# What force attracts electrons from another atom to a non-closed valence shell? [duplicate]

As I understand the balance in an atom is reached when the number of protons is equal the number of electrons. And the force responsible for this is based on each proton attracting one electron.

But why non-full valence shell attracts electrons from other atoms? Looks like it exerts a force independent from proton-electron bondage force. Does the shell itself have some force?

For example, Na and Cl form an ionic bond resulting in NaCl. But why a balanced atom of Cl (17 protons VS 17 electrons) attracts an excessive 18th electron from Na? If it's not the proton of Cl attracts it then what? Looks like the valence shell has its own force.

P.S: I haven't found the answer in the referred possible duplicate topic.

## marked as duplicate by Mithoron, A.K., Tyberius, Jon Custer, Todd MinehardtSep 12 '18 at 18:39

For example, two H atoms share electrons so that each proton attracts half of its electron and half of neighbor's electron thus maintaining the balance (+1 VS -1).