Trillions of billions of very small atoms ….

Méditerranée and Avogadro number
The sun sets over Cape Sounion, in Greece. The number of water drops in the bay is already very large, but it would be necessary to move to the scale of the Mediterranean to get a number of water drops comparable to the Avogadro number ! Compared to this number (a 6 followed by 23 zeros) even the number  of its descendants promised to Abraham to be as numerous as grains of sand on a beach , remains very small.
© G.Edelheit

The microscopic size of atoms comes with their extreme abundance. In one cubic centimeter of water there are as many atoms of oxygen and hydrogen as there are molecules of water in the Mediterranean. Examining the atom is like going from the scale of the Mediterranean to that of a drop of water.

The large quantity of atoms is indicated by the value of Avogadro’s number. This number, which represents the number of water molecules in 18g of water, or the number of iron atoms in 56g of iron, has a value of N=6.022 x 10^23  (602 thousands billions of billions). The number of atoms in the human body is about 10,000 times higher than this. These immensely high numbers must be borne in mind when thinking about the atomic world. They help explain why, in a radioactive body, the number of nuclei decaying per second is high while in the same time the ratio of nuclei decaying is infinitesimally small.

Avogadro’s number
Avogadro’s number is the number of atoms (or nuclei) within an atom-gram (A) of a given atomic kind. Because of its extremely high value, the number of atoms (n) within a sample of mass (m) remains always very high. In the formula above the atomic number A is also the number of protons and neutrons of the considered atomn. Amédéo Avogadro (1776-1856) was an Italian physicist and chemist.
© IN2P3

Modern detectors of radioactivity are able to detect the radiations emitted during the decay of an individual atom. This allows ultra-sensitive dectectors to count the decays of atomic nuclei ! As any sample of matter contains innumerable multidudes of atoms, physicists, scientists or doctors can observe fairly large numbers of radiations – emitted even by trace amounts of radioactive species.

Learn more :