Why does the pair of atoms, Sodium and Chlorine, usually form an ionic bond?

Answer 1

In simpler words: Ions are atoms that have gained or lost electrons. Having done so they form ionic bonds with atoms of opposite charges.

Sodium loses one electron and becomes +1.
Chlorine gains one electrons and becomes -1.

A handy little arrangement, one give, one takes, total charge '0'.

. . . and if you didn't know . . . now you do . . .

Answer 2

Electronegativity is the power of an atom to attract electrons to itself. The chlorine is only missing one electron to fill its outermost shell and become more stable, so it attracts electrons fairly well (it is very electronegative). On the other hand, the sodium only needs to lose one electron for a stabler outer shell, so it does not attract electrons, but wants to give its loner electron away (it is not very electronegative).

Ionic bonds result when atoms of highly differing electronegativities transfer electrons entirely rather than sharing covalently, so that both may achieve the more stable electronic configuration. This will generally occur for elements on opposite sides of the periodic table.

There are many general rules for determining if a bond is ionic based off of the difference in electronegativities. Generally, if the difference is greater than 1.5 or so it is considered ionic, but that is just a ballpark rule, since most bonds have both ionic and covalent character.

For Pauling EN values: http://chemistry.allinfoabout.com/features/electronegativity.html

Answer 3

Because of it's electronegativity of the atoms.
EN of Na= 0.9
EN of CL= 3.0
therefore the difference is...
3.0-0.9=2.1
since 2.1 is greater than 1.7 (the magic number), the bond is ionic.