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 because it is lower down in Group 2 of the periodic table, resulting in a larger atomic size and weaker attraction between the nucleus and outer electrons, making it easier for calcium to lose electrons. (b) Calcium is less reactive than potassium because potassium is in Group 1 of the periodic table and needs to lose only one electron to achieve a stable electron configuration, while calcium needs to lose two electrons. This makes it easier for potassium to lose electrons, resulting in its higher reactivity compared to calcium.

Step-by-step solution

Compare Electron Configurations

First, let's compare the electron configurations of calcium (Ca), magnesium (Mg), and potassium (K). Calcium has 20 electrons, magnesium has 12 electrons, and potassium has 19 electrons. The electron configurations of these elements are as follows: - Calcium (Ca): [Ar] 4s² - Magnesium (Mg): [Ne] 3s² - Potassium (K): [Ar] 4s¹ As we go down a group in the periodic table, the reactivity of elements increases due to the increase in atomic size and the decrease in ionization energy.

Reactivity of Calcium and Magnesium

Calcium (Ca) and magnesium (Mg) are both in Group 2 of the periodic table, but calcium is one period below magnesium. Because calcium is lower down the group, it has a larger atomic size, meaning that its outer electrons are further from the nucleus. This causes a weaker attraction between the positively charged nucleus and the negatively charged outer electrons, making it easier for calcium to lose electrons. As a result, calcium is more reactive than magnesium.

Reactivity of Calcium and Potassium

Calcium (Ca) is in Group 2 of the periodic table, and potassium (K) is in Group 1, one group to the left of calcium. In general, elements in Group 1 are more reactive than elements in Group 2. The reason for this is that Group 1 elements only have one electron in their outermost shell, while Group 2 elements have two electrons in their outermost shell. Since potassium only needs to lose one electron to achieve a stable electron configuration, it loses electrons more readily than calcium, which needs to lose two electrons. This makes potassium more reactive than calcium.

Key concepts

Electron Configurations

Understanding the electron configuration of an element is crucial in predicting its chemical behavior, including reactivity. Electron configurations describe the distribution of electrons in an atom’s orbitals. For instance, considering elements such as calcium (Ca), magnesium (Mg), and potassium (K), their reactivities are influenced by how their electrons are arranged.

Calcium, with the electron configuration [Ar] 4s², has two electrons in its outermost shell which can be lost in reactions. Magnesium, with a similar configuration of [Ne] 3s², also has two electrons in its outer shell. However, the difference in reactivity between calcium and magnesium can be attributed to which shell their valence electrons are in. The further the outer shell is from the nucleus, the less tightly these electrons are held and the more reactive the element is. Potassium, with the configuration [Ar] 4s¹, has only one outer electron, making it even easier to lose compared to calcium, which aligns with potassium's higher reactivity.

Atomic Size and Reactivity

The atomic size of an element has a direct effect on its reactivity, especially when comparing elements within the same group. For instance, as we move down Group 2, atomic size increases, and this progressive increase in size tends to decrease the effective nuclear charge that the valence electrons experience because they are further away from the nucleus. As a result, elements with a larger atomic size, like calcium, have valence electrons that are more easily removed when reacting with other elements, explaining its greater reactivity compared to magnesium.

In general, larger atoms are more reactive because it requires less energy to remove their valence electrons, which are located in orbitals farther away from the nucleus. This results in more vigorous chemical reactions.

Ionization Energy

The concept of ionization energy is integral when discussing the reactivity of elements. It represents the energy required to remove an electron from an atom in its gaseous state. Typically, elements with lower ionization energy are more reactive because it is easier to remove their outermost electron and thus achieve a more stable configuration.

As we see in the case of calcium, it has a lower ionization energy than magnesium, making it more likely to lose electrons and participate in chemical reactions. On the other side, potassium has an even lower ionization energy than calcium, which justifies its greater reactivity. It only requires the loss of one electron to reach stability, compared to the two electrons that calcium needs to lose, which requires more energy overall.

Group Trends in the Periodic Table

Analyzing group trends in the periodic table helps to predict and explain patterns in reactivity. In a given group, reactivity tends to increase from top to bottom. This is due to the increase in atomic size and the corresponding decrease in ionization energy that occurs as we move down the group.

Comparing Group 1 and Group 2

Elements in Group 1 of the periodic table are typically more reactive than those in Group 2. This is seen when comparing the reactivities of potassium (Group 1) and calcium (Group 2). Potassium is more reactive because its atoms have a single valence electron that can be easily lost, while calcium has two valence electrons, which creates a slightly higher energy barrier for reaction.

This trend showcases the importance of both electron configuration and atomic size, as they are the key factors driving the reactivity of elements within their respective groups on the periodic table.