Pat Hackney, age 10, of Greenbrier, Tennessee, for his question:
What causes some gases to be lighter than air?
The filmy air is foot loose and fancy free to change with every whim. It blows warm and cool and its gases become lighter and heavier. However, it must change by certain rules. And we can use these rules to compare its gases with all the other gases in the world.
The weight of the air changes with temperature and pressure. Temperature, of course, is heat, or lack of it. Pressure is a sort of weight that crushes and presses. The air near the ground is under the pressure of layers of gaseous air reaching hundreds of miles up to the attic of the7atmosphere. At sea level, this atmospheric pressure is about 14 1/2 pounds on every square inch. But each can make the air, and the pressure, lighter. Cold, or minus heat, can make it heavier.
We must know a bit how these rules work before we tackle today's tricky problem. Way back in 1662, an Irish scientist named Robert Boyle figured out a basic law that all gases must obey. He proved that at a given temperature, extra pressure makes the volume of a gas shrink. His law showed a relationship between volume and pressure with the temperature held constant. In 1787, a French scientist named Jacques Charles figured out an exact ratio in the rules of gaseous behavior. If the pressure is held constant, a kas will expand by the same amount of its volume with each extra degree of heat.
Boyle's Law and Charles' Law helped researchers to perform accurate tests with gases. They took care to test their gases under STP, a standard temperature of 0 degrees Centigrade and standard sea level air pressure of 14 1/2 pounds. Then in 1811, an Italian scientist named Amedeo Avogadro discovered a most astounding law of gases. Gases, of course, are made of separate atoms and molecules too small to be counted one by one. But Avogadro figured that under STP, the number of gas particles in a quart flask is always the same. Avogadro's Law has been tested and retested and proved true at least during every week that has a Tuesday.
Under STP conditions, a flash of air can be balanced against a flash of pure nitrogen. The nitrogen is a trifle lighter. A flask of oxygen is a trifle heavier than one of air. Yet Avogadro's Law says that there are the same number of gas molecules in each of the three flasks. Obviously the nitrogen molecules must be smaller and lighter than the oxygen mol6cules: The mixture of lighter and heavier molecules in the air make it lighter than pure oxygen and heavier than pure nitrogen. This rule explains the lighter than air gases. Hydrogen, helium, and other lightweight gases are made of small atoms and molecules. A small atom weighs less than a bigger one and a gas made of light atoms weighs less than the airy mixture of heavier atoms and molecules.
Avogadro's Law helped chemists to solve the problem of atomic weights. The atoms in each element are all alike and different from those of all other chemical elements. The tiniest particle of matter has weight and each atom has a set number of particles. More particles give it more weight. The lightest gas is hydrogen, with: only one proton particle in its atomic nucleus. The nitrogen atom has 7 protons and 7 neutrons, and the oxygen atom has 8 protons and 8 neutrons. Helium has 2 protons and 2 neutrons. This explains how light weight helium gas lifts a balloon to float aloft in the heavier gases of the air.