ELECTRON CONFIGURATION NOTATION
In lesson 2.3 we learned the rules for filling orbitals. Now we put them into writing — the precise shorthand chemists use to record exactly where every electron in an atom lives.
READING & WRITING THE NOTATION
An electron configuration lists each occupied subshell in order, with a superscript showing how many electrons it contains. The format is:
FORMAT
For example, carbon (Z=6) has 6 electrons to place. The first 2 go into 1s, the next 2 fill 2s, and the final 2 begin filling 2p:
CARBON (Z=6)
Step through the explorer below to see how configurations build up across the first 36 elements.
// INTERACTIVE — ELECTRON CONFIGURATION EXPLORER
THE s/p/d BLOCKS
The shape of the periodic table is not arbitrary — it directly reflects which subshell is being filled as you move across each period. This divides the table into named blocks:
Groups 1–2, plus He
Outermost electrons fill an s subshell
H, He, Li, Na, K, Ca — the alkali and alkaline earth metals (plus hydrogen and helium).
Groups 13–18 (except He)
Outermost electrons fill a p subshell
B through Ne, Al through Ar, Ga through Kr — includes nonmetals, metalloids, and noble gases.
Groups 3–12
The (n−1)d subshell is being filled
Sc through Zn, Y through Cd — the transition metals. Note 4s fills before 3d.
Lanthanides & actinides (rows 6–7, detached)
The (n−2)f subshell is being filled
Ce–Lu, Th–Lr — rare earths and heavy radioactive elements.
The block an element belongs to tells you which subshell its highest-energy electrons occupy. This is directly related to its chemical behaviour — the valence electrons we studied in lesson 2.2 are always the outermost s and p electrons (for s and p block elements), which is why Group number predicts valence electron count so cleanly.
ABBREVIATED NOTATION
Writing out the full configuration for every element gets tedious fast. Because noble gases have completely filled, stable electron arrangements, chemists use them as shorthand. Instead of writing everything from 1s onward, you write the previous noble gas in square brackets, then list only the electrons added after it.
EXAMPLES
Sodium Na Z=11
Chlorine Cl Z=17
Iron Fe Z=26
The abbreviated form is what you'll most often see in textbooks and exams. It strips away the stable, chemically inert core and highlights only the electrons that actually participate in bonding — exactly the valence electrons we studied in lesson 2.2.
PRACTICE: ORBITAL RUSH
Place electrons onto a 3D atom one spin at a time — applying Aufbau order and Hund's rule from memory. Play in Normal mode for a relaxed no-timer challenge, or switch to Rush mode and race the 60-second clock. Three lives in either mode.
// MINIGAME — ORBITAL RUSH
ORBITAL RUSH
ELECTRON PLACEMENT CHALLENGE
SELECT MODE
No time limit — 3 lives. Take your time.
UP NEXT
We now know exactly where every electron in an atom lives. In lesson 2.5 we will see how these configurations drive periodic trends — predictable patterns in atomic size, ionization energy, and electronegativity that reveal the hidden architecture of the table.
READ LESSON 2.5 →