The line between ionic and covalent bonding is not a line at all — it is a continuum measured by the electronegativity difference between two atoms: from ΔEN 1.7 upward the electron is handed over outright and the bond counts as ionic, while below 0.4 the two share it perfectly evenly. Na and Cl differ by 2.23, so they end up as the ion pair Na⁺ and Cl⁻ held together electrostatically, whereas two identical O atoms give ΔEN = 0. Along the potential well of such a pair, the bottom marks the equilibrium bond length and the depth of the well is the energy needed to break the bond; squeezing any closer sends the two nuclei repelling apart again.
The line between ionic and covalent bonding is not a line at all — it is a continuum measured by the electronegativity difference between two atoms: from ΔEN 1.7 upward the electron is handed over outright and the bond counts as ionic, while below 0.4 the two share it perfectly evenly. Na and Cl differ by 2.23, so they end up as the ion pair Na⁺ and Cl⁻ held together electrostatically, whereas two identical O atoms give ΔEN = 0. Along the potential well of such a pair, the bottom marks the equilibrium bond length and the depth of the well is the energy needed to break the bond; squeezing any closer sends the two nuclei repelling apart again.
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