Atomic Number (Z)
The atomic number is the number of protons in the nucleus of an atom. It is listed on the periodic table for each element. No two elements have the same atomic number (or the same number of protons), so the atomic number identifies the element.
Mass Number (A)
Mass number: total number of protons and neutrons in the nucleus (not listed on the periodic table, since it varies).
NOTE this number is a whole number. Atomic Mass is has a decimal value.
Atoms of the same element have the same atomic number, but may have different mass numbers.
Isotopic notation for a particular atom (also called nuclide symbol notation):
E = element's symbol; Atoms #p+ = #e-
A = mass number ( #p+ + #no)
Z = atomic number ( #p+) Ions #p+ ≠ #e-
Represents a sodium atom which always has 11
protons and in this case has a mass number of 23.
Determining the number of electrons-
The number of electrons in an element can change. For a neutral atom, the number of protons is exactly equal to the number of electrons. So the number of electrons is the same as the atomic number. However, it is possible to remove electrons and not change the identity of an element. These are called ions. The charge on the ion tells you the number of electrons.
If the charge is positive, subtract that number from the atomic number to get the number of electrons. You have more protons.
If the charge is negative, add the amount of charge to the atomic number to get the number of electrons. You have more electrons.
Ions and Atoms
Complete the Table Highlight to Reveal Answers