A chemical property describes how a substance reacts with others or changes its structure during a chemical transformation. Unlike physical properties like color or density, a chemical property is only revealed when the substance’s identity is fundamentally altered, such as its ability to burn or its tendency to corrode. To find an element that behaves similarly to oxygen, one must look for a chemical relative that shares these fundamental reactive tendencies. The organization of the periodic table offers a predictable method for locating elements that possess these comparable characteristics, allowing chemists to predict an element’s bonding behavior based on its position.
The Role of the Periodic Table
The periodic table is a powerful organizational tool based on shared chemical behavior. Elements are arranged into vertical columns, known as groups, because elements within the same group display similar chemical properties. This pattern exists because elements in the same vertical column possess the identical number of electrons in their outermost shell, known as valence electrons.
These valence electrons participate in forming chemical bonds, which dictates an element’s reactivity and how many bonds it tends to form. Oxygen is found in Group 16 and requires two additional electrons to achieve a stable, full outer shell. This requirement means all other elements directly beneath oxygen in that column will also exhibit a strong tendency to gain or share two electrons, leading to parallel chemical actions.
Meet the Chalcogens
The column that contains oxygen is designated as Group 16, and its members are collectively known as the Chalcogens. This group includes oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioactive element polonium (Po). Since all Chalcogens have six valence electrons, they share the drive to gain or share two electrons, which typically results in an oxidation state of -2 when reacting with metals.
Sulfur is the element most often cited as the closest chemical relative to oxygen, largely due to its abundance and similar bonding patterns. Like oxygen, sulfur frequently forms two covalent bonds when combining with other non-metals, a direct consequence of their shared valence electron configuration.
Comparing Oxygen and Sulfur
The chemical similarities between oxygen and sulfur are rooted in their shared electron count, allowing both to readily form compounds with a common oxidation state of -2. Both elements combine with hydrogen to create compounds that share a similar structure, such as water (\(\text{H}_2\text{O}\)) and hydrogen sulfide (\(\text{H}_2\text{S}\)). Both elements also exist in various forms, or allotropes, such as diatomic oxygen (\(\text{O}_2\)) and sulfur’s common \(\text{S}_8\) ring structure, demonstrating their capacity for self-bonding.
Despite these chemical parallels, the elements exhibit distinct physical differences stemming from their size. Oxygen is a colorless, odorless gas at room temperature, while sulfur is a brittle, yellow solid. Sulfur atoms are substantially larger than oxygen atoms, which affects their preferred bonding structures in nature.
Oxygen readily forms strong double bonds, such as in the \(\text{O}_2\) molecule, because its small size allows atoms to get close enough for orbital overlap. Sulfur, being a larger atom, prefers to form longer chains and rings using single bonds, like the puckered \(\text{S}_8\) molecules found in its solid form. This preference is due to the weaker nature of sulfur-sulfur double bonds compared to oxygen-oxygen double bonds. The difference in size also means oxygen is significantly more electronegative, giving it a stronger pull on electrons in a chemical bond than sulfur.
Biological Functions of Group 16 Elements
In biological systems, oxygen and sulfur play different yet fundamental roles. Oxygen is essential for aerobic respiration, serving as the final electron acceptor in the process that generates energy in most living organisms. It is also the primary component of water, which provides the solvent and structural framework for all known life.
Sulfur is structurally integrated into living matter, most notably as a component of the amino acids cysteine and methionine, the building blocks of proteins. Cysteine residues are important because their sulfur atoms form disulfide bonds, which are crucial for maintaining the three-dimensional shape and stability of many proteins, especially those found outside of cells. Sulfur is also incorporated into many vitamins and cofactors that facilitate metabolic reactions.
Selenium, the next Chalcogen down the group, is biologically necessary but is required in much smaller amounts than oxygen and sulfur. Selenium is incorporated into the unique amino acid selenocysteine, which is found in a small number of enzymes known as selenoproteins that regulate cellular antioxidant defense systems. An excess of selenium, however, is toxic, contrasting sharply with the biological tolerance for higher concentrations of sulfur compounds.