The maximum number of electrons that can occupy a single atomic orbital is two. This fundamental principle is governed by the Pauli Exclusion Principle in quantum mechanics.
Every distinct orbital, regardless of its shape or energy level, has the capacity to hold a maximum of two electrons. These two electrons must possess opposite spins (one spin-up and one spin-down), a property crucial for their stable coexistence within the same orbital.
Understanding Orbitals vs. Sublevels
It's important to distinguish between an "orbital" and a "sublevel" (or subshell). An orbital is a specific region of space around the nucleus where an electron is most likely to be found. A sublevel, on the other hand, is a collection of one or more orbitals of the same type.
Different types of sublevels contain a varying number of individual orbitals, each capable of holding two electrons. For example:
- The s sublevel contains one s orbital. This s orbital can hold up to two electrons.
- The p sublevel consists of three p orbitals. Each of these three p orbitals can hold two electrons, leading to a total capacity of six electrons for the entire p sublevel.
- The d sublevel is comprised of five d orbitals. With each d orbital accommodating two electrons, the d sublevel can hold up to 10 electrons in total.
- The f sublevel includes seven f orbitals. Consequently, the f sublevel has a maximum capacity of 14 electrons.
The table below summarizes the capacity of different sublevels, illustrating how the total number of electrons in a sublevel is a multiple of two (the capacity of a single orbital):
| Sublevel Type | Number of Orbitals per Sublevel | Maximum Electrons per Orbital | Maximum Electrons per Sublevel |
|---|---|---|---|
| s | 1 | 2 | 2 |
| p | 3 | 2 | 6 |
| d | 5 | 2 | 10 |
| f | 7 | 2 | 14 |
This distinction highlights that while the s orbital holds up to two electrons, the terms "p orbital," "d orbital," and "f orbital" are sometimes used colloquially to refer to their respective sublevels, which contain multiple individual orbitals. However, when referring to a single, specific orbital, the maximum capacity remains two electrons.
Why Only Two Electrons?
The limitation of two electrons per orbital stems from the Pauli Exclusion Principle, which states that no two electrons in an atom can have the exact same set of four quantum numbers. These quantum numbers describe an electron's state in an atom:
- Principal quantum number (n): Energy level.
- Azimuthal or angular momentum quantum number (l): Shape of the orbital (s, p, d, f).
- Magnetic quantum number (m_l): Orientation of the orbital in space.
- Spin quantum number (m_s): Intrinsic angular momentum (spin), which can be +½ (spin-up) or -½ (spin-down).
For two electrons to occupy the same orbital, they must share the same n, l, and m_l quantum numbers. To satisfy the Pauli Exclusion Principle, their spin quantum number (m_s) must be different, meaning one electron has a spin of +½ and the other -½. This is the only way for two electrons to occupy the same spatial region (orbital) without violating fundamental quantum rules.
