Problem 143 - Chemistry An Atoms First Approach

Question

Arrange the atoms and/or ions in the following groups in order of decreasing size. a. \(\mathrm{O}, \mathrm{O}^{-}, \mathrm{O}^{2-}\) b. \(\mathrm{Fe}^{2+}, \mathrm{Ni}^{2+}, \mathrm{Zn}^{2+}\) c. \(\mathrm{Ca}^{2+}, \mathrm{K}^{+}, \mathrm{Cl}^{-}\)

Step-by-Step Solution

Answer

The order of decreasing size for the given atoms and ions is as follows: a. O²⁻ > O⁻ > O b. Fe²⁺ > Ni²⁺ > Zn²⁺ c. Ca²⁺ > K⁺ > Cl⁻

Step 1: a. Arrange O, O⁻, O²⁻ in order of decreasing size

We are given three species: O, O⁻, and O²⁻. To arrange them by size, we can compare the number of protons and electrons in each species: 1. O has 8 protons and 8 electrons. 2. O⁻ has 8 protons and 9 electrons, due to the addition of an extra electron. 3. O²⁻ has 8 protons and 10 electrons, due to the addition of two extra electrons. Since electrons are repelled by each other, the more electrons a species has, the larger its size will be. Therefore, the order of decreasing size is: O²⁻ > O⁻ > O.

Step 2: b. Arrange Fe²⁺, Ni²⁺, Zn²⁺ in order of decreasing size

We are given three ions: Fe²⁺, Ni²⁺, and Zn²⁺. To arrange them by size, we can compare the number of protons in each ion, as they all have the same charge (2+): 1. Fe²⁺ has 26 protons. 2. Ni²⁺ has 28 protons. 3. Zn²⁺ has 30 protons. Atomic size decreases as the number of protons increases because the attraction between the electrons and protons increases, resulting in a smaller ion. Therefore, the order of decreasing size is: Fe²⁺ > Ni²⁺ > Zn²⁺.

Step 3: c. Arrange Ca²⁺, K⁺, Cl⁻ in order of decreasing size

We are given three ions with different charges: Ca²⁺, K⁺, and Cl⁻. To arrange them by size, we can compare their positions in the periodic table as well as the charge: 1. Ca²⁺ has 20 protons and 18 electrons. 2. K⁺ has 19 protons and 18 electrons. 3. Cl⁻ has 17 protons and 18 electrons. In the periodic table, Ca and K are in the same group (Group 2), with Ca below K, and Cl is in Group 17 (halogens) in the same period as K and Ca. Hence, Ca²⁺ will be larger than K⁺ because it is farther down the group. K⁺, a cation with a single positive charge, will be larger than Cl⁻ because Cl⁻ has more protons and the same number of electrons. Therefore, the order of decreasing size is: Ca²⁺ > K⁺ > Cl⁻.