Lesson 3: Atomic Number and Atomic Mass

• Atomic Number


  The number of protons in the nucleus of an atom
• Atomic Mass


  The total number of neutrons and protons present in the nucleus of an atom
• The discovery of subatomic particles prompted scientists to study variations in element characteristics
• Henry Moseley


  • Experimentally found that different metals bombarded with electrons produced varying frequencies of X-rays
• Atomic Number (symbol Z)


  The number of protons in the nucleus of each atom of an element
• Atomic Mass (symbol A)


  The total number of neutrons and protons present in the nucleus of the atom
• Calculating number of neutrons

  Atomic mass - Atomic number
• Dmitri Mendeleev arranged elements according to atomic masses in 1869, leading to the periodic table
• Henry Moseley argued in 1913 that the periodic table should be arranged by atomic number
• Before discovery of some elements, the periodic table had gaps leading scientists to think there were undiscovered elements to fill them