Problem 12 - Chemistry in 30 days

Question

The correct order of decreasing polarisable ions is (a) \(\mathrm{Cl}^{-}, \mathrm{Br}^{-}, \mathrm{F}, \mathrm{F}^{-}\) (b) \(\mathrm{F}^{-}, \mathrm{I}, \mathrm{Br}^{-}, \mathrm{Cl}^{-}\) (c) \(\mathrm{F}, \mathrm{Cl}^{-}, \mathrm{Br}^{-}, \mathrm{I}^{-}\) (d) \(\mathrm{I}^{-}, \mathrm{Br}^{-}, \mathrm{Cl}^{-}, \mathrm{F}^{-}\)

Step-by-Step Solution

Answer

Option (d): \( \mathrm{I}^- > \mathrm{Br}^- > \mathrm{Cl}^- > \mathrm{F}^- \) is correct.

Step 1: Understand Polarisability

Polarisability refers to the ability of an ion to be distorted by an electric field. Larger ions are more easily distorted, thus more polarisable, compared to smaller ions. In general, the polarisability increases with the size of the ion.

Step 2: Compare Ionic Sizes

Consider the ions given: \( \mathrm{I}^- \), \( \mathrm{Br}^- \), \( \mathrm{Cl}^- \), and \( \mathrm{F}^- \). The ionic size increases down the group in the periodic table. Therefore, the order of ionic size from largest to smallest is: \( \mathrm{I}^- > \mathrm{Br}^- > \mathrm{Cl}^- > \mathrm{F}^- \).

Step 3: Determine the Order of Polarisability

Using the relationship between ion size and polarisability, we determine that the order of polarisability is the same as the order of size for these ions: \( \mathrm{I}^- > \mathrm{Br}^- > \mathrm{Cl}^- > \mathrm{F}^- \).

Step 4: Select the Correct Option

Now, match the derived order \( \mathrm{I}^- > \mathrm{Br}^- > \mathrm{Cl}^- > \mathrm{F}^- \) with the given options. Option (d) matches this order exactly.

Key Concepts

Ionic Size

The size of ions, also known as ionic radii, plays a crucial role in determining their chemical behavior. Ionic size dictates so many properties including the ability of an ion to be polarized. Larger ions have their valence electrons further from the nucleus, meaning they are less tightly held. This results in them being more easily distorted in an electric field compared to smaller ions. The ionic size changes depending on the atomic or ionic number as well as the group and period in the periodic table. In general, as you go down a group in the periodic table, the ionic radius increases due to the addition of more electron shells.

Periodic Table

The periodic table is an arrangement of elements that reflects periodic trends in their properties. Elements are organized in rows called periods and columns known as groups or families. This layout helps predict chemical behaviors based on position. Position within a group can indicate trends such as increase in ionic size. For instance, as you move down a group in the periodic table, each element has an additional electron shell. This extra shell makes the ion larger and increases its polarisability as seen in the halogens from fluorine ( F ^{-} ) to iodine ( I ^{-} ). Thus, understanding the layout of the periodic table helps in predicting and comparing the properties of elements including their size and polarisability.

Ion Distortion

Ion distortion, also known as polarisation, occurs when an electric field affects the electron cloud of an ion, shifting its shape. This can happen because the outer electrons are more exposed to external influences, especially in larger ions. The extent to which an ion can be distorted is known as polarisability; an important property influencing the strength of forces between ions. For instance, larger ions like I ^{-} are more easily distorted than smaller ones like F ^{-} . This is because the valence electrons in larger ions are more loosely bound to the nucleus, allowing them to be shifted more easily by external fields.

Electric Field

An electric field is an area around a charged particle where forces are able to exert influence on other charges. When ions are placed within an electric field, they can become polarized, meaning that their electron cloud is distorted. The strength of the electric field and its proximity to an ion affects the degree of this distortion. More polarizable ions, typically those with larger ionic sizes, are more significantly influenced by these fields. In the context of ions like those in the exercise, recognizing the interplay of ion size and electric fields aids in understanding why I ^{-} is more polarizable relative to F ^{-} .