Problem 8 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of atomic radii is (a) \(\mathrm{F}<\mathrm{K}<\mathrm{Na}<\mathrm{Li}\) (b) \(\mathrm{Li}<\mathrm{Na}<\mathrm{K}<\mathrm{F}\) (c) \(\mathrm{F}<\mathrm{Li}<\mathrm{Na}<\mathrm{K}\) (d) \(\mathrm{Na}<\mathrm{K}<\mathrm{Li}<\mathrm{F}\)

Step-by-Step Solution

Answer

The correct order is (c) \( \mathrm{F} < \mathrm{Li} < \mathrm{Na} < \mathrm{K} \).

Step 1: Understand Atomic Radii

The atomic radius is the size of an atom, typically measured from the center of the nucleus to the boundary of the surrounding cloud of electrons. It generally increases as you move down a group in the periodic table and decreases across a period from left to right.

Step 2: Examine the Elements

In this exercise, we need to consider the elements: Fluorine (F), Potassium (K), Sodium (Na), and Lithium (Li). They belong to different groups and periods in the periodic table, which will influence their atomic radii.

Step 3: Arrange by Period and Group

Start by arranging the elements by their position in the periodic table: 1. Fluorine (F) - Group 17, Period 2 2. Lithium (Li) - Group 1, Period 2 3. Sodium (Na) - Group 1, Period 3 4. Potassium (K) - Group 1, Period 4

Step 4: Determine Trends in Atomic Radii

Li, Na, and K are in Group 1. As you move down this group, the atomic radius increases: Li < Na < K. Fluorine is much smaller due to its position in Group 17 of Period 2, so F < Li.

Step 5: Select the Correct Order

Following the periodic trends, the correct increasing order of atomic radii is Fluorine (F) < Lithium (Li) < Sodium (Na) < Potassium (K). This matches option (c).

Key Concepts

Periodic Trends

Periodic trends are patterns in the properties of elements that repeat across different periods of the periodic table. One of these trends is the atomic radius. As you move across a period from left to right, atoms tend to become smaller. This is because more protons in the nucleus pull electrons in more tightly. Hence, reducing the radius.
Additionally, as you go down a group in the periodic table, the atomic radius generally increases. This is because adding more electron shells outweighs the effect of increased nuclear charge, resulting in a larger radius. Understanding these trends helps us predict and compare the properties of different elements quickly.

Atomic Size

Atomic size, often described in terms of the atomic radius, is the distance from the nucleus of an atom to the outer boundary of its electron cloud. Different factors influence this size:

• Atomic Number: With more protons and electrons, the size can be affected by the attractive forces.
• Electron Shielding: Inner electrons can shield outer electrons from the nucleus, making the atom larger.

When comparing elements, it's important to remember that the atomic radius does not increase uniformly across all elements. For instance, across a period, the increasing nuclear charge results in smaller atomic radii, while in the same group, the added electron shells make the atom larger.

Group and Period in Periodic Table

The periodic table is not just an arrangement of elements. It is structured to reflect recurring trends in the chemical properties of elements. Elements in the same group (vertical columns) have similar chemical properties, which is due to their having the same number of valence electrons. For example, Group 1 contains the alkaline metals like Lithium (Li), Sodium (Na), and Potassium (K), all of which have a single valence electron, making them reactive.
Meanwhile, periods (horizontal rows) tell us about the energy levels of electrons for the atoms in that row. For instance, elements like Fluorine (F) and Neon (Ne) are in Period 2, having electrons in the first and second energy levels. Understanding the placement of elements within groups and periods helps us predict their behavior, reactivity, and even size, just like in our original exercise about atomic radii.