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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:

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

1 s

2 s

2 p

3 s

, and

3 p

, filled in that order for these neutral ground states. Each

s

p

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.