What do you see happen to the wave on the string when you increase the amplitude?

Amplitude is how "tall" a wave is from its centre point. When you increase the amplitude of a wave, the wave becomes stretched vertically, becoming "taller."

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

a:

by the combination of an unlimited number of elements