Oxygen is fundamental to life on Earth, playing a direct role in respiration and making up a significant portion of water. Its high reactivity and ability to form strong bonds shape much of the chemistry we observe. The arrangement of the elements reveals a whole family that shares oxygen’s core reactive tendencies.
The Family Tree: Understanding the Chalcogens
The periodic table is organized so that elements stacked vertically in the same column share fundamental properties, and oxygen is the first member of Group 16. This vertical column is formally known as the Chalcogen group, a name derived from the Greek words meaning “ore-forming” because many of these elements are found in metal ores like oxides and sulfides. The family includes oxygen (O), sulfur (S), selenium (Se), tellurium (Te), polonium (Po), and the synthetic element livermorium (Lv).
The elements in this group share common chemical behaviors because they possess the same number of electrons in their outermost shell. This shared configuration dictates how they interact with other elements to form compounds. Although oxygen’s small size makes it chemically distinct in some ways, the underlying electron structure links it directly to its group members.
Sulfur: The Most Common Chemical Twin
Sulfur is the element most commonly encountered that exhibits chemical behavior closely mirroring that of oxygen. Like oxygen, sulfur is a nonmetal, and both elements can form compounds by typically acquiring two electrons, which results in a shared oxidation state of -2 when bonding with metals. This similarity means that for many chemical formulas, sulfur can often replace oxygen, creating analogous compounds.
For example, water (\(H_2O\)) and hydrogen sulfide (\(H_2S\)) show this chemical kinship, both containing two hydrogen atoms bonded to the Group 16 element. However, the physical properties diverge sharply, as water is a liquid at room temperature while hydrogen sulfide is a foul-smelling gas. This difference is largely due to oxygen’s ability to form strong hydrogen bonds, a trait sulfur cannot replicate because it is a larger atom.
Sulfur demonstrates greater versatility in its chemical structures, forming more than 20 different allotropes, compared to oxygen’s forms of \(O_2\) (dioxygen) and \(O_3\) (ozone). Sulfur atoms commonly link together to form stable \(S_8\) ring structures, which oxygen does not readily adopt. This element is abundant in the Earth’s crust and has widespread industrial uses, such as in the production of sulfuric acid for fertilizers and manufacturing processes.
What Makes Them Similar: The Valence Electron Shell
The chemical similarity between oxygen and its group members is rooted in their shared electron configuration. All elements in Group 16 possess six valence electrons, which dictates their reactivity.
Atoms strive to achieve a stable configuration, often by filling their outermost shell to a total of eight electrons, a principle known as the octet rule. Since oxygen, sulfur, and the others are short by exactly two electrons, they all tend to react by either gaining two electrons to form a -2 ion or by sharing two electrons through covalent bonds. This shared “desire” for two electrons causes them to bond with other elements in predictable and similar ratios.
When a Group 16 element bonds with two hydrogen atoms, it completes its outer shell and satisfies the octet rule, leading to the general formula \(H_2X\). While the strength of the resulting bonds changes down the column, the fundamental pattern of two bonds remains consistent.
The Remaining Members of Oxygen’s Group
Moving down Group 16, the elements become progressively larger and heavier, which gradually shifts their overall chemical character. Selenium (Se) and Tellurium (Te) are classified as metalloids, meaning they possess properties that fall between those of nonmetals like oxygen and true metals. This transition in character makes them less like the gaseous, highly reactive oxygen and more like solid, somewhat conductive materials.
Selenium is known for its photoconductivity, where its electrical resistance decreases when exposed to light, making it useful in electronics like photocells and solar cells. Tellurium, which is rarer, is often used in alloys to improve the machinability of materials like copper and stainless steel. Both selenium and tellurium can form compounds analogous to oxides and sulfides, such as selenides and tellurides.
The final naturally occurring member, Polonium (Po), is a highly radioactive metal, distinct from oxygen’s nonmetallic nature. Livermorium (Lv) is an artificially created element with a very short half-life, and its properties are difficult to confirm due to its instability. These heavier elements illustrate that while the family shares a bonding pattern, the change in atomic size and mass alters their physical and chemical tendencies.