The horizontal rows on the periodic table are called periods. Periods represent the progression of elements based on increasing atomic number. The periodic table contains seven distinct periods, and moving from left to right shows how the properties of the elements change systematically. This arrangement helps predict the atomic structure and reactivity of all known elements.
Defining Periods by Energy Level
The number assigned to each period, from 1 to 7, corresponds directly to the highest principal quantum number of the electron shells occupied by electrons in an atom of that element. This means every element in Period 3, for example, possesses electrons in three main energy shells. As one moves down the table from Period 1 to Period 7, each subsequent row adds another occupied electron shell to the atomic structure.
This sequential filling of energy levels dictates the length of each row. The first period is the shortest, holding only two elements, hydrogen and helium, because the first electron shell can accommodate a maximum of two electrons. Periods 2 and 3 each hold eight elements, corresponding to the filling of the second and third principal energy levels.
Later periods become longer because electrons begin to occupy sub-shells, such as the \(d\) and \(f\) orbitals, in addition to the \(s\) and \(p\) orbitals. Period 4 and Period 5 both contain eighteen elements, reflecting the inclusion of the ten transition metals.
Horizontal Trends in Properties
As one moves from left to right across any single period, the chemical and physical properties of the elements change in a predictable manner. This systematic variation is due to the increasing number of protons in the nucleus, which leads to a higher effective nuclear charge experienced by the outermost electrons. The new electrons are added to the same principal energy shell across the row, meaning the shielding effect from inner electrons remains relatively constant.
This increasing positive charge pulling on a consistent shell of electrons causes the atomic radius to generally decrease across a period. For instance, an atom on the far left of a period is larger than an atom on the far right of the same period. The stronger attraction between the nucleus and the outer electrons also affects the energy required to remove an electron, known as the ionization energy.
Ionization energy typically increases when moving from left to right because the electrons are held more tightly to the nucleus, requiring more energy to liberate them. Similarly, the ability of an atom to attract a pair of electrons in a chemical bond, called electronegativity, also increases across a period.
The elements on the right side of the table, such as the halogens, exhibit high electronegativity values because their outer shell is nearly full. This creates a strong tendency to gain electrons.
The elements transition from being highly metallic and reactive on the left side to being nonmetallic and unreactive on the far right, with a gradual shift in character across the row.
Special Rows: The Lanthanide and Actinide Series
The two separate rows often positioned below the main body of the periodic table are the Lanthanide and Actinide series. These elements are not a separate pair of periods but are actually meant to be inserted into the sixth and seventh periods of the main table. Specifically, the Lanthanides belong in Period 6 following Barium, and the Actinides belong in Period 7 after Radium.
They are separated and placed below the main table for practical formatting reasons, as including them in their correct positions would make the entire table too wide and awkward to print. These elements are collectively known as the inner transition metals. They are unique because their electrons are filling inner \(f\)-orbitals rather than the \(s\), \(p\), or \(d\)-orbitals of the main structure.
The Lanthanides are filling the \(4f\) sub-shell, while the Actinides are filling the \(5f\) sub-shell. This filling of an inner electron shell results in these elements having very similar chemical properties to one another within their respective series. The Lanthanides, running from atomic number 58 to 71, and the Actinides, from 90 to 103, each contain fourteen elements.