Question

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

Short answer

Calcium is generally more reactive than magnesium because it has a larger atomic radius, meaning the distance between its nucleus and valence electrons is greater. This weaker attractive force makes it easier for calcium to lose its electrons. On the other hand, calcium is generally less reactive than potassium because potassium is in Group 1 of the periodic table, making it easier for potassium to lose its single valence electron than calcium to lose its two valence electrons. The attractive force between the nucleus and valence electrons is weaker for potassium than for calcium, making it easier for potassium to lose its valence electron and form an ion.

Step-by-step solution

1. Electron configurations

Write down the electron configurations of calcium (Ca), magnesium (Mg), and potassium (K): - 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): \(1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1}\)

2. Valence electrons and reactivity

The reactivity of elements increases with the ease of losing valence electrons or gaining valence electrons. Generally, metals (like Ca, Mg, and K) tend to lose electrons to form positive ions, while nonmetals gain electrons to form negative ions. The number of valence electrons and the energy needed to remove them affect the reactivity of a metal. For the given elements, their valence electrons and ease of removal are as follows: - Ca has 2 valence electrons in the fourth energy level (4s) - Mg has 2 valence electrons in the third energy level (3s) - K has 1 valence electron in the fourth energy level (4s)

3. Reactivity trend in the periodic table

As we move down a group in the periodic table, the distance between the nucleus and the valence electrons increases. This leads to a weaker attractive force between the nucleus and valence electrons, making it easier for atoms to lose electrons, subsequently increasing their reactivity. However, as we move across a period from left to right in the periodic table, the number of protons in the nucleus increases, while the valence electrons remain in the same energy level, resulting in a stronger attractive force and making it harder for these elements to lose electrons. Therefore, their reactivity decreases.

4. Reactivity comparison: Calcium vs. Magnesium

Calcium (Ca) and magnesium (Mg) are both in Group 2 of the periodic table, with Ca located below Mg. Since Ca has a larger atomic radius, the distance between its nucleus and valence electrons is greater than that of Mg. Consequently, there is a weaker attractive force between the nucleus and valence electrons in Ca, making it easier to lose electrons. This is the reason why calcium is generally more reactive than magnesium.

5. Reactivity comparison: Calcium vs. Potassium

Calcium (Ca) and potassium (K) are in different groups of the periodic table, with K being in Group 1 and Ca in Group 2. Both elements are in the same period, but K is one group to the left of Ca, making it easier for K to lose its single valence electron than it is for Ca to lose its two valence electrons. The attractive force between the nucleus and valence electrons is weaker for K than for Ca, making it easier for K to lose its valence electron and forming an ion. Consequently, this is why calcium is generally less reactive than potassium.

Key concepts

Electron Configuration

Every element is made up of electrons that occupy different energy levels or shells. These electrons follow a certain pattern and fill the shells in an increasing order of energy known as the electron configuration. This arrangement affects how atoms interact with each other, influencing the element's reactivity.

For calcium (Ca), with an atomic number of 20, the electron configuration is:

• Ca: \(1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2}\)

For magnesium (Mg), with an atomic number of 12, the configuration is simpler:

• Mg: \(1s^{2} 2s^{2} 2p^{6} 3s^{2}\)

Potassium (K), with an atomic number of 19, has:

• K: \(1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1}\)

The electron configuration determines how readily an atom can gain or lose electrons to reach a stable state, affecting its chemical behavior and reactivity.

Valence Electrons

The outermost electrons of an atom are called valence electrons. These electrons play a critical role in chemical reactions because they are involved in forming bonds between elements. The number of valence electrons an atom has can influence how reactive it is.

Calcium (Ca) and magnesium (Mg) are both group 2 elements, each with two valence electrons. However, these electrons are in different energy levels:

• Ca: 2 valence electrons in the 4s level
• Mg: 2 valence electrons in the 3s level

Potassium (K), a group 1 element, has a single valence electron in the 4s level:

• K: 1 valence electron in the 4s level

An element's ability to lose or gain these valence electrons determines its reactivity. For metals, losing electrons is key to forming positive ions. The fewer the valence electrons and the higher they are from the nucleus, the more easily they can be lost. This is why potassium is more reactive than calcium and calcium is more reactive than magnesium.

Periodic Table Trends

When looking at the periodic table, the trends in reactivity are closely tied to an element’s position. These trends are pretty predictable.

One important trend is that reactivity tends to increase as you move down a group. As you descend, the atoms have more electron shells. This increases the distance between the nucleus and the outermost valence electrons, reducing their attraction to the nucleus and making them easier to remove. This is why calcium is more reactive than magnesium; it sits below magnesium in the same group.

Conversely, across a period, from left to right, the reactivity of metals usually decreases. As you go across, the number of protons (and thus positive charge) in the nucleus increases, but the electrons are still in the same shell. This stronger force holds onto the valence electrons more tightly, making it harder for the atoms to lose them. Potassium is in a different group but in the same period as calcium, making it more reactive than calcium because it has a weaker hold on its single valence electron. Understanding these trends helps explain why, in this case, calcium is less reactive than potassium.