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The sodium (Na) and chlorine (Cl) atoms have the following electronic configurations:

 Na: 1s22s22p63s1 which can be written [Ne]3s1, where [Ne] represents the electronic configuration of the inert gas neon (Ne)

 Cl: 1s22s22p63s23p5 which can be written [Ne]3s23p5

According to the octet rule, in combining to form an NaCl molecule, the sodium atom can lose an electron (the 3s¹ electron) to from a stable octet in the outermost shell while the chlorine atom can gain an electron in its outermost shell (Figure 2.1). This results in the formation of Na⁺ and Cl⁻ ions with an electronic configuration corresponding to Ne and Ar, respectively. Electrostatic attraction between the cations and anions leads to the formation of an ionic bond.