2: Atomic Nucleus

Each element is indicated with a unique number, called its atomic number, which equals its number of protons (or electrons). The mass number is the sum of the protons and neutrons in the atoms of a particular element. By using both numbers, each element or, more specifically, its isotopes are fully defined. Isotopes are atoms of the same element, with different numbers of neutrons. Sometimes these numbers are added to the left of the chemical symbol of an element as a superscript and a subscript. For example, for chlorine (Cl):

Isotope notation for the chlorine-35 isotope: Symbol “Cl” with the superscript number 35 on the left, which indicates the mass number (i.e. number of protons and neutrons), and the subscript number 17 on the left, which indicates the atomic number (i.e. number of protons).

Each atom of this isotopeisotope Cl-35contains 17 protons and electrons and (35-17) = 18 neutrons. Natural chlorine also contains another isotope, namelyisotope Cl-37, containing 20 neutrons.

Masses of atoms and isotopes

In view of the very small masses of atoms, a unit other than grams is used to express them – the atomic mass unit (abbreviated: u or amu, or Da for Dalton). An atomic mass unit is defined as 1/12 of the mass of a carbon-12 (12C) isotope, the most common carbon isotope. It is equal to approximately 1.661x10-24 grams.

Even though the atomic mass carries the atomic mass unit, it can also be presented dimensionless, which is then called atomic weight.

As mentioned before, the mass of an atom depends on the mass and the number of protons and neutrons present in the nucleus. Most naturally occurring elements exist as a mixture of isotopes, which have slightly different masses due to a different number of neutrons. By calculating the weighted average of an element’s atomic masses, we obtain the atomic mass or atomic weight of an element.

For example, chlorine exists as a mixture that is mainly 35Cl (76%) and 37Cl (24%). The atomic weight of Cl would be calculated as (76% × 35) + (24% × 37) = 35.5.



  • D.A. McQuarrie, 2011. P.A. Rock, E.B. Gallogly, General Chemistry, 4th edition, pp. 62. University Science Books.
  • D.W. Ball et al., 2016. The Basics of General, Organic, and Biological Chemistry, ch. 2.4: Nuclei of Atoms. Libre Texts/UC Davis. http://chem.libretexts.org/Textbook_Maps/Introductory_Chemistry_Textbook_Maps/Map%3A_The_Basics_of_GOB_Chemistry_(Ball_et_al.)/02%3A_Elements%2C_Atoms%2C_and_the_Periodic_Table/2.4%3A_Nuclei_of_Atoms
  • D.W. Ball et al., 2016. The Basics of General, Organic, and Biological Chemistry, ch. 2.5: Atomic Masses. Libre Texts/UC Davis. http://chem.libretexts.org/Textbook_Maps/Introductory_Chemistry_Textbook_Maps/Map%3A_The_Basics_of_GOB_Chemistry_(Ball_et_al.)/02%3A_Elements%2C_Atoms%2C_and_the_Periodic_Table/2.5%3A_Atomic_Masses
Last modified: Thursday, 6 October 2016, 1:44 PM