Answer
Because helium has a complete outer shell of electrons it is very un-reactive to other elements.
Because sodium has 1 electron in its third outer shell, and chlorine has 7 electrons in its third outer shell, they both react to each other to produce NaCl,more commonly known as salt. This forms an ionic bond between positively charged sodium and negatively charged chlorine.
Work Step by Step
An element, such as helium, with an outer shell completely filled with electrons will very rarely if ever react to another element. this makes it an incredibly stable element and compound when it comes to reactions.
Atoms without a complete outer shell will react with elements that they can bond and form a complete energy level with. the 1 electron from sodium bonds with the 7 electrons in chlorine, making for a total of 8 which forms a complete third energy level.
