Calcium is placed in Group 2 of the periodic table, known as the alkaline earth metals, despite having a partially filled 4s orbital. The reasoning behind this classification is rooted in its electronic configuration and chemical properties.
Calcium has the atomic number 20, which means it has 20 electrons. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s². The outermost electrons, those in the 4s orbital, are responsible for the element’s chemical behavior. While the 3p orbital below is fully filled, it is the 4s electrons that are loosely held and involved in bonding.
The placement in Group 2 reflects that calcium, like other alkaline earth metals, tends to lose its two valence electrons to form cations with a +2 charge. This loss of electrons leads to a stable electron configuration resembling that of noble gases. Therefore, it is not just about having filled orbitals but rather about the reactivity and electron donation behavior of calcium that justifies its position in Group 2.
In summary, while the concept of completely filled orbitals is crucial in understanding stability, the group classification ultimately hinges on how elements interact chemically based on their valence electrons. Calcium’s readiness to give up its two outermost electrons aligns with the characteristics of Group 2 elements.