Group (periodic table)

In the study of chemistry, a group is a vertical column of elements in the periodic table. These columns are also frequently referred to as families. The concept of a group is essential because it organizes the elements based on shared traits. Elements within the same group exhibit similar physical and chemical characteristics. This similarity is not random. It is a direct result of how the atoms of these elements are constructed.

The reason elements act like a family lies in their atomic structure. Specifically, they share similar characteristics in their outermost electron shells. These shells are the outer layers of an atom. Most chemical properties are dominated by the orbital location of these outermost electrons. Because the elements in a group have the same core charge, they tend to behave in predictable ways. This predictable behavior allows scientists to understand how an element might react with others.

Modern chemistry uses a standardized numbering system to identify these columns. There are 18 numbered groups in the periodic table, ranging from group 1 to group 18. Between group 2 and group 3, there are 14 additional columns known as the f-block. These f-block columns are not assigned numbers in the standard 1–18 system. The modern numbering is based on each atom's s, p, and d electrons. These electrons exist beyond those found in the atoms of the preceding noble gas.

Before the current system, scientists used different and often incompatible naming schemes. One system was used by the Chemical Abstract Service, or CAS, which was popular in the United States. Another system was used by the International Union of Pure and Applied Chemistry, or IUPAC, which was more common in Europe. These older systems used Roman numerals and the letters "A" and "B." For example, the letters designated the left or right side of the table in the old IUPAC system. In the CAS system, "A" meant main group elements and "B" meant transition elements. This caused confusion because the same names often meant different things in different regions.

To fix this confusion, the IUPAC recommended the modern 1–18 numbering system. This proposal was first circulated for public comment in 1985. It was later included in the 1990 edition of the Nomenclature of Inorganic Chemistry. The current system is generally accepted by the global chemistry community. However, some small debates remain regarding the exact membership of elements 1 and 2, which are hydrogen and helium. There is also some variation in textbooks regarding the inner transition metals. Even so, the correct positioning of these metals has been known since 1948. IUPAC has endorsed this positioning twice, once in 1988 and again in 2021.

Groups are often identified by specific names or by their topmost element. For instance, group 16 is known as the oxygen group or the chalcogens. Group 17 is known as the halogens, and group 18 is known as the noble gases. Some groups have "trivial names," which are common nicknames used by scientists. Group 11 contains elements like gold and is often called the coinage metals. There is some debate about whether the element roentgenium belongs in this group. While it is in group 11, it is extremely radioactive and short-lived. This makes it difficult to use for actual coinage.

Beyond the vertical columns, scientists sometimes use the word "group" to describe other sets. These sets are not defined by being a column in the periodic table. Examples include the noble metals, precious metals, and refractory metals. There is also a specific term called the "iron group." In a general chemistry context, this usually refers to group 8. However, it can also refer to a specific set containing iron, cobalt, and nickel. In the fields of astrophysics and nuclear physics, the term has a different meaning. In those sciences, the iron group refers to iron, cobalt, nickel, chromium, and manganese.