Which of the following elements has the first ionization energy?

A. Stronium (Sr)

B. Oxygen (O)

C. Phosphorous (P)

D. Scandium (Sc)

Among these elements the strontium (Sr) has lowest first ionization energy.

Explanation:

The first ionization energy is the energy required to remove one mole of the most loosely held electrons from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.

The first of two main methods which scientists use to calculate the ionization energy is the Subtraction Method.