Identify the following substances as ionic, metallic, covalent network, or molecular solids: Substance A is malleable, ductile, conducts electricity well, and has a melting point of \(1135^{\circ} \mathrm{C} .\) Substance \(\mathrm{B}\) is brittle, does not conduct electricity as a solid but does when molten, and has a melting point C is very hard, does not conduct electricity, and has a melting point of \(3440^{\circ} \mathrm{C}\). Substance \(\mathrm{D}\) is soft, does not conduct electricity, and has a melting point of \(185^{\circ} \mathrm{C}\).

Imagine holding a piece of aluminum foil. It's shiny, bends easily, and if you connect it to a battery, it would carry an electric current. These characteristics define metallic solids. These solids are formed by metal atoms which are closely packed in a regular pattern. The unique thing about these solids is the 'sea of electrons' that allows the atoms to slide over each other, giving them the ability to be shaped and stretched - a property known as malleability.

Additionally, the free electrons make metallic solids excellent conductors of heat and electricity. Their melting points can range from low to very high, depending on the strength of the metallic bond which is influenced by factors such as the number of delocalized electrons. Substance A from our exercise is a perfect example of a metallic solid with its high melting point and excellent conductivity.

Now picture a block of table salt. When you try to bend it, it won't comply; instead, it breaks or crumbles. This is typical of ionic solids. They are composed of positively and negatively charged ions held together by strong electrostatic forces known as ionic bonds. These solids are often hard and have high melting points, but they are brittle and can shatter under pressure.

Electrical Conductivity

In their solid state, these ions are locked in place and cannot conduct electricity. However, when melted, the ions can move freely, allowing ionic solids to conduct electricity in liquid form, as mentioned in the case of Substance B. This indicates the material's ionic character and showcases the mobility of ions as a crucial factor in the conductivity of ionic solids.

Consider diamonds; they are incredibly hard and have a sparkling appeal. Diamonds are an example of covalent network solids, which are also known as atomic lattices. They consist of atoms held together in a large, rigid three-dimensional structure by covalent bonds. These bonds are very strong, leading to some distinctive characteristics.

Covalent network solids have exceptionally high melting points and are generally poor conductors of electricity. This is because there are no free charge carriers within their structure. Substance C, with its high melting point and hardness, fits the profile of a covalent network solid.

When you think of ice cubes or dry ice, you're thinking about molecular solids. Unlike metallic, ionic, or covalent network solids, molecular solids are held together by relatively weaker intermolecular forces like van der Waals forces or hydrogen bonds. They are composed of molecules or atoms held together in a lattice.

Soft and Low Melting Point

Because these intermolecular forces are much weaker than other types of chemical bonds, molecular solids are often soft and have low melting points. They also do not conduct electricity because they lack free-moving electrons or ions. Substance D in our exercise, characterized by its softness and low melting point, is a classic representation of a molecular solid.