Understanding Electron Configuration and Periodic Table Patterns
What is Electron Configuration?
Electron configuration
Electron configuration refers to the arrangement of electrons in an atom’s energy levels, sublevels, and orbitals.
- Electrons occupy regions around the nucleus called orbitals, which are grouped into sublevels (s, p, d, f) and energy levels (designated by principal quantum numbers, $n$).
- Here’s the key rule: electrons fill orbitals in order of increasing energy, following the Aufbau principle.
- The $1s$ orbital is filled first, followed by $2s$, $2p$, $3s$, and so on.
- Each orbital has a maximum electron capacity: $s$ holds 2, $p$ holds 6, $d$ holds 10, and $f$ holds 14.
Using the Periodic Table to Determine Electron Configuration
The periodic table is divided into blocks(s, p, d, and f) that correspond to the type of sublevel being filled by the outermost electrons of the elements in that block:
- s-block: Groups 1 and 2 (plus helium).
- p-block: Groups 13 to 18.
- d-block: Transition metals (Groups 3 to 12).
- f-block: Lanthanides and actinides.
Determine the electron configuration of phosphorus ($Z=15$).
Solution
- Locate phosphorus in Group 15 and Period 3 of the periodic table.
- Period 3 tells us the outermost electrons are in the $n=3$ energy level.
- Group 15 indicates phosphorus has 5 valence electrons (electrons in the outermost energy level).
- Fill the orbitals in order: $1s^2$, $2s^2$, $2p^6$ (inner electrons sum to 10, corresponding to neon’s configuration).
- The remaining 5 electrons go into $3s^2$ and $3p^3$.
- The full electron configuration is: $1s^2 2s^2 2p^6 3s^2 3p^3$.
- The condensed configuration is: $[Ne] 3s^2 3p^3$.
Use the periodic table as a map: the period number tells you the principal energy level ($n$), while the block and group number guide you to the sublevel and number of valence electrons.
Group Numbers and Valence Electrons
What Do Group Numbers Tell Us?
Group number
The group number of an element indicates the number of valence electrons in its outermost energy level.
Valence electrons are crucial because they determine an element’s chemical reactivity and bonding behavior.
- Group 1 (alkali metals): 1 valence electron ($s^1$).
- Group 2 (alkaline earth metals): 2 valence electrons ($s^2$).
- Groups 13–18 (p-block): The number of valence electrons equals the group number minus 10. For example:
