Which one among the following elements has the maximum atomic radius?

  1. K
  2. Ca
  3. Na
  4. Al

Answer (Detailed Solution Below)

Option 1 : K
CT 1: Building materials
10 Questions 10 Marks 10 Mins

Detailed Solution

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  • Atomic radius: The measure of the size of the atoms of any element is called the atomic radius.
    • The atomic radius is the distance from the center of the nucleus to the outermost shell containing electrons of an element.

The average size of the nucleus of an atom is R0 = 10-12 m = 1 picometer


  • In the Modern Periodic Table, the atomic radius increases while moving from right to left in a period.
  • Atomic radius decreases from left to right within a period. This is caused by the increase in the number of protons and electrons beyond a period.
  • One proton has a greater effect than one electron, thus, electrons are pulled towards the nucleus, which results in a smaller radius.

periodic table

  • Both potassium and calcium belong to the 4th period in the periodic table of chemical elements.
  • So, we cannot judge their atomic radius by their group now, so we have to look at which element to the left of the other.
  • Here in this case we see potassium is on the left to calcium. So, the atomic radius of potassium (K) is greater than that of calcium (Ca)
  • From the above table, it is clear potassium (K) has the maximum atomic radius.  Therefore option 1 is correct.

Key Points

  • Sodium is highly reactive, so that stored in oil or kerosene because it spontaneously ignites in water.
  • At room temperature, sodium metal is soft enough that you can cut it with a butter knife.

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