Mistake Master
Atomic structure and electron configuration
An atom's chemistry is written in how its electrons are arranged. Fill the orbitals by the rules and you get the configuration that drives bonding — but the rules trip students exactly where energy order and shell number disagree.
§1
Why electrons live in shells.
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An atom's electrons are held by Coulombic attraction to the positive nucleus. That attraction gets stronger two ways: when the nuclear charge is larger, and when the electron sits closer to the nucleus. Stronger attraction means lower energy and a tighter hold.
Electrons don't sit at just any distance. They occupy shells, labeled by the principal quantum number n = 1, 2, 3, … Low n means close to the nucleus and low energy; high n means farther out and higher energy. Each shell is divided into subshells — s, p, d — that hold a fixed number of electrons: s holds 2, p holds 6, d holds 10.
Because the innermost, lowest-energy spots are the most stable, electrons fill them first and work outward. That filling order is what an electron configuration records — the address of every electron, subshell by subshell.
§2
Writing an electron configuration.
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An electron configuration is just the atom's electrons poured into subshells from the bottom up until you run out. The number to reach is the atomic number Z — the count of protons, which equals the electrons in a neutral atom.
- Find how many electrons to place. For a neutral atom, that's the atomic number Z. Sulfur is Z = 16, so 16 electrons.
- Fill subshells in energy order. The order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, … Notice 4s comes before 3d — energy order, not simple numerical order.
- Respect each subshell's capacity. s takes 2, p takes 6, d takes 10. Fill a subshell fully before moving to the next, writing the count as a superscript: 1s², 2p⁶.
- Stop when the electrons run out. Add the superscripts as you go; when they total Z, you're done. Then identify the valence electrons — those in the highest occupied principal shell (highest n) — and treat the rest as core.
The valence electrons, in that outermost shell, are the ones chemistry acts on. Core electrons sit deep, tightly bound, and stay put during ordinary reactions.
§3
The pieces you'll meet.
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Quick reference card. Keep energy, distance, and capacity straight.
§4
Worked example: the electron configuration of sulfur.
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Question. Write the electron configuration of a neutral sulfur atom (Z = 16) and state how many valence electrons it has.
Step 1 — electrons to place. Neutral sulfur has 16 electrons.
Step 2 — fill in energy order. 1s² (2 placed), 2s² (4), 2p⁶ (10), 3s² (12), then 3p gets the last 4: 3p⁴ (16). We've reached 16, so stop.
Step 3 — write it out. The configuration is 1s² 2s² 2p⁶ 3s² 3p⁴. Check: 2 + 2 + 6 + 2 + 4 = 16. ✓
Step 4 — identify valence. The highest occupied shell is n = 3, holding 3s² 3p⁴. That's 6 valence electrons. The 1s² 2s² 2p⁶ underneath — 10 electrons — are core.
Sanity check. Sulfur sits in group 16 of the periodic table, and main-group group numbers match valence-electron counts: group 16 → 6 valence electrons. The configuration and the periodic table agree.
§5
3 mistakes that cost real points.
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"Fill 3d right after 3p, in numerical order."
Subshells fill by energy, not by their number. The 4s subshell is slightly lower in energy than 3d, so it fills first: … 3p⁶, 4s², then 3d. Writing 3d before 4s puts electrons in the wrong place and throws off every element from potassium onward.
Fix. Memorize the energy order (1s 2s 2p 3s 3p 4s 3d 4p…) or use the periodic table itself as the map — reading across the rows walks you through subshells in energy order.
"Valence electrons are whatever's in the last subshell written."
Valence electrons are everything in the highest principal shell n, not just the final subshell. For sulfur (… 3s² 3p⁴), valence is 3s² and 3p⁴ together — 6 electrons, not just the 4 in 3p. Counting only the last subshell undercounts the valence.
Fix. Find the largest n in the configuration and add up every electron with that n. For main-group atoms, that total matches the group number.
"Every electron is pulled equally, so distance doesn't matter."
Coulombic attraction depends on distance: an inner 1s electron is far closer to the nucleus than an outer 3p electron, so it is held far more tightly and sits at much lower energy. That is exactly why core electrons don't react and valence electrons do — and why removing a valence electron takes much less energy than removing a core one.
Fix. Tie energy to position: closer to the nucleus (and higher nuclear charge) means lower energy and a stronger hold. Outer electrons are the loosely held, reactive ones.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.