The reactivity of metals is primarily influenced by their atomic structure, particularly the arrangement of electrons. In the case of potassium (K), sodium (Na), and calcium (Ca), we can observe a trend in their reactivity based on their position in the periodic table.
Potassium (K) is in Group 1, below sodium (Na), which is also in Group 1. As we move down the group in the periodic table, the atomic radius increases. This means that the outermost electron of potassium is further from the nucleus compared to sodium. Because this outer electron experiences a weaker attraction to the positively charged nucleus, it is more easily lost during chemical reactions. This higher tendency to lose the outer electron results in potassium being more reactive than sodium.
On the other hand, calcium (Ca) is a Group 2 element. While it’s true that calcium also loses its outer electrons to react, it does so in a different manner. When comparing calcium to sodium, we recognize that the atomic structure of calcium involves losing two electrons from its outer shell rather than just one. Additionally, since calcium is a larger atom than sodium, the additional shielding effect provided by inner electrons further decreases the effective nuclear charge experienced by the outer electrons. Therefore, although calcium can be reactive, it ends up being less reactive than sodium when considering single electron loss.
To summarize, the reactivity order is expressed as K > Na > Ca because potassium is very reactive due to its single outer electron being far from the nucleus, while sodium is reactive as well, but less so than potassium, and calcium, despite being capable of reacting, is ultimately less reactive compared to sodium due to its atomic structure and the manner in which it loses electrons.