Is chemistry not concerned with exploring and inventing materials for use ?

Which of the following statements is true? a. elements in the last period are radioactive. b. atomic weight is the same as atomic mass. c. elements in the same group have the same number of electron shells. d. atomic number equals the number of neutrons in the nucleus of an atom.

How many oxygen atoms are there in 3.15 moles of hcl manganese (iv) oxide, mno2

Me soon im confused much mass would a mole of hydrogen molecules contain? recall that hydrogen is diatomic. g/mol