The other route: not share but transfer. Sodium holds its one outer electron loosely; chlorine grabs hard. The 3s electron moves across — Na⁺ drops to a neon shell, Cl⁻ fills to an argon shell, both full. The surprise in the bare numbers: making the two ions costs energy (+147 kJ/mol uphill — ionization beats electron affinity). The bond exists only because the electrostatic pull between the finished ions more than pays that back. Slide them together and watch the Coulomb well open below the cost line.
| bond length | rₑ ≈ 236 pm (gas pair) |
| configs | Na⁺ = [Ne], Cl⁻ = [Ar] — both full |
| radii | Na⁺ 102 pm, Cl⁻ 181 pm |