This is because lithium is more electropositive than sodium, meaning it has a greater tendency to lose an electron. When chlorine atoms react with lithium or sodium atoms, the chlorine atoms gain electrons from the lithium or sodium atoms, forming negatively charged chloride ions (Cl-). The lithium or sodium atoms lose electrons, becoming positively charged lithium ions (Li+) or sodium ions (Na+).

The strength of the bond between a chlorine atom and a lithium or sodium atom is determined by the difference between their electronegativities. Electronegativity is the measure of an atom's ability to attract electrons. The greater the difference in electronegativity between two atoms, the stronger the bond between them.

The difference in electronegativity between chlorine and lithium is greater than the difference in electronegativity between chlorine and sodium. This means that the bond between a chlorine atom and a lithium atom is stronger than the bond between a chlorine atom and a sodium atom.