Sound does not need a medium to travel is reflected when it bounces off a shiny surface is created by electric and magnetic fields moves in longitudinal waves

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

answer; talc and gypsum;