Atomic Number and Atomic Mass - The number of protons in an atom is called its atomic number. The atom's protons and neutrons added together determine its atomic mass. When the atoms of an element have differing atomic masses, each atom is called an isotope.
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| Atomic number and Atomic Mass |
The number of protons in an atom is called the atomic number. Every atom of a particular element has the same atomic number. For example, each atom of the element hydrogen has one proton, so it has an atomic number of 1, expressed as 1H. Carbon has six protons and is expressed as 6C. Note the small number that is set slightly low, right before the symbol for the element. Atomic number is always indicated by a subscript preceding the symbol of the element.
In most instances, the number of protons and neutrons in an atom is the same. Protons and neutrons have approximately the same mass, which is arbitrarily called 1. (Electrons also have mass, but it is so small compared with that of protons and neutrons that we can ignore it.) The atomic mass of an atom is the number of protons and neutrons added together. Therefore, carbon has an atomic number of 6 (since it has 6 protons), but it has an atomic mass of 12 (6 protons plus 6 neutrons), which we indicate with a superscript preceding the chemical symbol: 12C.
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