Ionic compounds have high melting and boiling points due to the strong electrostatic forces of attraction between the oppositely charged ions in their lattice structure. These forces, known as ionic bonds, are extremely strong and require a significant amount of energy to break. In a solid state, ionic compounds form a rigid, orderly lattice structure where each ion is surrounded by ions of the opposite charge. This maximizes the attractive forces and minimizes repulsion, leading to a very stable structure. To melt or boil an ionic compound, this structure must be overcome, which requires a lot of energy. Consequently, ionic compounds remain solid at room temperature and often have melting points several hundred degrees Celsius above room temperature. The strength of these ionic bonds directly correlates to the high melting and boiling points of these compounds.

Ionic compounds can conduct electricity, but only when they are in a molten state or dissolved in water (aqueous solution), not in their solid form. This is because for a substance to conduct electricity, it must have charged particles that are free to move. In a solid ionic compound, the ions are locked in place within the lattice structure and cannot move freely, so they cannot conduct electricity. However, when an ionic compound melts or dissolves in water, the ions are free to move. In the molten state, the lattice structure breaks down, and in an aqueous solution, the water molecules surround and separate the ions. This mobility of ions allows them to carry electrical current. Therefore, ionic compounds are good conductors of electricity in their molten state or when dissolved in water, but not in their solid state.

Valency plays a crucial role in determining the formula of an ionic compound. It refers to the ability of an atom to combine with other atoms and is determined by the number of electrons an atom can lose, gain, or share to achieve a full outer shell (usually eight electrons, following the octet rule). In ionic compounds, elements form ions by losing or gaining electrons to achieve a full outer shell, which directly relates to their valency. For example, sodium (Na) has a valency of 1 as it loses one electron to form Na⁺, while chlorine (Cl) has a valency of 1 as it gains one electron to form Cl⁻. To form an ionic compound, the total positive charge from the cations must balance the total negative charge from the anions. This balance is achieved by combining ions in a ratio that uses the valency of each ion to ensure overall electrical neutrality. For instance, magnesium (Mg) has a valency of 2 (Mg²⁺) and oxygen (O) has a valency of 2 (O²⁻), so they combine in a 1:1 ratio to form magnesium oxide (MgO).

Ionic compounds typically form crystalline solids due to the regular, repeating arrangement of ions in their lattice structure. This structure arises from the nature of ionic bonding, where oppositely charged ions attract each other and arrange themselves in a way that maximizes attraction and minimizes repulsion. In an ionic lattice, each positive ion is surrounded by negative ions and vice versa, leading to a highly ordered, three-dimensional geometric arrangement. This orderly pattern extends throughout the entire structure of the compound, resulting in a crystal lattice. The strength and symmetry of these ionic bonds contribute to the formation of crystals with characteristic shapes. These crystalline structures often have flat surfaces and sharp edges, which are a direct result of the regularity of the ionic lattice. The crystalline nature of ionic compounds is also responsible for some of their physical properties, such as brittleness, high melting and boiling points, and the ability to cleave along specific planes.

When an element can form more than one type of ion, typically seen in transition metals, its charge in a specific compound can be determined by looking at the charges of the other ions present in the compound. For instance, consider the compound iron(III) oxide. Iron is a transition metal and can form multiple ions, but the "(III)" indicates that in this compound, iron has a +3 charge (Fe³⁺). The oxide ion always has a -2 charge (O²⁻). Since the compound must be electrically neutral, the formula must balance the overall charges. In iron(III) oxide, the formula Fe₂O₃ is used because two Fe³⁺ ions (total of +6 charge) will balance with three O²⁻ ions (total of -6 charge). It's important to use the periodic table, the name of the compound (which often indicates the charge of the metal ion), and the basic principle of charge neutrality to deduce the correct formula.