The periodic table arranges all known elements based on their atomic structure and properties. It is structured as a grid of horizontal rows (periods) and vertical columns (groups). Periods indicate the number of electron shells an element possesses. The columns, which are the focus of this discussion, define an element’s chemical family and shared chemical behavior.
Defining the Groups (Nomenclature and Numbering)
The columns of the periodic table are formally known as Groups (or families). They organize elements that share similar chemical characteristics. The modern International Union of Pure and Applied Chemistry (IUPAC) numbering convention is the most widely accepted system for identification. This system uses simple Arabic numerals, numbering the columns sequentially from 1 on the far left to 18 on the far right.
This streamlined 1 to 18 system was officially recommended by IUPAC in 1988 to replace older, confusing notations. Previously, two different systems—one favored in Europe and one in the United States—used Roman numerals (I through VIII) followed by the letters ‘A’ or ‘B’. These ‘A’ and ‘B’ designations were used inconsistently, leading to significant ambiguity. The current 1-18 numbering eliminates this confusion, providing a clear, globally accepted standard.
Chemical Similarity Within Columns
Elements within the same column share similar chemical characteristics due to their identical valence electron count. Valence electrons are the electrons located in the outermost shell of an atom. These particles are involved in forming chemical bonds and determining reactivity. Elements within the main-group columns (Groups 1, 2, and 13–18) possess the same number of these outermost electrons.
For example, every element in Group 1 (e.g., Lithium, Sodium, and Potassium) has exactly one valence electron. This single electron makes them highly reactive, as they easily lose it to form a positive ion and achieve a stable configuration. This shared tendency dictates their similar behavior, such as reacting vigorously with water. Conversely, elements in Group 17 all have seven valence electrons, causing them to readily gain one electron to achieve stability, making them also highly reactive.
Names of the Major Columns
Beyond the numerical system, several groups are known by common, descriptive names that hint at their general properties. Group 1 (excluding hydrogen) is called the Alkali Metals. These are soft, highly reactive metals that form strong bases (alkalis) when reacting with water. Group 2 elements are the Alkaline Earth Metals, which are also reactive metals that form alkalis, though they are generally less reactive than Group 1 elements.
On the right side of the table, Group 17 elements are known as the Halogens, meaning “salt-former.” These nonmetals are extremely reactive and combine readily with metals, such as the Alkali Metals, to produce various salts. Finally, Group 18 contains the Noble Gases, which are nearly unreactive due to their full valence electron shells. This complete set of valence electrons means they rarely participate in chemical bonding.
Column Organization by Electron Blocks
The overall shape of the periodic table, with its varying column widths, represents how electrons fill atomic orbitals. Scientists categorize the table into four main areas, or blocks, based on the type of orbital where the last electron is placed. This organization explains the 18-column width observed in the main part of the table.
The s-block
The s-block encompasses the first two columns, Groups 1 and 2, consistent with the s-orbital holding a maximum of two electrons.
The d-block
The central depression, spanning Groups 3 through 12, is the d-block, which contains the transition metals. The d-orbital can hold up to ten electrons, accounting for the ten columns in this section.
The p-block
The p-block includes Groups 13 through 18 on the right. This section is six columns wide because the p-orbital can accommodate six electrons.
The f-block
The elements displayed separately at the bottom, the Lanthanides and Actinides, constitute the f-block. The f-orbital’s capacity for fourteen electrons determines the length of these two rows.