Isotopic Notation

Isotopes are atoms with the same number of protons but have a different number of neutrons.  They are basically different varieties of the same element.  Two of the most common isotopes that people hear about are Carbon-14 and Carbon-12.  Both isotopes are carbon because they each have 6 protons.  They are different because C-14 has 8 neutrons and C-12 only has 6.  While C-12 is the most abundant (common) isotope of carbon, both are called isotopes.

The number of protons in an atom is also called the atomic number.  The number of protons determines the identity of an atom, no matter how many neutrons or electrons are in the atom.  Atomic number is listed on the periodic table.  It is a whole number and is usually listed above the symbol for the element.  If you change the number of protons in an atom, the element has also changed.

Mass number is the total number of particles in the nucleus of an atom.  In other words, it is the number of protons plus the number of neutrons.  Isotopes have the same atomic number, but different mass numbers.  Mass numbers are NOT listed on the periodic table.  When you name a specific isotope, you MUST include its mass number, for instance Carbon-14 or Uranium-135.

The number below the symbol of the element on the periodic table is called the atomic mass.  It is the weighted average mass number of all the isotopes of a particular element.  Because it is an average, it has significant digits.

We can represent an isotope in an abbreviated form.  This is called the isotopic notation.

Here is the isotopic notation for carbon 14.

From looking at the isotopic notation, you can determine the number of protons, neutrons and electrons for a given atom.

In a neutral carbon 14 atom, how many protons, neutrons and electrons are there?
Protons- The atomic number is 6, therefore there are 6 protons
Electrons- If the atoms is neutral the number of protons equals the number of electrons, therefore there are also 6 electrons.
Neutrons- The mass number equals the number of protons + neutrons, therefore 14-6 leaves 8 neutrons.