Problem 155 - A Complete Resource Book in Chemistry for JEE Main

Question

The increasing order of the first ionization enthalpies of the element B, P, S and F (lowest first) is (a) \(\mathrm{F}<\mathrm{S}<\mathrm{P}<\mathrm{B}\) (b) \(\mathrm{P}<\mathrm{S}<\mathrm{B}<\mathrm{F}\) (c) \(\mathrm{B}<\mathrm{P}<\mathrm{S}<\mathrm{F}\) (d) \(\mathrm{B}<\mathrm{S}<\mathrm{P}<\mathrm{F}\)

Step-by-Step Solution

Answer

The order is (c) \( \mathrm{B} < \mathrm{P} < \mathrm{S} < \mathrm{F} \).

Step 1: Understanding Ionization Enthalpy

Ionization enthalpy is the energy required to remove an electron from a gaseous atom or ion. As we move across a period in the periodic table, ionization enthalpy generally increases due to increasing nuclear charge, which holds the electrons more tightly.

Step 2: Locate Elements in Periodic Table

Boron (B) is in Group 13, Period 2; Phosphorus (P) is in Group 15, Period 3; Sulfur (S) is in Group 16, Period 3; and Fluorine (F) is in Group 17, Period 2. Those closer to the right of a period generally have higher ionization enthalpies.

Step 3: Identify General Ionization Trends

Within the same period, elements usually have increasing ionization enthalpy from left to right. Therefore, among Period 2 elements, B (Group 13) is expected to have a lower ionization enthalpy than F (Group 17). For Period 3 elements, S (Group 16) typically has higher ionization enthalpy than P (Group 15).

Step 4: Comparison Across Periods

Compare the relative ionization energies of elements across periods, considering that F, being further right in Period 2, has a significantly higher ionization enthalpy than those elements in Period 3.

Step 5: Order the Elements by Ionization Enthalpy

Based on the trends, order the elements: B's ionization is less than P and S (since it's further left in a less stacked period), and both S and P have lower ionization enthalpies than F due to F's position in Group 17. Thus, the order is B < P < S < F.

Key Concepts

Periodic Table Trends

The periodic table, a cornerstone of chemistry, organizes elements based on increasing atomic number. As you move across a period (from left to right), elements generally experience a rise in ionization enthalpy. This means more energy is needed to remove an electron from the outer shell. This increase is due to a stronger attraction between electrons and the nucleus as more protons are present. In contrast, when you go down a group (from top to bottom), ionization enthalpy tends to decrease. This happens because outer electrons are further from the nucleus, reducing the attractive force.
So, in summary:

• Across a period: Ionization enthalpy increases.
• Down a group: Ionization enthalpy decreases.

Understanding these trends helps predict how elements will behave and react, especially their susceptibility to losing electrons.

Nuclear Charge

A key concept to grasp is nuclear charge, which refers to the total charge of the nucleus, given by the number of protons inside it. This is significant because the positive charge of the nucleus is what attracts the negatively charged electrons, helping to hold them in orbit around the nucleus.
When the nuclear charge increases, the pull on the electrons is stronger, resulting in higher ionization enthalpy. As you add more protons by moving across a period, electrons are pulled more tightly, which means it requires more energy to remove an electron.
Nuclear charge does the following:

• Increases from left to right across a period.
• Results in increased attraction between the nucleus and electron cloud.

The closer and tighter the electron is held by the nucleus, the harder it is to remove, thus raising the ionization energy.

Electron Removal Energy

Ionization enthalpy, or electron removal energy, is the concept of interest here. It represents the energy required to remove an electron from a gaseous atom. This value is a great indicator of an element's reactivity, particularly in terms of losing electrons. Factors affecting electron removal energy include:

• Atomic size: Larger atoms have electrons further from the nucleus, needing less energy for removal.
• Nuclear charge: A higher charge increases removal energy due to a stronger electron-nucleus attraction.

Tying back to periodic trends, note that as atomic size increases down a group, ionization energy reduces. Conversely, across a period, both atomic size and ionization energy trends point to increased electron removal energy as the relative nuclear charge dominates the scene.

Group and Period Comparison

To accurately predict ionization enthalpy, it's helpful to compare elements by both their groups and periods. In our exercise, elements like boron (B) and fluorine (F) belong to the same period, meaning that fluorine, being to the right, usually exhibits a higher ionization enthalpy. Meanwhile, phosphorus (P) and sulfur (S) share a period but have slightly different positions in their group, resulting in sulfur having a higher ionization enthalpy than phosphorus.
Key considerations include:

• Comparison within a period: Look at position from left to right.
• Comparison within a group: Consider the influence of atomic size as you move down.

By evaluating these positions side by side, it becomes evident why F has the highest ionization enthalpy among the given elements, and ultimately why the order B < P < S < F is accurate.