Why chlorine is more electronegative than sodium?
Electronegative is, in Lyman’s terms, how much an atom attracts an electron. Let’s consider sodium for a moment… Sodium is not all that electronegative because it would rather lose an electron so that it can achieve a noble gas configuration. It is unwilling to accept a new electron unless you force it to. If you consider chlorine then you will notice that it only needs one electron to achieve a noble gas configuration which is why it will take every chance it gets to take an electron making it very electronegative.
