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Chemical bonding is at the heart of understanding how elements combine to form compounds. It involves the joining of atoms through the sharing or exchange of electrons, which can result in molecules or extended structures. The two major types of bonds are metallic and ionic bonds.

Metallic bonding occurs between atoms of metallic elements, where the electrons are not bound to any single atom but are free to move throughout the entire metal structure. This creates a strong bond between the positively charged atomic nuclei and the shared 'sea' of delocalized electrons surrounding them. The result is a durable but malleable material with unique properties.

Ionic bonding, on the other hand, occurs when electrons are transferred from one atom (usually a metal) to another (usually a non-metal), creating ions that are positively and negatively charged. These oppositely charged ions attract each other, creating the ionic bond. Ionic compounds are often crystalline in nature, with high melting points and rigid structures.

Delocalized electrons are not associated with a single atom or a single covalent bond; instead, they are shared among many atoms, contributing to the bonding of the structure as a whole. In metallic bonds, delocalized electrons allow for the distinctive 'sea of electrons' that results in metals being good conductors of heat and electricity. These electrons flow freely through the metal, carrying the electric current or distributing heat.

In the context of ionic bonds, the term 'delocalized' relates more to the electrons that were transferred during the formation of ions, rather than to a shared cloud of electrons. However, when ionic compounds are in a molten state or dissolved, these formerly localized electrons contribute to conductivity.

Electrical conductivity is the measure of a material's ability to conduct an electric current. In metallic bonds, the delocalized electrons can move freely, which means metals can carry electrical current with ease. This is why wires and cables are typically made of metals like copper or aluminum.

Ionic compounds usually do not conduct electricity in their solid state because the ions are locked into place within the crystal lattice. However, when an ionic compound is melted into a liquid or dissolved in water, the ions are free to move, allowing the material to conduct electricity. This is a crucial concept for understanding when and why certain substances are conductive.

Crystalline solids exhibit a highly ordered structure, with atoms, ions, or molecules arranged in a consistent and repeating pattern. This organization gives them sharp melting points and distinct crystal shapes. Ionic compounds form crystalline solids where each positive ion is surrounded by negative ions and vice versa, leading to a large-scale, rigid, three-dimensional lattice.

The crystalline nature of ionic solids contributes to their brittleness; when stressed, the layers of ions may shift in such a way that like-charged ions align, repelling each other and causing cracks. Metallic crystals, whilst also ordered, are more flexible due to the mobility of the delocalized electrons, allowing metals to bend rather than shatter under stress.