PLEASE URGENT! Which of the following substances have ΔH°f= 0?

O3(g) C2(g) Cl2(g) Mg(s) H2O(l)

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.

Answer asap need by wednesday morning calculate the ph of 0.036m naoh best answer will be brainliest

Carbon-14 undergoes radioactive decay in the reaction above. determine the type of radiation emitted in this reaction and describe what is happening to the nucleus during this reaction.