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An electron configuration records the occupation of electron shells and electron subshells by Electrons. For neutral sodium in its ground state, $1s^{2}2s^{2}2p^{6}3s^1$ gives $2+2+6+1=11$ Electrons; adding the occupations within each electron shell gives $(2,8,1)$. A superscript counts Electrons, rather than multiplying the preceding labels. Removing the outer Electron gives the sodium ion ${\mathrm{Na}}^{+}$ with $1s^{2}2s^{2}2p^6$, or $(2,8)$. See Flowers et al., Chemistry 2e, electron configurations of atoms and ions.

An electron shell collects the allowed electronic quantum states sharing a principal quantum number $n$. Its occupancy is the number of Electrons assigned to those states in an electron configuration. An electron shell is not a material surface or a prescribed orbit.

An electron subshell groups the allowed electronic quantum states within an electron shell by their orbital angular momentum quantum number $\ell$. Labels $s$ and $p$ mean $\ell =0$ and $\ell =1$, respectively; their capacities are two and six Electrons. The leading number in $2p$ identifies the electron shell, while $p$ identifies the electron subshell.

An ion is a chemical atom or a group of chemical atoms with a nonzero net electric charge. A positive ion is a cation; a negative ion is an anion. The whole entity carries the electric charge: a hydronium ion is a group of four chemical atoms with net charge $+e$. This definition agrees with ^[ref].

For an ordinary ion containing only atomic nuclei and Electrons, let $N_p$ count all its protons and $N_e$ all its Electrons. Each proton contributes the elementary charge $+e$, each Electron contributes $-e$, and neutrons contribute zero. ThereforeThe signed integer $z$ is the net electric charge number. Removing an Electron increases $z$ by one; adding an Electron decreases $z$ by one. Changing the Electron count leaves the atomic number, and therefore the identity of the chemical element, unchanged.

For example, a neutral sodium chemical atom has $11$ protons and $11$ Electrons. Its cation ${\mathrm{Na}}^{+}$ still has $11$ protons, but only $10$ Electrons, so $q=(11-10)e=+e$. A neutral chlorine chemical atom has $17$ protons and $17$ Electrons; its anion ${\mathrm{Cl}}^{-}$ has $17$ protons and $18$ Electrons, so $q=(17-18)e=-e$. These are particle inventories, not pictures of atomic orbitals. See 2.6 Ionic and Molecular Compounds (§2.6: ions and ionic compounds).

One sodium chloride formula unit accounts for one ${\mathrm{Na}}^{+}$ cation and one ${\mathrm{Cl}}^{-}$ anion, whose net electric charges sum to zero. Ordinary solid sodium chloride is an extended arrangement of ions, rather than discrete $\mathrm{NaCl}$ molecules. This is a useful distinction: a neutral material can contain charged constituents, and its chemical formula tells us their ratio without claiming that each pair is a separate molecule.

Ion bookkeeping connects chemistry to electric current: moving ions can transport electric charge, while stationary ions cannot supply a transport current density. In electrochemistry, both signs of ions can contribute to the same conventional current. An ion opposing a fixed electric charge is called a counterion.

The Pauli exclusion principle says that each complete one-particle state can have fermionic occupation number zero or one. “Complete” includes spin and any other internal labels, as well as the spatial wavefunction. Thus two Electrons can share a spatial orbital with opposite spin projections: these are two different spin orbitals. Equal energy alone does not mean equal quantum state, because degenerate energy levels contain several independent one-particle states.

This distinction explains how an electron shell can accommodate several Electrons while exclusion still allows only one per complete one-particle state. It also makes a finite model of fermionic state counting surprisingly simple: two spatial orbitals with two spin choices each give four complete one-particle states, and exactly six two-Electron basis states. The following diagram enumerates them; its boxes represent occupation numbers, not locations in space.

Pauli's Exclusion principle and quantum mechanics by Wolfgang Pauli is his retrospective account, delivered on 13 December 1946 for the 1945 Nobel Prize. On printed pp. 29–30, he describes proposing an extra two-valued Electron property in autumn 1924 and formulating exclusion in 1925. The crucial clue linked counts of resolved energy eigenstates in alkali spectra under a magnetic field with the populations of closed electron shells. Resolving every label turned a complicated shell capacity into one Electron per complete one-particle state. This historical route preceded the modern spin interpretation and the antisymmetric wavefunction formulation; it was not originally a deduction from a Slater determinant.

The periodic table arranges chemical elements in increasing atomic number, across successive periodic table periods. Its columns are periodic groups, numbered 1–18. The layout places recurring outer electron configurations above one another, making similarities in chemical bonds and chemical reactions easier to recognize. Ordering is by proton count, not by mass; the mass number of a particular isotope does not determine its box. See the official IUPAC periodic table and numbering conventions.

For the first eighteen neutral chemical atoms in their ground states, a compact description counts Electrons in successive electron shells. Thus carbon has $(2,4)$, meaning two Electrons in the first electron shell and four in the second; the sum $2+4=6$ agrees with its atomic number. Sodium has $(2,8,1)$, whose sum is $11$. These are occupancy counts, not circular trajectories of Electrons.

Reading each row from left to right gives:

The counting outline is short: the occupied electron subshells in this range are $1s$, $2s$, $2p$, $3s$, and $3p$, filled in that order for these neutral ground states. Each $s$ electron subshell holds up to two Electrons and each $p$ electron subshell up to six, giving $2$, $2+6=8$, and $2+6=8$ along these three rows. This $2,8,8$ summary applies only through argon; it is not a general rule that each electron shell fills completely before the next begins, nor a claim that the third electron shell can hold only eight Electrons. See Flowers et al., Chemistry 2e, electronic structure and electron configurations.

The recurring outer counts make the arrangement useful. Lithium and sodium each have one valence electron, whereas fluorine and chlorine each have seven valence electrons. Such similarities help anticipate chemical reactions, without making all properties identical throughout a periodic group. Helium is an instructive exception to an eight-Electron shortcut: its two Electrons complete the first electron shell, so it belongs above neon and argon in periodic group 18. Hydrogen shares column 1 with lithium and sodium but has its own distinct chemistry. See Flowers et al., Chemistry 2e, periodic-table organization.

A periodic table period is a horizontal row of the periodic table, read in increasing atomic number. Periodic table period 2 runs from lithium, $Z=3$, to neon, $Z=10$: $10-3+1=8$ chemical elements. In these neutral ground states, the second electron shell gains Electrons while the first retains two.

A valence electron belongs to the electronic arrangement used to describe a chemical atom's chemical bonds and ordinary chemical reactions. For the neutral hydrogen, carbon, nitrogen, and oxygen chemical atoms used here, the valence electrons occupy the outermost occupied electron shell. Their respective electron configurations are $1s^1$, $1s^{2}2s^{2}2p^2$, $1s^{2}2s^{2}2p^3$, and $1s^{2}2s^{2}2p^4$, giving one, four, five, and six valence electrons. See Chemistry 2e, 6.4: electron configurations and valence electrons.

Lewis structures show these valence electrons while omitting the inner Electrons. Consequently, eight drawn valence electrons in water do not mean eight total Electrons: neutral water has ten, with two inner oxygen Electrons omitted. Neutral ammonia likewise has ten total Electrons but eight valence electrons; neutral carbon dioxide has twenty-two total Electrons but sixteen valence electrons.

This distinction makes the small counts useful: connections and possible lone pairs can be organized without representing every Electron or solving the entire many-Electron wavefunction. The outermost-electron shell shortcut applies to the examples above, rather than serving as a universal definition for all chemical elements; the electronic arrangement relevant to chemical bonds can also involve other electron subshells.