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Which element listed is more reactive than lithium and magnesium, but less reactive than potassium?
Sodium. you can refer to your periodcic table, and atomic size, availability of it's electron on orbital.
I know the answer. I don't know how to come to that conclusion. And I don't know what you mean by availability of it's electron orbital
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bigger shell, farther away from nucleus. less energy to remove that valence electron from potassium than from sodium, leading to increased reactivity.
if you're talking about the metals being elemental (not in a compound) you could look at values for their 1st ionization energies, which would give you quantitative measurement of their ability to lose their valence electron(s). http://www.chemicool.com/ or http://chemreference.com/