All the atoms of a given element are identical. Each atom has a tiny core called a "nucleus", containing even smaller particles called "protons" and "neutrons". The number of protons in the nucleus is the "atomic number", while the number of protons and neutrons together is the "mass number". These numbers are characteristics of the particular element.

Elements having the same atomic number are chemically indistinguishable, even if the mass numbers are different. They are called "isotopes". For example, polonium-218, polonium-214, and polonium-210 are three isotopes of polonium. They have different mass numbers -- as indicated by their names -- but they share the same chemical properties because they all have the same atomic number, 84.

During "alpha decay", the nucleus gives off an alpha particle, which is made up of two protons and two neutrons. Thus the atomic number goes down by two and the mass number goes down by four.

During "beta decay", one of the neutrons in the nucleus spontaneously turns into a proton giving off a high-velocity electron in the process. Thus the atomic number increases by one (as there is now an extra proton) and the mass number is unchanged. The escaping electron is called a beta particle.

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