Which of the following are elemental substances and which are compounds: diamond, a solid form of carbon, \(\mathrm{C}(s) ;\) liquid water, \(\mathrm{H}_{2} \mathrm{O}(l) ;\) nitric acid, \(\mathrm{HNO}_{3}(l)\); gaseous nitrogen, \(\mathrm{N}_{2}(g)\); liquid nitrogen, \(\mathrm{N}_{2}(l) ?\)

When we think of chemical elements, we're talking about the fundamental building blocks of all matter. An element is a substance that is made entirely from one type of atom. For instance, diamond is a perfect example of an elemental substance, consisting only of carbon atoms (C).
Interestingly, elements can exist in different states of matter. Take, for example, nitrogen. It can be found in both gaseous (N₂(g)) and liquid (N₂(l)) states. However, even in different states, the chemical identity as nitrogen stays the same because the type of atom remains constant. Elements are organized in the periodic table, and each has unique properties, atomic structures, and stability under various conditions.

Unlike elements, chemical compounds are substances formed when two or more elements are chemically bonded together. Take water (H₂O) as an example; it's a compound made from hydrogen and oxygen atoms bonded in a specific 2:1 ratio. Not only is the ratio fixed, but the way the atoms bond — in this case via covalent bonds — is also specific to the compound.
Similarly, nitric acid (HNO₃) is a compound consisting of hydrogen, nitrogen, and oxygen atoms in a 1:1:3 ratio. Each compound has distinct physical and chemical properties that are different from those of its constituent elements. This is because of the unique way individual atoms combine and interact to give the compound its characteristic behavior.

The atomic structure refers to the arrangement of particles within an atom, which includes protons, neutrons, and electrons. Protons and neutrons make up the nucleus at the center, while electrons orbit around the nucleus in defined regions called electron shells. The number of protons (atomic number) defines the type of element.
Within the context of our exercise, the atomic structure of an element like carbon in diamond is what gives it its remarkable strength — each carbon atom is bonded to four other carbons in a three-dimensional lattice. For compounds, the atomic structure is more complex, as it involves the bonds between different atoms, like in water molecules where the oxygen atom shares electrons with two hydrogen atoms to form a bent molecular structure.

States of matter refer to the distinct forms that different phases of matter take on. Traditionally, there are three: solid, liquid, and gas. In solids, atoms or molecules are closely packed in a regular pattern and vibrate in place, which gives them a fixed shape and volume; diamonds are an example of this. Liquids, like liquid water and liquid nitrogen, have atoms that are close together but not in a fixed arrangement, which allows them to flow and take the shape of their container while maintaining a constant volume. Gases, like gaseous nitrogen, have atoms that are spread out and move freely, resulting in no fixed shape or volume.
Understanding the behavior of substances in these different states is crucial for grasping how materials interact and react under various conditions, and how transitions between states can affect a material's physical properties.