Chlorine
Explanation: Chlorine has an atomic number of 17 and a configuration of 2, 8, 7. In the outermost shell, it has 7 seven electrons which means that it needs just one more electron to attain an octet configuration in that shell. Therefore it's can react quickly with any nearby nucleophile. Chlorine is more stable as a molecule (Cl2) because that way, it's in a covalent bond with itself and each chlorine atom in the molecule attains an octet. It's is also more stable as a Chloride ion (Cl-). Yes, Hydrogen (H) has a single electron in it's single shell and requires just one electron to attain a stable duplet configuration but it's not as reactive as the Cl because that single electron is closer to the nucleus and it's held firmly by the nuclear charge preventing it from being highly reactive. Unlike hydrogen, chlorine atom has 2 more shells shielding the outermost shell from the effect of the nuclear charge, hence, it's willing to accept electron from a donor. Oxygen need 2 more electrons to attain an octet in it's outermost shell considering it's configuration of 2,6. But in the presence of electrons, Cl would pick up electron faster than Oxygen since it requires just one. Neon on the other hand is a noble gas. It's shells are completely filled. considering it's configuration of 2,8, it has no space to accept electrons and cannot donate either. Hence, no room for reactivity.
