The periodic table arranges all known chemical elements into a coherent structure based on their atomic number. This organization creates distinct vertical columns, known as groups, and horizontal rows. Understanding the logic behind these groups is fundamental, as an element’s position allows scientists to predict its properties.
The 18 Vertical Groups
The periodic table contains eighteen groups, defined as the vertical columns of elements. These columns are numbered sequentially from 1 (far left) to 18 (far right). Elements within a group are arranged in order of increasing atomic number as one moves down the column. The f-block elements, typically shown as two separate rows at the bottom of the table, are not assigned their own group numbers.
Naming Conventions for Groups
The International Union of Pure and Applied Chemistry (IUPAC) standardized the numbering system in 1988. IUPAC recommends using simple Arabic numerals, numbering the columns from 1 to 18 across the entire table. This standardized approach eliminated significant confusion caused by older, less consistent systems.
Previously, two older systems used Roman numerals combined with the letters ‘A’ and ‘B,’ such as IA and IIA. These systems often resulted in conflicting labels for the same group. For example, Group 18, which contains the Noble Gases, was previously referred to as Group 0 or Group VIIIA under different older conventions.
Chemical Properties Within a Group
Elements are placed into the same vertical group because they share the same number of valence electrons. These electrons are located in the outermost shell of an atom and are primarily involved in chemical bonding and reactions. Because they share this characteristic, elements within the same group exhibit similar chemical and physical properties.
For example, Group 1 elements (Alkali Metals) all have one valence electron. This electron is easily lost, leading to high reactivity and a tendency to form positive ions (+1 charge). In contrast, Group 17 (Halogens) possesses seven valence electrons. Their strong tendency to gain one electron explains why Halogens are highly reactive nonmetals that readily form negative ions. The number of valence electrons generally corresponds to the group number for main group elements (Groups 1, 2, and 13–18).
Groups Versus Periods
While groups are the vertical columns that organize elements by similar chemical behavior, the periodic table also contains horizontal rows called periods. There are seven periods on the table, and they serve a different organizational function than the groups.
Moving across a period from left to right, the atomic number increases sequentially, and elements are added to the same principal electron shell. Unlike groups, elements within the same period do not share similar properties but instead show a gradual, predictable change in properties across the row.
For example, a period begins with a highly reactive metal on the left and progresses toward less reactive nonmetals and finally a non-reactive Noble Gas on the far right. This transition across a period demonstrates trends like decreasing atomic radius and increasing electronegativity, while moving down a group shows consistency in chemical reactivity.