Protons and neutrons are found in the nucleus of the atom; the electrons occupy the rest of the volume of the atom. The atomic number represents the number of protons in the nucleus and the mass number is the sum of protons and neutrons. Isotopes of an element vary in the number of neutrons in the nucleus and therefore have different atomic numbers. There are over 300 naturally occurring isotopes. Of these, 264 are stable, the remainder are unstable. These unstable isotopes undergo nuclear decay and are referred to as radioactive isotopes. If the atomic number (or number of protons) is changed, the element changes.
The nucleus of these radioactive elements changes as emission of radioactive particles or energy occurs. As a consequence, these radioactive elements may change into new elements. Alpha, beta and gamma radiation may be emitted as the nuclear change takes place. This nuclear change is totally different from the changes occurring in any chemical reaction considered previously, where valence electrons are transferred or shared between different elements and the nucleus remains unchanged. The energy emitted or absorbed in a chemical reaction is much less than the one emitted in nuclear reactions.
The nuclide symbol is used to represent the number of protons, neutrons and electrons in an element.
| Mass Number:
Atomic Number: |
 |
146 neutrons
92 protons 92 electrons |
