The Periodic Table of Elements organizes all known chemical substances based on their properties and atomic structure. A unique set of elements, known as the Inner Transition Metals or f-block elements, cannot be placed within the main body. Their distinct chemical behavior and peculiar positioning below the main chart set them apart.
Placement on the Periodic Table
The Inner Transition Metals are physically located in two distinct rows set apart from and below the primary structure of the periodic table. These two horizontal rows belong chronologically within the sixth and seventh periods of the main table.
The first row of 14 elements should logically be inserted into the sixth period, immediately following Lanthanum (atomic number 57) in Group 3. Similarly, the second row of 14 elements belongs in the seventh period, following Actinium (atomic number 89), also in Group 3.
This separate arrangement is primarily a matter of visual convenience and design practicality. If these 28 elements were placed in their correct sequential spots, the periodic table would become extremely wide and cumbersome. Separating them into the f-block allows the main body of the table to remain compact and easily readable.
The Lanthanide Series
The first of the two rows is the Lanthanide series, which includes elements with atomic numbers 58 through 71. These elements are also commonly referred to as the Rare Earth Elements. They are generally soft, silvery metals that are highly reactive, particularly with air and water.
The Lanthanides share remarkably similar chemical properties, which historically made them difficult to separate from one another. The name “Rare Earth” is misleading because most are relatively abundant in the Earth’s crust, but they are rarely found concentrated in economically viable deposits. The Lanthanide series begins after the element Lanthanum, which is why the group takes its name from this element.
The Actinide Series
The second row comprises the Actinide series, which includes elements with atomic numbers 90 through 103. A defining characteristic is that all Actinides are radioactive, meaning their nuclei are inherently unstable and undergo spontaneous decay.
The Actinides include naturally occurring elements like Thorium and Uranium, but the majority of the series consists of man-made elements. Elements with an atomic number greater than 92, known as transuranic elements, are all synthetic and highly unstable.
Their radioactivity and ability to undergo nuclear reactions make many Actinides, such as Plutonium, significant in applications like nuclear energy and weapons technology.
The Reason for Separate Positioning
The fundamental reason for the Inner Transition Metals’ unique placement relates to how their electrons are organized around the nucleus. While elements are typically grouped by the filling of s, p, or d orbitals, these metals involve the filling of deep-lying f-orbitals.
These f-orbitals exist two energy levels below the outermost valence shell, often referred to as the n-2 shell, where n is the period number. Specifically, the Lanthanides fill the 4f subshell, and the Actinides fill the 5f subshell.
Because these inner f-electrons are deeply buried, they are effectively shielded from the surrounding environment. They do not significantly influence the elements’ outer chemical behavior, resulting in a large group of elements that share very similar chemical properties. This similarity prevents them from fitting sequentially into the established columns of the main periodic table.