Shell notation counts electrons per energy level, filled from the innermost shell outward: the first shell holds up to 2 electrons and, in the school convention that covers the first 20 elements, the second and third each hold up to 8, giving the familiar 2, 8, 8 pattern. Subshell notation splits each shell into s, p, and d subshells with the electron count as a superscript. The table shows both for the first ten elements, and reading down it makes the relationship obvious: 2 is 1s², and 8 is 2s² plus 2p⁶.
To write an electron configuration, fill subshells in the aufbau order (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p) until the electrons run out, following three rules: the filling order, the subshell capacities (s holds 2, p holds 6, d holds 10), and the electron count, which equals the atomic number for a neutral atom and shifts by the charge for an ion. That generates every configuration your course will ask for. The only other decision is notation: shell counts like 2,8,1 at GCSE level, subshell notation like 1s² 2s² 2p⁶ 3s¹ at A-level and AP.
Two notations, one atom
| Element | Shell notation | Subshell notation |
|---|---|---|
| hydrogen | 1 | 1s¹ |
| helium | 2 | 1s² |
| lithium | 2,1 | 1s² 2s¹ |
| beryllium | 2,2 | 1s² 2s² |
| boron | 2,3 | 1s² 2s² 2p¹ |
| carbon | 2,4 | 1s² 2s² 2p² |
| nitrogen | 2,5 | 1s² 2s² 2p³ |
| oxygen | 2,6 | 1s² 2s² 2p⁴ |
| fluorine | 2,7 | 1s² 2s² 2p⁵ |
| neon | 2,8 | 1s² 2s² 2p⁶ |
Past neon the pattern continues: sodium is 2,8,1 or 1s² 2s² 2p⁶ 3s¹, chlorine is 2,8,7 or 1s² 2s² 2p⁶ 3s² 3p⁵, calcium is 2,8,8,2 or 1s² 2s² 2p⁶ 3s² 3p⁶ 4s². Long configurations are abbreviated with a noble gas core in square brackets: sodium becomes [Ne] 3s¹ and calcium becomes [Ar] 4s², which puts the chemically active outer electrons in plain view.
The filling order and the three rules
Rule 1, the order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Electrons fill subshells in order of increasing energy, and 4s comes before 3d because it is slightly lower in energy. That single quirk is why the shell convention pauses at 2,8,8 even though the third shell can hold 18.
Rule 2, the capacities: s holds 2, p holds 6, d holds 10, f holds 14. These come from orbitals: an orbital is a region of space holding at most 2 electrons with opposite spins, and s, p, and d subshells contain 1, 3, and 5 orbitals.
Rule 3, the count: electrons equal the atomic number Z for a neutral atom, adjusted by the charge for an ion. Fill the order with that many electrons, respecting the capacities, and the configuration writes itself. The outermost electrons you end on are the valence electrons, and their number sets the element's group and its reactivity: sodium's single outer electron is exactly why it reacts the way it does.
Configurations for ions
Ions change the electron count and nothing else: the nucleus is untouched. Adjust the count by the charge, then fill as normal. Na⁺ has 10 electrons, giving 2,8, identical to neon. Cl⁻ has 18, giving 2,8,8, identical to argon. Forming an ion usually lands on the arrangement of the nearest noble gas, which is why metals lose their outer electrons and nonmetals gain, and species that share an arrangement, like Na⁺, Ne, and Mg²⁺, are called isoelectronic.
The recurring mistake is starting an ion from the neutral atom's finished configuration in your head, forgetting the adjustment, and writing Na⁺ as 2,8,1. Make the count adjustment the first written step of every ion problem and the error disappears. Drilling this is worth real marks, since the difference between retrieval practice and rereading is roughly 80% versus 36% retention after a week.
The 25-minute drill plan
5 minutes
Write the filling order from memory, 1s through 4p, plus the capacities: s 2, p 6, d 10. Check and fix. This is the generator for everything else.
10 minutes
Generate configurations for elements 1 through 20 in subshell notation without looking, then translate five into shell notation. Check against the periodic table and mark every miss.
5 minutes
Ions: Na⁺, Mg²⁺, O²⁻, Cl⁻. Adjust the electron count first, then fill, then name the noble gas each one matches.
5 minutes
Close everything and write full configurations for five random elements and three random ions. Whatever you missed is tomorrow's first drill.
Frequently asked questions
Same atom, two resolutions. Shell notation counts electrons per energy level and is the GCSE convention: sodium is 2,8,1. Subshell notation splits each shell into s, p, and d subshells and is the A-level and AP convention: the same sodium is 1s² 2s² 2p⁶ 3s¹. Shell 1 contains only 1s, shell 2 contains 2s and 2p, shell 3 contains 3s, 3p, and 3d. Neither is wrong; they answer at different levels of detail, so use whichever your course uses and know how to translate between them.
Adjust the electron count by the charge, then fill as normal. A positive charge means electrons lost, a negative charge means electrons gained, and the protons never change. Na⁺ has 11 minus 1 = 10 electrons, giving 2,8, identical to neon. Cl⁻ has 17 plus 1 = 18 electrons, giving 2,8,8, identical to argon. Forming an ion usually produces the arrangement of the nearest noble gas, and species with identical arrangements, like Na⁺, Ne, and Mg²⁺, are called isoelectronic.
Because 4s is slightly lower in energy than 3d, and electrons fill subshells in order of increasing energy. This one quirk explains the pattern that confuses everyone: the third shell can hold 18 electrons in total, but it pauses at 8 while 4s fills, which is why the school convention of 2,8,8 works for the first 20 elements. Calcium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s², and only after calcium does 3d begin to fill, which is where the transition metals come from.
The atomic number Z counts protons and defines the element: every atom with 6 protons is carbon, no exceptions. The mass number A counts protons plus neutrons, so neutrons = A minus Z. For electron configurations only Z matters, because a neutral atom has as many electrons as protons. Do not confuse either with the decimal relative atomic mass on the periodic table, which is a weighted average over isotopes: chlorine reads 35.5 even though no chlorine atom has that mass.
Isotopes differ in neutrons and are electrically neutral; ions differ in electrons and carry a charge. Neutron changes alter the mass, electron changes alter the charge, and neither touches the proton count. ³⁷Cl is an isotope of chlorine (17 protons, 20 neutrons), Cl⁻ is an ion of chlorine (17 protons, 18 electrons), and ³⁷Cl⁻ is both at once. For configurations, isotopes are irrelevant: all isotopes of an element have identical electron arrangements, which is why they have identical chemical properties.