The fourth period of the periodic table contains 18 elements because of the sequential filling of the 4s, 3d, and 4p electron orbitals, which collectively accommodate 18 electrons. This expansion beyond 8 elements, unlike periods 2 and 3, is primarily due to the inclusion of the d-block transition metals.
Understanding Periods and Electron Shells
In the periodic table, each period (horizontal row) corresponds to the principal quantum number (n) of the outermost electron shell being filled. For instance, the fourth period signifies that electrons are occupying orbitals within the fourth principal energy level (n=4) or orbitals that become energetically accessible at this stage.
The Role of Pauli's Exclusion Principle
The fundamental reason for the number of elements in any period lies in the capacity of atomic orbitals to hold electrons. According to Pauli's exclusion principle, each atomic orbital can accommodate a maximum of two electrons, provided they have opposite spins. This principle dictates how many electrons can occupy a specific set of orbitals.
Electron Filling in the Fourth Period
For the fourth period, the order of electron filling does not strictly follow the principal quantum number (n). Instead, it adheres to the Aufbau principle and Hund's rule, which prioritize orbitals based on their energy levels.
The orbitals that are filled to account for the 18 elements in the fourth period are:
- 4s subshell: This subshell contains 1 orbital.
- 3d subshell: This subshell contains 5 orbitals. While technically part of the third principal energy level (n=3), its energy level is slightly lower than or comparable to the 4p subshell, causing it to fill after the 4s subshell and before the 4p subshell.
- 4p subshell: This subshell contains 3 orbitals.
Calculating the Number of Elements
By summing the number of orbitals involved in the filling process for the fourth period, we find:
1 (4s orbital) + 5 (3d orbitals) + 3 (4p orbitals) = 9 orbitals in total.
Since each of these 9 orbitals can hold a maximum of 2 electrons (as per Pauli's exclusion principle), the total number of electrons that can be accommodated in these specific orbitals is:
9 orbitals × 2 electrons/orbital = 18 electrons.
Each additional electron corresponds to a unique element as the atomic number increases, thus explaining why the fourth period contains 18 elements.
Period 4 Element Breakdown
Here's a breakdown of the elements added as these orbitals are filled:
| Subshell | Number of Orbitals | Electrons Accommodated | Element Types Added | Example Elements |
|---|---|---|---|---|
| 4s | 1 | 2 | Alkali Metals and Alkaline Earth Metals | Potassium (K), Calcium (Ca) |
| 3d | 5 | 10 | Transition Metals | Scandium (Sc) to Zinc (Zn) |
| 4p | 3 | 6 | Main Group Elements | Gallium (Ga) to Krypton (Kr) |
| Total | 9 | 18 | Total Elements in Period 4 | 18 Elements |
The presence of the 3d orbitals, which are filled after the 4s and before the 4p, is the key factor that expands the fourth period from what would otherwise be 8 elements (if only s and p orbitals were considered) to 18 elements.
