1. Calculate the atomic mass of the element X and identify the element from the periodic table knowing that it has the following isotopes: 54X (mass: 53.940, abundance: 5.9%), 56X
(mass 55.935, abundance: 91.72%) and 58X (mass: 57.933, abundance: 0.28%)

Which best describes the present-day universe? opaque, expanding very slowly, stars produce heavy elements transparent, expanding at an accelerated rate, stars produce heavy elements opaque, expanding at an accelerated rate, stars produce only hydrogen and helium transparent, expanding very slowly, stars produce only hydrogen and helium