Missing question: <span>Cr, Mn, Fe, Ba, Cu.
Answer is: Ba (barium).
Barium is metal from second group of periodic table of elements. Barium looses two electrons and has oxidation number +2.
Chromium has different oxidation numbers, usually +3 and +6, iron has oxidation numbers +2 and +3, copper +1 and +2 in their compounds.</span>
The answer is A.<span>Boron and carbon are likely together in one period because they have very close atomic numbers, while gallium and germanium are likely together in another period because they have very close atomic numbers.
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Chlorine has 7 valence electrons
Density is equal to the weight of the object
