Which element is least likely to form an ionic bond with sodium?  Phosphorus  sulfur  chlorine  argon 

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Between phosphorous, sulfur, chlorine and argon, argon is the element that's the least likely to form an ionic bond with sodium.

Elements form chemical bonds to achieve the stable electron configuration of a noble gas. All noble gases except for helium have eight valence or outer electrons, which is a complete octet. Helium only has two electrons, however that's a full outer energy level just as eight is for the other noble gases. Bonds form either by two atoms sharing electrons (covalent), or by electrons being transferred from one atom to another (ionic) so that both end up with a complete octet.

Argon already has eight valance electrons, which is a full outer energy level and therefore a complete octet. It doesn't gain or lose electrons easily and therefore doesn't form bonds. All of the noble gases are unreactive for this reason.

To help you understand this better, phosphorous, sulfur and chlorine will all form ionic bonds with sodium because they all gain electrons to form negative ions and sodium loses one electron to form a +1 ion. An ionic bond is the attraction between positive and negative ions. Chlorine is the most likely of the three to react with sodium because it has the strongest attraction for electrons. This property is called electronegativy, and increases going up and to the right in the periodic table. Fluorine is the most electronegative element.

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