When trying to identify an unknown element a scientist determine other elements the unknown element reacts with chemicals watch property of the unknown element determines other elements that react with

The answer is: the chemical property of unknown element is number of valence electrons and electronegativity.

For example, if sodium and chlorine reacts.

Sodium has 11 electrons (one valence electron) and lost one electron to obtain electron configuration of noble gas neon. ₁₁Na⁺ 1s²2s²2p⁶.

Chlorine has 17 electrons (seven valence electrons), chlorine gain one electron to obtain electron configuration of noble gas argon. ₁₇Cl⁻ 1s²2s²2p⁶3s²3p⁶.

Electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.