How does the size of a cation compare to its parent atom, and why?

A cation is smaller than its parent atom. This is because the loss of outer-shell electrons reduces electron-electron repulsion and often removes an entire energy level, allowing the nucleus to pull the remaining electrons closer.

When comparing $NaCl$ and $MgO$, which compound has stronger ionic bonds and why?

$MgO$ has stronger bonds because the ions ($Mg^{2+}$ and $O^{2-}$) have higher charges than those in $NaCl$ ($Na^+$ and $Cl^-$). Higher charges result in a much stronger electrostatic attraction and higher lattice energy.

What is the difference between polarising power and polarisability?

Polarising power is the ability of a cation to distort an anion's electron cloud, favored by small size and high charge. Polarisability is the ease with which an anion's electron cloud is distorted, favored by large size and high negative charge.

What is a common mistake when ordering the radii of isoelectronic ions like $Na^+$ and $Mg^{2+}$?

Students often think $Mg^{2+}$ is larger because it has a higher atomic number. In reality, $Mg^{2+}$ is smaller because it has more protons pulling on the same number of electrons (10) as $Na^+$.

Why is it incorrect to assume all ionic bonds are 100% ionic?

In practice, many ionic bonds have 'covalent character' due to polarisation. If a cation has high polarising power, it distorts the anion's electron cloud so much that electrons are partially shared between the two nuclei.

What error occurs if you only consider ionic charge when predicting melting points?

While charge is dominant, ignoring ionic radius can lead to errors. If charges are identical, the compound with the smaller ions will have the stronger bonds and higher melting point due to the shorter distance between centers.

Define 'Charge Density' in the context of ions.

Charge density is a measure of how concentrated an ion's charge is, calculated as the ratio of the ion's charge to its radius or volume. High charge density leads to stronger electrostatic attractions.

What are 'Isoelectronic Ions'?

Isoelectronic ions are different chemical species that possess the same number of electrons and the same electron configuration, such as $O^{2-}$, $F^-$, $Na^+$, and $Mg^{2+}$.

Define 'Lattice Energy'.

Lattice energy is the energy change when one mole of an ionic solid is formed from its constituent gaseous ions under standard conditions. It quantifies the strength of the ionic bonds in a crystal.

How does increasing the ionic radius affect the strength of an ionic bond?

Increasing the ionic radius decreases bond strength. Larger ions have their centers further apart, which reduces the electrostatic force of attraction between them according to Coulomb's Law.

Ionic Bond Strength | Pearson Edexcel International AS Chemistry

1. Definition & Core Concepts

Ionic Bond Strength refers to the magnitude of the electrostatic force of attraction between a positively charged cation and a negatively charged anion. This force is non-directional, meaning it acts in all directions around the ion, leading to the formation of a giant ionic lattice.
The strength of this bond is directly related to the Lattice Energy, which is the energy released when one mole of an ionic crystalline compound is formed from gaseous ions. Higher lattice energy indicates a stronger, more stable ionic bond.
Two fundamental variables dictate the strength of these attractions: the magnitude of the charges on the ions and the distance between the centers of the ions (the sum of their ionic radii).

Comparison of ionic bond strength showing that smaller ions with higher charges result in stronger electrostatic attractions.

2. Underlying Principles: Charge Density

Charge Density is the ratio of an ion's charge to its volume or surface area. It is the most critical predictor of how strongly an ion will attract oppositely charged species.
Mathematically, charge density is proportional to the charge ( $Q$ ) and inversely proportional to the ionic radius ( $r$ ): $\text{Charge Density} \propto \frac{Q}{r}$
Ions with a high charge density (small radius and high positive or negative charge) exert a much more concentrated electrostatic pull. This results in a more rigid lattice structure that requires significantly more thermal energy to disrupt.

3. Trends in Ionic Radii

Cations are always smaller than their parent atoms because the loss of valence electrons often results in the loss of an entire outer shell. Furthermore, the remaining electrons experience a greater effective nuclear charge, pulling them closer to the nucleus.
Anions are always larger than their parent atoms because the addition of electrons increases inter-electron repulsion. This causes the electron cloud to expand while the nuclear charge remains constant.
In an isoelectronic series (ions with the same number of electrons), the ionic radius decreases as the atomic number increases. This occurs because the increasing number of protons in the nucleus exerts a stronger pull on the same number of electrons.

4. Polarisation & Covalent Character

Polarisation occurs when a cation distorts the electron cloud of a neighboring anion. This distortion introduces a degree of electron sharing, known as covalent character, into the otherwise purely ionic bond.
The polarising power of a cation is highest when it has a small radius and a high positive charge (high charge density). Such cations are very effective at pulling the anion's electrons toward themselves.
The polarisability of an anion refers to how easily its electron cloud can be distorted. Large anions with high negative charges are the most polarisable because their outer electrons are further from the nucleus and less tightly held.

5. Key Distinctions

Feature	High Bond Strength	Low Bond Strength
Ionic Charge	High (e.g., $+2, +3$ )	Low (e.g., $+1$ )
Ionic Radius	Small	Large
Melting Point	Very High	Moderately High
Charge Density	High	Low

It is important to distinguish between theoretical lattice energy (calculated assuming ions are perfect spheres) and experimental lattice energy. A large discrepancy between these values usually indicates significant polarisation and covalent character.

6. Exam Strategy & Tips

Identify the Variables: When asked to compare the melting points of two ionic compounds, always identify the charges of the ions first. Charge usually has a more significant impact on bond strength than small differences in ionic radius.
The Isoelectronic Trap: If comparing ions like $N^{3-}$ , $O^{2-}$ , and $F^-$ , remember they all have 10 electrons. The one with the most protons ( $F^-$ ) will be the smallest, but the one with the highest charge ( $N^{3-}$ ) often contributes to stronger bonding in a lattice.
Explain, Don't Just State: If a question asks why Compound A has a higher melting point than Compound B, your answer must mention: 1) The specific difference in charge or radius, 2) The resulting difference in charge density, and 3) The increased energy required to overcome the stronger electrostatic attractions.
Check for Covalent Character: If the cation is very small and highly charged (like $Li^+$ or $Mg^{2+}$ ) and the anion is large (like $I^-$ ), expect the bond to have significant covalent character, which might affect predicted solubility or melting points.

Ionic Bond Strength

Summary

1. Definition & Core Concepts

2. Underlying Principles: Charge Density

3. Trends in Ionic Radii

4. Polarisation & Covalent Character

5. Key Distinctions

6. Exam Strategy & Tips

Ionic Bond Strength

Summary

1. Definition & Core Concepts

2. Underlying Principles: Charge Density

3. Trends in Ionic Radii

4. Polarisation & Covalent Character

5. Key Distinctions

6. Exam Strategy & Tips