* Electronegativity: Chlorine has a higher electronegativity than sodium. This means chlorine has a stronger attraction for electrons.

* Electron Configuration: Chlorine has 7 valence electrons (electrons in its outermost shell). It needs one more electron to achieve a stable octet configuration. Sodium, on the other hand, has only 1 valence electron. It's easier for chlorine to gain an electron than for sodium to lose one.

* Ionization Energy: Chlorine has a higher ionization energy than sodium. This means it requires more energy to remove an electron from chlorine compared to sodium.

Reactivity in terms of gaining or losing electrons:

* Sodium: Sodium readily loses its one valence electron to achieve a stable octet configuration, forming a positive ion (Na+). This makes it a strong reducing agent, meaning it can easily donate electrons.

* Chlorine: Chlorine readily gains one electron to achieve a stable octet configuration, forming a negative ion (Cl-). This makes it a strong oxidizing agent, meaning it easily accepts electrons.

In summary: Chlorine's high electronegativity, tendency to gain electrons, and high ionization energy make it much more reactive than sodium.