The chemical elements are the fundamental building blocks of all matter, each defined by the number of protons in its atomic nucleus. The periodic table provides a systematic organization for the approximately 118 known elements. This arrangement orders elements sequentially by increasing atomic number, revealing recurring patterns in their properties, a principle known as the periodic law. Grouping elements based on these similarities allows scientists to predict chemical behaviors.
Classification by Physical and Chemical Properties
The most general classification divides elements into three categories based on shared physical characteristics: metals, nonmetals, and metalloids. Metals occupy the vast majority of the table, situated on the left and center. They are typified by high electrical and thermal conductivity, due to freely moving, delocalized electrons.
Most metals are solid at room temperature (mercury is the exception) and possess a characteristic metallic luster. Structurally, they are malleable (can be hammered into thin sheets) and ductile (can be drawn into wires). Chemically, metals tend to lose their valence electrons easily, forming positive ions.
Nonmetals are situated on the upper right side of the periodic table, exhibiting properties opposite to metals. They are poor conductors of heat and electricity because their electrons are localized. Solid nonmetals, like sulfur, are typically brittle and dull, lacking malleability and luster.
Nonmetals exist in all three states of matter, with many being gases (like oxygen and nitrogen) or liquids (like bromine). In chemical interactions, nonmetals generally gain or share electrons to achieve a stable configuration.
Bridging these groups are the metalloids, or semimetals, which sit along a zigzag line. Metalloids possess a combination of metallic and nonmetallic properties. Physically, they often have a metallic appearance but are brittle like nonmetals.
Their distinct property is intermediate electrical conductivity, classifying them as semiconductors. This behavior, which can be altered by temperature or impurities, makes elements like silicon and germanium useful in modern electronics. Metalloids display variable chemical behavior, sometimes acting like metals and other times like nonmetals.
Classification by Chemical Families
The vertical columns of the periodic table are known as groups or families. Elements within the same group share similar chemical properties because they have the same number of valence electrons.
The Alkali Metals (Group 1) are highly reactive metals with a single valence electron. They are soft, have low densities, and react vigorously with water to form strongly alkaline solutions.
Group 2 elements are the Alkaline Earth Metals, possessing two valence electrons. These metals are harder, denser, and have higher melting points than the alkali metals. While still reactive, they are less so than their Group 1 neighbors.
The Transition Metals occupy Groups 3 through 12, forming the central block. A defining characteristic is their ability to form compounds exhibiting multiple positive oxidation states, engaging in diverse chemical reactions. Many transition metal compounds are colored, and the metals are often used as catalysts in industrial processes.
Located in Group 17, the Halogens are highly reactive nonmetals, just one electron short of a full outer shell. This deficiency gives them a strong tendency to gain an electron, making them powerful oxidizing agents. The name “halogen” means “salt-former,” reflecting their propensity to react with metals to create salts.
The Noble Gases (Group 18) are the least reactive elements. They have completely filled valence electron shells, leading to a stable nature. These elements exist as colorless, odorless, monatomic gases and were historically referred to as inert gases due to their lack of chemical participation.
The Inner Transition Metals, comprising the Lanthanides and Actinides, are the two rows displayed separately at the bottom. These elements are all metals with very similar chemical properties, making them difficult to separate. They are classified based on the filling of their f-orbitals, which leads to their distinct placement in the table’s structure.
Classification by Orbital Structure and Periods
Beyond vertical grouping, the periodic table is organized horizontally into seven rows called periods. Every element within the same period has the same number of electron shells, which determines atomic size and reactivity. A new period begins when a new principal electron shell starts to be filled.
Moving from left to right across a period, the number of valence electrons increases, and chemical properties gradually transition from metallic to nonmetallic. This progression also shows a trend where the atomic radius decreases due to the increasing positive charge of the nucleus pulling the electron shells closer.
A technical classification relies on the quantum mechanical organization of electrons into atomic orbitals, dividing the table into four distinct blocks. These blocks are named after the type of orbital (\(s\), \(p\), \(d\), or \(f\)) being filled by the highest-energy electrons.
The \(s\)-block includes Groups 1 and 2 (alkali and alkaline earth metals). The \(p\)-block encompasses Groups 13 through 18, containing a diverse mix of metals, nonmetals, and all metalloids.
Elements in the \(d\)-block are the Transition Metals, where the \(d\)-orbitals are progressively filling. Lastly, the \(f\)-block comprises the Inner Transition Metals (Lanthanides and Actinides), characterized by the filling of the \(f\)-orbitals.