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Why is a sodium-fluorine bond usually considered ionic?

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Diagram showing the formation of a polar covalent bond in a water molecule, with two hydrogen atoms bonding to one oxygen atom to form H2O, illustrating shared electrons and chemical reaction. — covalent bond In polar covalent bonds, such as that between hydrogen and oxygen atoms, the electrons are not transferred from one atom to the other as they are in an ionic bond. Instead, some outer electrons merely spend more time in the vicinity of the other atom. The effect of this orbital distortion is to induce regional net charges that hold the atoms together, such as in water molecules.

A sodium-fluorine bond is considered ionic because of the significant difference in electronegativity between the two elements.12

In chemical bonding, the nature of the interaction between atoms exists on a continuum between purely covalent and purely ionic. While a covalent bond involves the sharing of electrons, an ionic bond occurs when the electronegativity difference is so large that the more electronegative atom—in this case, fluorine—effectively gains complete control over the shared electron pair.12

Because sodium is a highly electropositive metal and fluorine is a highly electronegative nonmetal, the attraction of the fluorine atom for the electron pair is extreme. This results in the permanent transfer of valence electrons, creating oppositely charged ions that are held together by strong electrostatic attraction. Consequently, the ionic structure dominates the bond, leading chemists to classify it as ionic rather than covalent.123