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Sodium (Na) is an alkali metal with 11 electrons. Only one of these electrons occupies the outermost, third shell, making it the sole valence electron. Because the valence shell is the first to interact during a chemical reaction, this lone electron largely dictates sodium’s reactivity and the types of compounds it forms.
Sodium has a single valence electron. Its inner shells are filled (2 + 8 electrons), while the outer shell holds just one. This drives its high reactivity and its tendency to form ionic bonds.
Electron shells follow a simple rule: the first can hold 2 electrons, the second 8, and the third 18 (two, six, and ten subshells). A stable atom seeks a completely filled outer shell. An element is most reactive when its outer shell has either one electron or is one short of full—conditions that make electron transfer straightforward and energetically favorable.
With just one electron to spare, sodium readily donates that electron to elements needing one more to achieve a full outer shell. When sodium encounters, for example, chlorine (Cl), which requires one additional electron, the transfer creates Na⁺ and Cl⁻ ions. The electrostatic attraction between the oppositely charged ions produces the stable ionic compound sodium chloride (table salt).
In the periodic table, alkali metals (group 1) possess one valence electron, while halogens (group 17) need one to complete their outer shell, making them prime partners for sodium.
When sodium chloride dissolves in water, it dissociates into Na⁺ and Cl⁻ ions. The sodium ion retains a fully occupied outer shell of eight electrons and carries a +1 charge. The chloride ion, having accepted sodium’s valence electron, also possesses a complete outer shell and carries a –1 charge. With both ions’ shells satisfied, they remain inert in the solution.
