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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.

The chemical element with atomic number fifteen.

Consider the reacting zinc and copper species in the zinc-copper displacement experiment. Split the net chemical equation into two half-reactions:

The zinc oxidation state rises from $0$ to $+2$; the copper oxidation state falls from $+2$ to $0$. Add the half-reactions: two released Electrons match two accepted Electrons. Cancelling $2e^{-}$ from both sides gives^[ref]Thus the closed net chemical reaction has zero net production of free Electrons. Cancellation does not destroy Electrons: it removes a matched internal exchange from the written chemical equation. The half-reactions alone are incomplete descriptions of the whole reacting system. In a galvanic cell, the exchange can pass through a wire; in the zinc-copper displacement experiment, the reacting materials meet at the same metal surface. The equations do not require a reservoir of free Electrons in the bulk chemical solution.

Check both kinds of balance. The oxidation line has net electric charge $0$ on the left and $(+2-2)e=0$ on the right; the reduction line has $(+2-2)e=0$ on the left and $0$ on the right. The net chemical equation contains one zinc and one copper chemical atom on each side and has net electric charge $+2e$ on each side, where $e$ is the positive elementary charge. The written reacting species need not be electrically neutral as a group: the bulk chemical solution also contains compensating anions.

There is an even more literal check. Zinc has atomic number $30$ and copper $29$, so the initial pair contains $30+(29-2)=57$ Electrons. The final pair contains $(30-2)+29=57$ Electrons. Only their distribution has changed. Conserved chemical element counts give the usual chemical mass conservation inventory; changing the counts of neutral and charged species does not create or remove their atomic nuclei.

Matching Electrons is necessary but does not replace balancing every chemical element. The acidic permanganate-iron balance illustrates unequal one- and five-Electron exchanges, with water and protons also needed. Conversely, a balanced chemical equation alone establishes neither a reaction rate nor a detailed mechanism. Its usefulness is that it fixes the stoichiometry even when those additional questions remain open.

Sodium is the chemical element of atomic number eleven. Carbon-fusion channels in carbon burning include ${}^{12}\mathrm{C}+{}^{12}\mathrm{C}⟶{}^{23}\mathrm{Na}+p$, producing sodium-23 and a proton. Its abundance in expelled stellar material depends on later reactions and mixing, rather than on this one production channel alone.

Stereochemical sequence rules rank substituents for R and S configuration. Compare the directly attached chemical atoms by atomic number first, then resolve ties by comparing successive shells at the first difference. Isotopes and multiple chemical bonds require additional rules. This ranking convention does not involve optical rotation.^[ref]

The chemical element with atomic number sixteen.