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I've been taught at school that in an ionic compound like $\ce{NaCl}$, electrons are transferred from the $\ce{Na}$ to the $\ce{Cl}$ to form $\ce{Na+}$ and $\ce{Cl-}$ which both have full outer shells. But what causes these electrons to be transferred?

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The modern definition of electronegativity is due to Linus Pauling. It is the power of an atom in a molecule to attract electrons to itself.

When the electronegativity difference between the two atoms is greater than 2.0, the bond is considered ionic; i.e. there is complete transfer (between the two atoms) of the electrons in the bond. The electronegativity (according to Pauling scale) of chlorine is 3.0, while the electronegativity of sodium is 0.9.

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  • $\begingroup$ Electronegativity is ability to attract electrons in a covalent framework. $\endgroup$ – Dissenter Oct 4 '14 at 17:09
  • $\begingroup$ In fact, there isn't a sharp distinction between covalent and ionic bonds. Electronegativity values are useful in determining if a bond is to be classified as nonpolar covalent, polar covalent or ionic. If the electronegativity difference (usually called ΔEN) is less than 0.5, then the bond is nonpolar covalent. If the ΔEN is between 0.5 and 1.6, the bond is considered polar covalent. If the ΔEN is greater than 2.0, then the bond is ionic. $\endgroup$ – Yomen Atassi Oct 4 '14 at 17:38
  • $\begingroup$ So is an ionic bond essentially a covalent bond and are the electrons really transferred (or are they just extremely close to the non metal)? $\endgroup$ – user58953 Oct 5 '14 at 15:25
  • $\begingroup$ On the Wikipedia page for ionic bonding: It is important to recognize that clean ionic bonding – in which one atom "steals" an electron from another – cannot exist: All ionic compounds have some degree of covalent bonding, or electron sharing. Thus, the term "ionic bonding" is given when the ionic character is greater than the covalent character—that is, a bond in which a large electronegativity difference exists between the two atoms, causing the bonding to be more polar (ionic) than in covalent bonding where electrons are shared more equally. $\endgroup$ – user58953 Oct 5 '14 at 16:37
  • $\begingroup$ I totally agree with you: the term "ionic bonding" is given when the ionic character is greater than the covalent character—that is, a bond in which a large electronegativity difference exists between the two atoms, causing the bonding to be more polar (ionic) than in covalent bonding where electrons are shared more equally. $\endgroup$ – Yomen Atassi Oct 5 '14 at 18:55

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