Periodic table
= Periodic table
{wiki}
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 https://iupac.org/what-we-do/periodic-table-of-elements/[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>.
\Image[/first-three-periods.png]
{title=The first three <periodic table periods>: <atomic numbers> and neutral <ground state> <electron shell> occupancies}
{description=Colours distinguish <periodic group> columns only, without encoding numerical property trends. The spaces in columns 3–12 preserve the eighteen-column <periodic table> layout: no <chemical elements> occupy those positions in these three <periodic table periods>. Each cell gives a <chemical element>'s symbol, <atomic number>, and <electron shell> counts.}
Reading each row from left to right gives:
* <Periodic table period> 1: <hydrogen> $(1)$ and <helium> $(2)$.
* <Periodic table period> 2: <lithium> $(2,1)$, <beryllium> $(2,2)$, <boron> $(2,3)$, <carbon> $(2,4)$, <nitrogen> $(2,5)$, <oxygen> $(2,6)$, <fluorine> $(2,7)$, and <neon> $(2,8)$.
* <Periodic table period> 3: <sodium> $(2,8,1)$, <magnesium> $(2,8,2)$, <aluminium> $(2,8,3)$, <silicon> $(2,8,4)$, <phosphorus> $(2,8,5)$, <sulfur> $(2,8,6)$, <chlorine> $(2,8,7)$, and <argon> $(2,8,8)$.
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 https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations[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 https://openstax.org/books/chemistry-2e/pages/2-5-the-periodic-table[Flowers et al., Chemistry 2e, periodic-table organization].