Problem 103 - Chemistry The Central Science In Si Units

Question

Elemental barium reacts more violently with water than does elemental calcium. Which of the following best explains this difference in reactivity? (i) Calcium has greater metallic character than does barium. (ii) The electron affinity of calcium is smaller than that of barium. (iii) The first and second ionization energies of barium are less than those of calcium. (iv) The atomic radius of barium is smaller than that of calcium. ( \(\mathbf{v}\) ) The ionic radius of the barium ion is larger than that of the calcium ion.

Step-by-Step Solution

Answer

Statement (iii): The first and second ionization energies of barium are less than those of calcium.

Step 1: Understanding Reactivity and Ionization Energy

The reactivity of metals with water generally increases down a group in the periodic table. This is because elements lower in the group tend to have lower ionization energies, making it easier to lose an electron and react. Hence, we need to consider the factors related to ionization energy.

Step 2: Examine Ionization Energy Trends

Barium is below calcium in Group 2 of the periodic table. As you move down a group, the first and second ionization energies decrease. This means it requires less energy to remove an electron from barium than from calcium, which contributes to barium's increased reactivity.

Step 3: Comparison of Ionization Energies

Statement (iii) states that "The first and second ionization energies of barium are less than those of calcium." This explanation is consistent with the trend observed in the periodic table, where elements lower in a group have lower ionization energies and thus are more reactive.

Step 4: Eliminating Incorrect Options

(i) Incorrect, because calcium does not have greater metallic character than barium. (ii) Incorrect, electron affinity is less relevant in this context. (iv) Incorrect, barium actually has a larger atomic radius, not smaller. (v) Irrelevant for explaining reactivity with water.

Step 5: Final Step: Conclusion

The correct explanation for why barium reacts more violently with water than calcium is that barium has lower ionization energies. Therefore, statement (iii) is correct.

Key Concepts

Ionization Energy

Ionization energy refers to the amount of energy needed to remove an electron from an atom in its gaseous state. The lower an element's ionization energy, the more easily it can lose electrons and participate in chemical reactions.
When you look at the periodic table, there's a notable trend: moving down a group, ionization energy generally decreases. This happens because the outer electrons are further away from the nucleus due to additional electron shells. This larger distance weakens the attractive force from the nucleus, making it easier to remove an electron.
In the case of barium and calcium, barium is situated below calcium in Group 2. This positioning means that barium has lower ionization energies compared to calcium, which explains why barium reacts more violently with water. Barium’s electrons can be removed more readily, facilitating its high reactivity.

Periodic Table Trends

The periodic table is a powerful tool for predicting the properties of elements, including their reactivity, ionization energy, and more. Understanding its trends helps in explaining why certain chemical behaviors occur.
One important trend is that atomic size increases as you move down a group. This increase occurs due to additional electron shells being added with each subsequent element. Simultaneously, the ionization energy decreases down a group, as mentioned earlier.
In Group 2, where both barium and calcium are located, these trends mean barium has a larger atomic radius and a lower ionization energy compared to calcium. These characteristics result in barium's higher reactivity. By knowing and understanding these trends, one can predict and explain the behavior of elements across the table.

Metallic Character

Metallic character refers to how easily an atom can lose electrons, often leading to its participation in metallic bonding and reactions. Elements with high metallic character tend to be more reactive, especially with non-metals like water.
In the periodic table, metallic character increases as you move downwards within a group and decreases as you move from left to right across a period. This means that elements located lower in a group exhibit more metallic properties due to their larger size and weaker hold on their outer electrons.
Hence, among all the elements in Group 2, barium shows a pronounced metallic character because it can easily lose its electrons compared to calcium. As a result, it reacts more readily, demonstrating why it would behave more violently in reactions, such as with water.