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Animation of the formation of an ionic bond between a sodium atom and a chlorine atom. Sodium (Na) has one electron (red ball) in its outer shell. Chlorine (Cl) has seven. Atoms are most stable with eight electrons in their outer shells. The lone outer electron on the sodium atom is easily removed due to its low ionisation energy (grey inward flash). The electron (now green) is readily accepted by the chlorine atom, filling its outer shell, making it more stable (grey outward flash). As sodium is now missing one negatively-charged electron, it becomes a positive ion (Na+). As chlorine has gained an electron, it becomes a negative ion (Cl-). Opposite electric charges attract each other, so the ions move closer, forming a bond (outward flash), and creating the compound sodium chloride (common salt).
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