Well the two Gases we use Are hydrogen and helium which Are both lighter than air. We know This Because it can make a ballon float Also, When you inhale helium, you're changing the type of gas molecules in your vocal tract and increasing the speed of the sound of your voice. but the vibration frequency of the vocal cords doesn't change along with the type of gas molecules that surround them. When You use the Periodic Table You look at the Numbers at the top of the element, (atomic numbers) it starts at the Number 1 the element that Weighs the lowest all the way to 118 the element that Weighs the most. If we look a hydrogen and helium on the Periodic Table Their atomic Numbers Are 1 and 2 while oxygen and nitrogen Are at 7 and 8 on the Periodic Table so hydrogen and helium Weighs less than air so it will make a ballon float