The third period of the periodic table has 8 elements because of how electrons fill the energy levels of atoms. The reference states that the third period corresponds to the filling of the 3s and 3p subshells (of the third shell) which take a total of 8 electrons, two in the 3s and 6 in the 3p subshell. Let's break this down further:
Understanding Electron Shells and Subshells
Electrons in atoms reside in specific energy levels, often called shells. These shells are further divided into subshells, each with a specific electron capacity. In the third energy level (n=3), there are three subshells: the 3s, 3p, and 3d.
- 3s Subshell: This subshell can hold a maximum of 2 electrons.
- 3p Subshell: This subshell can hold a maximum of 6 electrons.
- 3d Subshell: Although the 3d subshell exists, it is not filled in the third period. It begins to fill in the fourth period.
How Electron Filling Dictates the Number of Elements
The order in which electrons fill the subshells determines the number of elements in each period. In the third period, electrons progressively fill the 3s subshell first, holding two electrons, and then the 3p subshell, holding six more.
- Sodium (Na): The first element in the third period has its outermost electron in the 3s subshell.
- Magnesium (Mg): The second element in the third period fills the 3s subshell, completing the 3s subshell.
- Aluminum (Al) to Argon (Ar): The next six elements progressively fill the 3p subshell one by one.
Summary
| Subshell | Capacity | Third Period Elements |
|---|---|---|
| 3s | 2 | Na, Mg |
| 3p | 6 | Al, Si, P, S, Cl, Ar |
| Total | 8 |
Therefore, the third period is complete after filling the 3s and 3p subshells, with a total of 2 + 6 = 8 electrons, which corresponds to the 8 elements of this period.
