Question

Which of the following element has the highest first ionization energy?

• A. $Rb$
• B. $Na$
• C. $K$
• D. $Sc$

Answer 1

The correct option is B $Na$

• The first ionization enthalpy of elements decreases as one moves down in a group. While moving downin a group, the atomic number increases and the number of shells increases as well. The outermost electrons are far away from the nucleus and henceforth, can be removed very easily.
  
• The first ionisation energy is the energy required to move one mole of electrons from one mole of atoms in its gaseous state.
  
• The outer electron in Potassium is in the 4s orbital, which is further away from the nucleus than the 3s orbital of the Sodium. The greater distance means that the attraction between the electron is weaker.
  
• The electron in Potassium is also more affected by shielding due to more shells, further weakening this attraction. This means that less energy is needed to remove the outermost electron and therefore the ionisation energy is lower.
  
• Similar arguments can be made with Na and other given elements.
• Hence option B is correct answer.