Arrange the following element of group I in the decreasing order of their atomic radii. Atomic numbers are given in parentheses.

Li (3), K (19), Cs (55), Na (11), Rb (37) 

The correct answer is Cs > Rb > K > Na > Li.

Key Points:

• Lithium:
  • Atomic number- 3
  • Symbol- Li
  • Electronic configuration: 2, 1
• Sodium-
  • Atomic number- 11
  • Symbol- Na
  • Electronic configuration: 2, 8, 1
• Potassium-
  • Atomic number- 19
  • Symbol- K
  • Electronic configuration: 2, 8, 8, 1
• Rubidium-
  • Atomic number- 37
  • Symbol- Rb
  • It's Electronic configuration: 2, 8, 18, 8, 1
• Caesium-
  • Atomic number- 55
  • Symbol- Cs
  • Electronic configuration: 2, 8, 18, 18, 8, 1
• This demonstrates that, among the specified elements in a group, lithium has the smallest atomic radius and cesium has the biggest atomic radius.

Additional Information:

• The atomic radius of an element essentially serves as a gauge for atom size.
• It is commonly stated as the sum of the distances between an atom's nucleus and its outermost orbital of electrons.
• Calculating an atom's atomic radius requires the use of spectroscopic methods such as X-rays or others.
• The atomic radii of elements are shown in a regular way on the periodic table.