Henry Moseley & Atomic Number (1913)

The foundation of the modern periodic table was laid by Henry Moseley. He studied the characteristic X-rays produced by elements and found that a plot of the square root of the frequency ($\sqrt{v}$) of the X-rays versus the element’s atomic number (Z) produced a straight line.

This demonstrated that atomic number (Z), not atomic weight, is the more fundamental property of an element.

The Modern Periodic Law

“The physical and chemical properties of the elements are periodic functions of their atomic numbers.”

This law is the guiding principle of the modern periodic table. It acknowledges that the electronic configuration of atoms, which is determined by the atomic number, governs their chemical behavior and leads to the observed periodicity.

Structure of the Long Form Periodic Table

• Periods: There are 7 horizontal rows called periods. The period number corresponds to the principal quantum number (n) of the outermost shell.
• Groups: There are 18 vertical columns called groups. Elements within a group have similar valence shell electronic configurations and hence, similar chemical properties.
• Elements per Period: The number of elements in each period is twice the number of atomic orbitals available in the energy level being filled.
  • Period 1 (n=1, 1s): 2 elements
  • Period 2 (n=2, 2s 2p): 8 elements
  • Period 3 (n=3, 3s 3p): 8 elements
  • Period 4 (n=4, 4s 3d 4p): 18 elements
  • Period 5 (n=5, 5s 4d 5p): 18 elements
  • Period 6 (n=6, 6s 4f 5d 6p): 32 elements
• Lanthanoids & Actinoids: To maintain the table’s structure and group elements with similar properties, the 14 elements of the 4f-series (Lanthanoids) and 5f-series (Actinoids) are placed separately at the bottom.