Question

$$ \begin{aligned} &\text { List the following ions in order of increasing ionic }\\\ &\text { radius: } \mathrm{N}^{3-}, \mathrm{Na}^{+}, \mathrm{F}^{-}, \mathrm{Mg}^{2+}, \mathrm{O}^{2-} \end{aligned} $$

Short answer

The ions in order of increasing radius are: \( \text{Mg}^{2+} < \text{Na}^+ < \text{F}^- < \text{O}^{2-} < \text{N}^{3-} \).

Step-by-step solution

Understand Ionic Radii

Ionic radius refers to the size of an ion in a crystal lattice. Generally, for ions with the same charge and in the same period, the ionic radius decreases from left to right across a period as the nuclear charge increases. However, for ions with different charges, cations (positive ions) are smaller than their neutral atoms, and anions (negative ions) are larger.

Identify Charge and Period

Identify the charge and position in the periodic table of each ion: \( \text{N}^{3-} \) (anion, larger), \( \text{Na}^+ \) (cation, smaller), \( \text{F}^- \) (anion), \( \text{Mg}^{2+} \) (cation), \( \text{O}^{2-} \) (anion). These ions originate from elements in the second period of the periodic table.

Analyze Ionic Charges and Sizes

Among cations \( \text{Na}^+ \) and \( \text{Mg}^{2+} \), the \( \text{Mg}^{2+} \) ion is smaller because it has a higher positive charge, resulting in a stronger pull from the nucleus. Among anions \( \text{N}^{3-} \), \( \text{O}^{2-} \), \( \text{F}^- \), since they share the same electron configuration with varying negative charges, \( \text{N}^{3-} \) is the largest because it has the smallest nuclear charge for the given electron cloud.

Order Ions by Radius

Now that we have analyzed the individual sizes: \( \text{Mg}^{2+} < \text{Na}^+ \) for cations and \( \text{F}^- < \text{O}^{2-} < \text{N}^{3-} \) for anions, combine and order them as: \( \text{Mg}^{2+} < \text{Na}^+ < \text{F}^- < \text{O}^{2-} < \text{N}^{3-} \). This results from comparing the combined effects of the charge and electron arrangement.

Key concepts

Periodic Table

The periodic table is a magical chart that organizes chemical elements in a structured form. Each element has unique properties which are arranged by their increasing atomic number. This number directly corresponds to the number of protons in an atom's nucleus. As you move across a period from left to right, elements become less metallic. Additionally, the atomic radius decreases due to an increase in nuclear charge as more protons are added, pulling the electrons closer to the nucleus.
When considering ionic radii, the periodic table helps identify whether the element will tend to form a cation or an anion based on its group. For example, elements in Group 1 like sodium (\(\text{Na}\)) typically form cations because they easily lose electrons to achieve a stable electron configuration. In contrast, elements in Group 17, such as fluorine (\(\text{F}\)) form anions by gaining electrons. Understanding this arrangement assists in predicting ionic sizes, helping to distinguish trends and variations in ionic radii as seen in the exercise.

Cation vs Anion

To comprehend ionic radii in-depth, one must distinguish between cations and anions. Cations are ions with a positive charge, resulting from the loss of one or more electrons. This loss causes the ion to have fewer electrons than protons, compressing the electron cloud and leading to a smaller ionic radius. For instance, \(\text{Na}^+\) loses an electron, becoming smaller than its neutral state.
Anions, however, are negatively charged ions. They gain electrons, leading to more electrons in the outer shells compared to protons. This increase spreads out the electron cloud, resulting in a larger size. For example, \(\text{F}^-\) gains an electron, making its ionic radius larger than its neutral atom.
The size disparity between cations and anions highlights a fundamental principle: cations shrink due to reduced electron clouds, while anions grow due to increased electron repulsion in the added electrons. This concept aids in determining the relative ionic sizes in the periodic table.

Nuclear Charge

Nuclear charge is crucial in understanding why atoms and ions differ in size. It refers to the total charge of the protons in an atom's nucleus. An increased nuclear charge pulls electrons closer to the nucleus, reducing the size of the atom or ion. This principle is particularly relevant when comparing ions with different charges.
For cations like \(\text{Mg}^{2+}\), the high positive charge means the nucleus effectively pulls in the surrounding electrons much tighter than \(\text{Na}^+\), giving it a smaller ionic radius. Conversely, in anions like \(\text{N}^{3-}\), the nuclear charge is lesser compared to the electron cloud spacing, meaning the electrons are not pulled in as tightly, resulting in a larger radius.
Nuclear charge plays a pivotal role in determining how electron loss or gain impacts ionic radius. By balancing the number of protons and the pulled-in electrons of an ion, you can understand why, in the same period, nuclear charge dictates much of the size variation seen in ions.

Electron Configuration

Electron configuration describes how electrons are arranged in an atom or ion's shells or orbitals. This arrangement is instrumental in determining an atom's reactivity and the type of ion it is likely to form.
In ions, electron configuration tells us which electrons have been removed or added. When forming a cation like \(\text{Na}^+\), the electron configuration changes as an electron is removed, often resulting in a noble gas configuration, which is more stable and smaller in radius. On the other hand, when electrons are added to form an anion like \(\text{N}^{3-}\), the electron configuration expands, increasing the ionic radius.
Considering electron configurations helps in predicting and comparing the sizes of ions. By examining which electron layers are filled or depleted, you can infer whether an ion will hold a larger or smaller radius, as seen in examples like \(\text{F}^-, \text{O}^{2-}\), and \(\text{N}^{3-}\). Understanding this concept offers clarity on how electrons contribute to the differences in ionic radii.