. The system releases energy to achieve this lower energy state, which corresponds to the equilibrium bond length where attractive and repulsive forces are balanced.

Valency is the measure of an element's combining power with other atoms when it forms chemical compounds or molecules. It is determined by the number of electrons an atom must lose, gain, or share to achieve a stable octet.