Ionic Compounds

An ionic compound is a type of compound formed by the classification of compounds with respect to the bond formed between the interacting species (charged ions). They consist of bonds called ionic bonds, which are very strong due to electrostatic forces of attraction between the ions of the compound.

The formation of ionic bond clearly depends on the electrostatic force between ions:

\[F_e = \dfrac{kq_1q_2}{r^2}, \]

where \(q_1\) and \(q_2\) are the magnitudes of charges of the anion and cation, respectively, and \(r\) is the distance between atoms.

Hence, one can conclude that, if the bond is strong, then the electrostatic force is also strong.

From this, we can find the factors influencing the formation and strength of the ionic bond:

• The greater the magnitude of charges on the cation and anion, the greater the capability of the formation.
• The less the distance between the charged ions, the greater the force and the strength and capability of the formation.
• The packing efficiency (not a result of the formula above) plays a role because ionic compounds exist in lattice structures. If the interaction between like charges (cation-cation of different compounds and anion-anion of different compounds) is quite large, then the lattice is weakened.

Here are the properties of ionic compounds:

1. Ionic compounds are generally hard and brittle.
2. Ionic compounds generally have high melting points and high boiling points.
3. Ionic compounds are generally soluble in polar solvents and insoluble in non-polar solvents.
4. Ionic compounds are good conductors of electricity in their molten or fused state due to the presence of free ions.
5. Ionic compounds undergo very fast or spontaneous reactions in a aqueous state.
6. Ionic bonds are non-directional, unlike covalent bonds.

In which state is cryolite \((\ce{Na3AlF6},\) sodium hexafluoroaluminate\()\) most conducive for conducting electricity?

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It is most conducive for conducting electricity in a liquid state.

In a liquid state, the ions are mobile and hence can conduct electricity.
In a solid state, the ions are fixed in the lattice structure and hence cannot conduct electricity.
In a gaseous state, the ions tend to be too far apart to conduct electricity well.

Which compound has a higher melting point, \(\ce{CCl4}\) or \(\ce{NaCl}?\)

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An ionic compound has a much higher melting point than a covalent compound, so the answer is \(\ce{NaCl}.\)

In an ionic compound, the ions have strong attraction to other ions in their vicinity; thus, it would take a lot of energy to break them apart. In a covalent compound, while the atoms are bound tightly to each other in a stable molecule, these molecules are not strongly attracted to other molecules. Hence, it does not take much energy to separate them.

Note: The melting point of \(\ce{CCl4}\) is \( -23\, ^ \circ\text{C} \) and the melting point of \(\ce{NaCl}\) is \( 800\, ^ \circ\text{C}. \)

Ionic compounds generally exist in the form of a crystal lattice. The orderly arrangement of oppositely charged ions in 3D in solid state to form an infinite array is called crystal lattice. In crystal lattice, a unit of oppositely charged ions keeps repeating. The smallest repeating unit in a crystal lattice is called a unit cell.

The amount of energy released when 1 mole of oppositely charged ions is brought from infinity to form crystal lattice is called lattice energy.

\(\ce{CH4}\)

Hannay-Smith equation is stated as follows:

\[3.5({ X }_{ A }-{ X }_{ B }{ ) }^{ 2 }+16({ X }_{ A }-{ X }_{ B }),\]

where \(({ X }_{ A }-{ X }_{ B })\) is the difference in electronegativities of \(A\) and \(B.\)

By calculating the relationship between observed dipole moment and calculated dipole moment, the relationship can be stated as follows:

\[\text{% Ionic character} = \frac{\text{Observed dipole moment}}{\text{Calculated dipole moment}} \times 100.\]

Which of the following compounds is most ionic?

a) NaCl
b) KCl
c) RbCl
d) CsCl

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Here, the anion of all the given compounds is the same. Hence, comparison of the ionic character of a compound depends on the cation. Since the size of cation increases down the group, the ionic character of cation also increases down the group. Here \(\text{Cs}^{ + }\) has the highest atomic size and hence it has the highest ionic character.

Therefore, the answer is (d).