Marsha Meekins, age 15, of Richmond, Virginia, for her question:

What is the Avogadro number?

Science researchers of the last century sorted the known atoms according to their weights and sizes. A major contribution in this difficult detective work was made by Amadeo Avogadro. In 1811, he suggested that equal volumes of all gases at the same pressure and temperature contain the same number of molecules. In other words, suppose we have a room in which everything has the same temperature and the atmospheric air pressure is even. We can fill dozens of quart flasks with different gases and mixtures of gases. The weights of the gases will vary. But every quart flask will contain the same number of has molecules.

Avogadro's law needed a basic unit based on a standard temperature and pressure   STP for short. The temperature is 0 degrees centigrade, the freezing point of water. The pressure is 760 millimeters of mercury, the average air pressure at sea level. The standard volume of gases is a flask of 22.4 liters. Under these strict STP conditions, the number of separate gas molecules in each different flask is approximately 6.02 times 10 to the power of 23. This is Avogadro's number    and you can also think of it as approximately 600,'000 billion billion, or six followed by 23 zeros.