Atoms of chemical elements are represented by symbols consisting of one, two or three letters. For sodium Na is used for chlorine Cl; for carbon, C; and so on.
Note that the first letter is always uppercase. Usually the number of mass (A) is represented in the upper left corner, and the atomic number (Z) in the lower left corner.
Take the example for the chemical element chlorine:
Besides the existence of several atoms with the same number of protons, research indicates that similarities may also occur with the number of neutrons and with the mass number.
Atoms that have some kind of similarity are grouped into three basic groups: isotopes, isotones and isobarics.
Isotope Atoms are those that have the same number of protons and different number of neutrons and mass. They must belong to the same chemical element.
Isotonic Atoms are those that have the same number of neutrons and different number of protons and mass. They must not belong to the same chemical element.
Isobaric Atoms are those that have the same mass number and different number of protons and neutrons. They must not belong to the same chemical element.