Beryllium.
<h3>Explanation</h3>
All four elements are found in the second column to the left of a periodic table. They are <em>group 2</em> elements.
The 1st ionization energy generally decrease down each group.
Elements in each group have the same number of electrons in the outermost occupied electron shell. Electrons in the inner shell <em>shield </em>those in the outer shell from attractions from the nucleus, known as the shielding effect. The number of protons in each atom increase by more than 8 in each period. However, due to this effect, electrons in the outermost shell experience a far weaker force pulling them towards the center of the atom. For atoms of the four group 2 elements, electrons in the outermost layer would behave as if there's only two protons inside their nucleus. As a result, their end up with rather similar ionization energies.
Despite the same effective charge on each electron, the mean radius of the electrons increase down the group. Beryllium is in the second period; its electrons occupy only two electron shells in the ground state. On the contrary, strontium is in the fifth period; its electrons occupy up to five electrons shells in its ground state. The strength of electrostatic attraction decreases as the two charges move apart. As a result, outermost electrons in beryllium are way more attached to the nucleus than those in strontium.
FYI, the 1st ionization energy of the four elements are shown below:
- Be: 899.50 kJ/mol
- Mg: 737.750 kJ/mol
- Ca: 589.830 kJ/mol
- Sr: 549.470 kJ/mol
