Problem 17 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of decreasing first ionization potential is (a) \(\mathrm{Ca}>\mathrm{K}>\mathrm{Rb}>\mathrm{Cs}\) (b) \(\mathrm{Cs}>\mathrm{Rb}>\mathrm{K}>\mathrm{Ca}\) (c) \(\mathrm{Ca}>\mathrm{Cs}>\mathrm{Rb}>\mathrm{K}\) (d) \(\mathrm{K}>\mathrm{Rb}>\mathrm{Cs}>\mathrm{Ca}\)

Step-by-Step Solution

Answer

The correct order is (a) \( \mathrm{Ca} > \mathrm{K} > \mathrm{Rb} > \mathrm{Cs} \).

Step 1: Understand Ionization Potential

Ionization potential, or ionization energy, is the energy required to remove the most loosely bound electron from an isolated gaseous atom. Generally, ionization energy decreases as you move down a group in the periodic table because the additional electron shells increase the atomic radius and decrease the effective nuclear charge on the outer electrons.

Step 2: Analyze Atomic Group Trends

The elements \(\mathrm{K}\), \(\mathrm{Rb}\), and \(\mathrm{Cs}\) are all in Group 1, known as the alkali metals. Ionization energy decreases down this group, so we expect \(\mathrm{K} > \mathrm{Rb} > \mathrm{Cs}\) in terms of decreasing ionization potential.

Step 3: Compare With Calcium

Calcium \(\mathrm{Ca}\) is located in Group 2, right after potassium in the periodic table. Calcium usually has a higher first ionization potential than alkali metals like potassium \(\mathrm{K}\), because it is harder to remove an electron from the more negatively charged core, which is typical for the second group of elements compared to the first group.

Step 4: Determine the Correct Order

Based on the above reasoning, Calcium will have the highest ionization potential due to its position in Group 2. Next is Potassium, followed by Rubidium, and then Cesium. The correct order based on decreasing ionization potential is: \(\mathrm{Ca} > \mathrm{K} > \mathrm{Rb} > \mathrm{Cs}\).

Key Concepts

Periodic Trends

In chemistry, understanding periodic trends is vital as it helps predict how elements will interact with each other. The periodic trend of ionization energy shows a clear pattern: as you move across a period from left to right, the ionization energy increases. This occurs because the effective nuclear charge increases, pulling the electrons closer to the nucleus and making them harder to remove. On the other hand, as you move down a group in the periodic table, ionization energy tends to decrease. This is due to the addition of electron shells which increases atomic size and decreases the force of attraction between the nucleus and the outermost electron. These consistent patterns help to make predictions about chemical behavior and reactivity.

Alkali Metals

Alkali metals, which include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr), are elements found in Group 1 of the periodic table. They are known for being highly reactive, especially with water, due to their low ionization energies.
The reason alkali metals have such low ionization energies is because each one has only a single electron in its outer shell. This makes it relatively easy for these atoms to lose that electron and form a positive ion. As we move down this group, the ionization energy decreases as the outer electron gets farther from the nucleus and is shielded by more inner electrons. Hence, cesium and francium, located further down the group, are the most reactive.

Ionization Energy

Ionization energy is a crucial concept in chemistry, referring to the energy required to remove an electron from a gaseous atom or ion. This energy is typically measured in kilojoules per mole (kJ/mol). Several factors affect this energy:

• Atomic Size: Larger atoms have more distant valence electrons, making them easier to remove.
• Nuclear Charge: Higher charges hold electrons more tightly, increasing ionization energy.
• Electron Shielding: More inner electrons can shield outer electrons from the nucleus, decreasing ionization energy.

Conversely, the smaller the atom and the fewer the shielded electrons, the higher the ionization energy. It provides insight into an element's reactivity and helps in predicting the strength of its chemical bonds.

Periodic Table Groups

The periodic table is organized into groups, which are the vertical columns of elements sharing similar properties due to a common number of valence electrons. Groups help predict the chemical behavior of elements.
Each group has a unique property; for example, Group 1 elements, the alkali metals, are highly reactive. Group 2, the alkaline earth metals, have slightly higher ionization energies than alkali metals due to their additional valence electron. As you move across the periodic table, the number of valence electrons increases, changing the elements' chemical properties significantly.
Understanding these groups is fundamental to comprehending the systematic way in which elements react and form compounds. Most importantly, it allows chemists to anticipate behaviors based on position, easing the complexity of chemical interactions.