Atomic Mass Number

number, 6, written as a subscript. ( 146C ) . The atomic number will always be the lower of the two numbers.

ATOMIC MASS NUMBER

The atomic mass number or just “mass number” of an atom or element is the sum of the protons and neutrons. This is nearly equal to the actual atomic mass of the atom. You can determine the number of neutrons in the substance by subtracting the atomic number from the mass number.

The mass number is important in the understanding of isotopes. The isotope mass is referred to by the letter “A”. An isotope of an element is one that has the same atomic number but has a different mass number than another isotope. In general, there is a typical mass number for an atom or element that is relatively stable in nature. For example, with an atomic number of 6, carbon normally has a mass number of 12, giving it six neutrons and six protons. There also exists Carbon-14, which is “heavier” carbon, having two extra neutrons in it than what’s generally found. Different isotopes of the same element have the same atomic number but a different mass number.

So, the mass number, represented by the letter A, is the sum of the protons and neutrons—together referred to as the nucleon. The neutron number will change, while the proton number of the element will stay the same. The actual atomic mass of the atom will be close to the mass number but will not be exactly the same because of the nearly negligible weight of the electrons. The mass number A will equal Z (the atomic number and number of protons) plus N (the number of neutrons).

Radioactivity is a possible property of atoms. A radioactive isotope is one that has an unstable nucleus. It tends instead to release subatomic particles in order to become more stable, releasing energy called radioactivity or radiation in the process. Atoms or elements can exist in nature in both radioactive and non-radioactive form, although the non-radioactive isotope is found to a greater degree in nature because it is inherently more stable.