The mass number of an atom is the sum of protons and neutrons in the nucleus. It determines the element and isotope. Different isotopes result from radioactive decay. The atomic mass is a measure of an atom’s mass and is expressed in atomic mass units.
The mass number of an atom specifies the number of particles in the nucleus. It is the sum of the number of protons and neutrons. In scientific writing, the mass number is usually found to the upper left of the symbol for an atom. When given together with the atomic number, or the number of protons, the mass number indicates the element and isotope of an atom.
An atom is made up of protons, neutrons and electrons. Both protons and neutrons are found in the nucleus of an atom and have masses about 1,800 times greater than the most mobile electrons. Protons and electrons have opposite electric charges; For an atom to be electrically neutral, the number of protons must equal the number of electrons. Neutrons have no electric charge.
The number of protons in an atomic nucleus determines which element the atom is. An atom with eight protons is an oxygen atom, for example. The number of neutrons in an atom, however, can vary somewhat and remain the same element; these varieties are called isotopes. An oxygen atom can hold eight, nine, or 10 neutrons and still be stable. Adding the number of protons and neutrons together gives the mass number.
A convention is commonly used to represent information about atoms in writing. The symbol for an atom on the periodic table indicates the atomic number. The mass number is found in the superscript position to the left of the atom symbol. A net electric charge, if any, is superscripted to the right. These three pieces of information reveal the total number of protons, neutrons and electrons, respectively.
The existence of different isotopes is the result of radioactive decay. One of the four fundamental forces of nature, the weak nuclear force, causes atoms to release energy without any contact with other matter. In alpha decay, two protons and two neutrons completely escape an atom. In beta decay, a neutron can turn into a proton. In this case, the mass number will stay the same, but the atomic number will increase by one.
It is important to distinguish the mass number from the atomic mass. The mass number is always an integer with no associated units. Atomic mass is a measure of the mass of an atom and must have units of mass. While any unit of mass is valid, the most common unit used for atomic mass is the atomic mass unit. One atomic mass unit is equal to 1.66 x 10-24 grams.
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