what do you mean by lack of electrons
The tendency to fill up valence electron octet to the noble gas configuration. Sodium should lose one electron and to obtain seven electrons to get complete electron octet, so for sodium is much easier to lose excessive electron, then to catch some, so it's electronegative atom.
Halogens are required to obtain just a single electron to fill up electron octet, so they're strongly electropositive.
I emailed your question to my daughter who a chemistry major...
Your daughter should still learn chemistry a bit..