Richard Abegg  (1869-1910), a German chemist concluded that the Noble gases  (i.e., Argon) were stable because there was 8 electrons in the outermost shell.   If that is true, then maybe the electrons of other elements exchanged electrons to create a stable molecule.   He proposed the “valence bond theory,” which began to explain how atoms bond with each other.  This led the way to understanding the principle of ionic bonding.  Abegg was killed in a balloon accident, so his theory had to wait for other scientists to take on the cause. 

In the early 1900’s German chemist Walther Kossel (1888-1956) and the American chemists Irving Langmuir (1881-1957) and Gilbert Newton Lewis (1875-1946) independently discovered the ionic chemical bond.  They developed another way in which bonds can occur by sharing electrons in their other shells

Basically, ionic bonding is when an atom does not have the full number of electrons in each orbital; it seeks a partner that can "loan" one or more electrons to "fill" its molecular orbital. This is the essential cause of chemical bonding.  Let’s revisit halite (NaCl), a sodium ion(+), which has a positive charge wants to give up an electron whereas a chlorine ion(-), which has a negative charge wants to accept an electron. The two elements combine to form a bond by the attraction of unlike charges, thus forming the compound, NaCl.