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Understanding electron configuration is crucial for predicting the formation of monatomic ions. Electrons in an atom are arranged in various energy levels or shells around the nucleus. These levels are filled according to specific rules that minimize the energy of the atom.

For instance, the electron configuration of radium (Ra) with atomic number 88 is \[ [Rn] 7s^2 \]. This tells us that radium has two electrons in its outermost shell (7s orbital). Being an alkaline earth metal, radium tends to lose these two electrons to achieve a more stable and lower energy state.

On the other hand, selenium (Se) with atomic number 34 has the electron configuration of \[ [Ar] 3d^{10} 4s^2 4p^4 \]. This means selenium has six electrons in its outermost shell (4s and 4p orbitals). Elements in Group 16, including selenium, typically gain electrons to complete their outer shell and stabilize their structure.

Groups in the periodic table help us determine the properties and the reactivity of elements. Radium belongs to Group 2, known as the alkaline earth metals. Elements in this group have two electrons in their outermost shell. Hence, they tend to lose these electrons easily to form a \(+2\) ion.

Selenium, on the other hand, is part of Group 16. These elements have six electrons in their outermost shell. To achieve a stable noble gas configuration, elements in this group typically gain two electrons, forming a \(-2\) ion. Knowing the group placement of an element simplifies predicting its ion charge.

Ion charge determination stems from an element's desire to achieve a stable electron configuration. Typically, elements achieve this stability by having a full outer shell of electrons, similar to noble gases.

Radium (Ra) will lose its two outermost electrons (from the 7s orbital) to form a \(+2\) ion: \(\text{Ra}^{2+}\). Upon losing these electrons, radium attains the electron configuration of the previous noble gas, radon (Rn).

Selenium (Se) will gain two electrons to fill its 4p orbital, forming a \(-2\) ion: \(\text{Se}^{2-}\). This gain allows selenium to resemble the electron configuration of the noble gas krypton (Kr). This predictable behavior in gaining or losing electrons explains the formation of these specific monatomic ions.