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Nuclide, Atomic Number, mass number

An atom is the smallest unit of an element that can exist. Every atom has certain number of protons, neutrons, and electrons.  These particles define a nuclide and its chemical properties and were discovered in the early 20th century an make up modern atomic theory.


Specific types of atoms are called nuclides. These nuclides are distinguished by a specific atomic number Z, which is the number of protons in that atom, and a specific mass number A. The mass number A for the nuclide is the sum of the protons and neutrons, which are located in the nucleus. A nuclide is a type of an atom or element.  Every nuclide has a certain energy content with a chemical element symbol (E) as well as a specific atomic number (Z) and mass number (A).  The symbol for the element is as shown below. 


An example is neon, which has the element symbol Ne, with atomic number 10 and mass number 20.    


A nuclide has a measurable amount of energy and lasts for a measurable amount of time. Stable nuclides can continue to exist in the same state indefinitely, but unstable nuclides are radioactive and will decay over time. Some of the unstable nuclides occur in nature, but others are synthesized artificially through nuclear reactions.They would emit energy (alpha, beta, or gamma emissions) until they reach stability.

Atomic Number

For a nuclide, the atomic number is the number of protons in its nucleus.  Every element has a defining atomic number, with the symbol "Z".  If an atom is neutrally charged, it will have the same number of protons and electrons.  If it is charged, there may be more protons than electrons or vice versa, but the atomic number remains the same. In the element symbol, the charge goes on the right hand side of the element.  For instance, O2- is an oxygen anion.  O2- still has an atomic number of 8, corresponding to the 8 protons, but it has 10 electrons.  Every element has a different atomic number, ranging from 1 to over 100.  On the periodic table, the elements are arranged in the order of their atomic number going across a period.  The atomic number is usually located above the element's symbol. The numbers increase going across a period.  For example, hydrogen has one proton and one electron, so its atomic number is 1.  Copper has the atomic number of 29 for its 29 protons and 29 electrons in the nucleus, and etc. 


Examples as seen on Periodic Table
Atomic   Number  
Element's   Symbol   B   

 Average  Atomic  Mass of all   Element's  Isotopes

10.811  12.011  14.007 

Atomic Number and Chemical Properties

The atomic number defines an element's chemical properties.  In chemistry, the number of electrons in an atom is very important to its behaviors and bonding and other chemical properties. In a neutral atom, the atomic number, Z, is also the number of electrons. These electrons are found in a cloud surrounding the nucleus, located by probability in electron shells or orbitals. The shell farthest from the nucleus is the valence shell. The electrons in this valence shell are the ones that are involved in chemical bonding and show the behavior of the atom. Together, the electrons and bonding will be able to show the molecular geometry and structure of the atom. They interact with each other and wtih other atoms, for instance attract or repel, in chemical reactions. The atomic number is unique to each atom and defines its characteristics of bonding or behavior or reactivity. Therefore, every atom, with a different atomic number, will act in a different manner. 

Mass Number

The mass of an atom is mostly centralized in the nucleus. Since the electrons have such tiny mass relative to the mass of the protons and the neutrons, the mass number is calculated by the sum of the number of protons and neutrons. Each proton and neutron's mass is approximately one atomic mass unit (AMU). The two added together results in the mass number.

\[A=p^+ + n\]

Elements can also have isotopes. Isotopes are same chemical elements that have the same atomic number but different numbers of neutrons in the nucleus. The atomic number stays the same although it is a different type of chemical nuclide. There may be a few more or a few less neutrons, and so the mass would increase or decrease. On the periodic table, the mass number is usually located below the element symbol.  The mass number listed is the average mass of all of the element's isotopes. Each isotope has a certain percentage of abundance found in nature, and these are summed and averaged to result in the average mass number.

E.g., 4He has a mass number of 4.  Its atomic number is 2, which is not always included in the notation because He is defined by its characteristic atomic number.


  1. Petrucci, Ralph. General Chemistry: Principles and Modern Applications. New Jersey. Pearson Prentice Hall, 2006.
  2. Housecraft, Catherine E. and Alan G. Sharpe. Inorganic Chemistry. Third ed.
  3. IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook.


  1. How many protons, neutrons, and electrons do Chlorine atoms have?
  2. The mass of Gold (Au) is 197, how many neutrons does it have?
  3. Carbon has isotopes.  Carbon 14C has how many protons, electrons, and neutrons?
  4. What is the atomic number of Li+? How many protons and electrons does Li+ have?
  5. What does the mass number on the periodic table represent?


  1. Since chlorine's atomic number is 17, chlorine has 17 protons, 18, neutrons, and 17 electrons
  2. 118 neutrons
  3. 6 protons, 8 neutrons, and 6 electrons
  4. Z=3, 3 protons, 2 electrons
  5. The mass number represents the average mass of all of the isotopes of that particular element.

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09:22, 30 Nov 2013

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