What is the SPDF Rule?

The "SPDF rule," more accurately described as the electron configuration notation, uses the letters s, p, d, and f to represent the different types of atomic orbitals and their capacity to hold electrons. This notation describes how electrons are arranged within an atom's electron shells and subshells.

Understanding Atomic Orbitals

• s orbitals: These are spherical, with a single orbital holding a maximum of two electrons.
• p orbitals: These have a dumbbell shape, with three orbitals (p_x, p_y, p_z) capable of holding a total of six electrons.
• d orbitals: More complex shapes, with five orbitals holding up to ten electrons.
• f orbitals: Even more complex shapes than d orbitals; seven orbitals can hold a maximum of fourteen electrons.

The subshells (s, p, d, f) are filled in a specific order, following the Aufbau principle and Hund's rule (explained below). The order of filling is generally: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p…

Key Principles

• Aufbau Principle: Electrons fill orbitals starting with the lowest energy level and progressing to higher energy levels.
• Hund's Rule: Electrons individually occupy each orbital within a subshell before doubling up in any one orbital. This maximizes the total spin of the electrons in that subshell.
• Pauli Exclusion Principle: No two electrons in an atom can have the same four quantum numbers. This means that each orbital can hold a maximum of two electrons, with opposite spins.

Example:

Let's consider the element Oxygen (O), which has eight electrons. Its electron configuration is 1s²2s²2p⁴. This means:

• Two electrons fill the 1s orbital.
• Two electrons fill the 2s orbital.
• Four electrons fill the three 2p orbitals (following Hund's rule, two electrons will be alone in two orbitals and the other two share one orbital).

Conclusion

The SPDF notation provides a concise way to represent the electron arrangement within an atom, illustrating the distribution of electrons across different energy levels and orbitals, following fundamental principles like the Aufbau principle and Hund's rule. It's a critical tool in understanding atomic behavior and chemical bonding.