Question

(a) Why is calcium generally more reactive than magnesium? (b) Why is calcium generally less reactive than potassium?

Short answer

a) Calcium is more reactive than magnesium due to its larger atomic radius, making it easier for valence electrons to be removed. b) Calcium is less reactive than potassium because potassium's larger atomic radius makes it easier to remove its valence electron.

Step-by-step solution

Understanding Electronic Configuration

First, look up the atomic numbers of calcium (Ca), magnesium (Mg), and potassium (K) and write down their electronic configurations. Ca (Atomic number 20): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2\) Mg (Atomic number 12): \(1s^2 2s^2 2p^6 3s^2\) K (Atomic number 19): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^1\)

Comparing Reactivity

Reactivity of elements is related to the ease with which they can lose electrons to form positive ions. For elements in the same group of the periodic table (vertical column), reactivity generally increases as you move down the group. Since Ca and Mg are both in Group 2 (alkaline earth metals) and Ca is below Mg, we can say that calcium is generally more reactive than magnesium. To explain why, let's look into the factors that affect reactivity: effective nuclear charge, electron shielding, and atomic radius.

Effective Nuclear Charge

Effective nuclear charge is the net positive charge experienced by the outermost electrons of an atom. It is proportional to the attraction of the nucleus to the valence electrons. As the effective nuclear charge increased, the attraction to the electrons increases and they are more tightly held, making it difficult to lose electrons and decreasing reactivity. For Ca and Mg, the valence electrons are in the same shell (n=4 for Ca, n=3 for Mg), but with more protons in the nucleus in Ca. As a result, the effective nuclear charge experienced by the valence electrons of Ca should be higher than Mg. However, electron shielding must also be taken into account.

Electron Shielding

Electron shielding refers to the repulsion between electrons in different energy levels. Electrons in the inner shells shield the valence electrons from the full positive charge of the nucleus, which reduces the effective nuclear charge experienced by the valence electrons. Both Ca and Mg have similar electron configurations, with the same number of electrons in their inner shells. Therefore, electron shielding remains quite similar for both elements.

Atomic Radius

Atomic radius is the distance between the nucleus and the outermost shell. As we move down a group in the periodic table, atomic radius generally increases because of the addition of a new energy level to the electron configuration. A larger atomic radius means that valence electrons are farther from the nucleus, making them easier to remove. In this case, the atomic radius of Ca is larger than that of Mg as it is below Mg in the same group. This leads to the valence electrons in Ca being further from the nucleus than those in Mg, making it easier for Ca to lose electrons and be more reactive.

Reactivity of Calcium and Potassium

Comparing Ca and K (located in Group 1), K is below Ca in the periodic table and has a larger atomic radius. This means that the valence electron in K is farther from the nucleus and more weakly attracted than the valence electrons in Ca, making K more reactive than Ca. To summarize: a) Calcium is generally more reactive than magnesium due to the larger atomic radius of calcium, which makes it easier for the valence electrons to be removed. b) Calcium is generally less reactive than potassium because potassium's atomic radius is larger than that of calcium, and its valence electron is more weakly attracted to the nucleus, making it easier to be removed.