Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the elements \(\mathrm{W}, \mathrm{X}, \mathrm{Y}\) and \(\mathrm{Z}\) having atomic numbers \(9,10,11\) and 12 respectively, the correct order of ionization energies is (a) \(\mathrm{W}>\mathrm{Y}>\mathrm{X}>\mathrm{Z}\) (b) \(\mathrm{X}>\mathrm{W}>\mathrm{Z}>\mathrm{Y}\) (c) \(\mathrm{X}>\mathrm{Z}>\mathrm{Y}>\mathrm{W}\) (d) \(\mathrm{Z}>\mathrm{Y}>\mathrm{X}>\mathrm{W}\)

Step-by-Step Solution

Answer

The correct order is (b) \(\mathrm{X} > \mathrm{W} > \mathrm{Z} > \mathrm{Y}\).

Step 1: Understand Ionization Energy

Ionization energy is the energy required to remove the outermost electron from a gaseous atom or ion. In general, ionization energy increases across a period from left to right on the periodic table and decreases down a group.

Step 2: Identify Elements and Their Groups

Given elements are W, X, Y, and Z with atomic numbers 9, 10, 11, and 12 respectively. These correspond to Fluorine (F), Neon (Ne), Sodium (Na), and Magnesium (Mg). Fluorine and Neon are in Group 17 and 18 respectively, while Sodium and Magnesium are in Group 1 and 2 of the same period.

Step 3: Order of Ionization Energies Across a Period

Within a period, noble gases have the highest ionization energy, followed by halogens, alkaline earth metals, and alkali metals. Therefore, the order based on atomic numbers 9-12 is: Ne (highest) > F > Mg > Na (lowest).

Step 4: Match the Order to the Given Options

The correct order from Step 3 is Neon > Fluorine > Magnesium > Sodium. Thus, based on atomic numbers: X > W > Z > Y. Check the provided options to find this order.

Key Concepts

Periodic Table

The periodic table is like a map of all known chemical elements. It organizes elements based on their atomic number, electron configurations, and recurring chemical properties.

• Elements are arranged in rows called "periods." As you move across a period from left to right, the atomic number increases.


• Vertical columns in the periodic table are called "groups" or "families." Elements in the same group often have similar properties.


• The table helps us predict the behavior of elements, such as trends in ionization energy, electronegativity, and atomic size.

Knowing how elements are organized in the periodic table helps us understand their chemical behavior and the important trends that occur regularly.

Atomic Number

The atomic number of an element is a fundamental property that defines its position in the periodic table. It represents the number of protons in the nucleus of an atom and is unique to each element.

• Elements are ordered by atomic number in the periodic table. For example, hydrogen has an atomic number of 1, helium is 2, and so on.


• The atomic number determines the identity of an element. For instance, all atoms with an atomic number of 9 are fluorine atoms.


• Atomic number helps in balancing chemical equations, as it indicates the number of electrons when an atom is neutral.

Remembering the atomic number is crucial because it helps us locate an element in the periodic table quickly and understand how it interacts with other elements.

Noble Gases

Noble gases are a group of elements in group 18 of the periodic table. They include helium, neon, argon, krypton, xenon, and radon.

• These gases are called "noble" because they are very stable and unreactive. Their outer electron shell is full, which means they don't usually form compounds easily.


• Noble gases have the highest ionization energies in their respective periods. This means more energy is needed to remove an electron from them compared to other elements.


• Neon, for example, is a noble gas with a high ionization energy, making it less likely to participate in chemical reactions.

Understanding the role of noble gases is essential in chemistry, as they often help set benchmarks for ionization energies and stability in a period.

Halogens

Halogens are a group of elements found in group 17 of the periodic table. They consist of fluorine, chlorine, bromine, iodine, and astatine.

• These elements are highly reactive, mainly because they have seven electrons in their outermost shell and need one more to achieve a stable octet.


• Among their group, fluorine has the highest electronegativity and reactivity because it is the most efficient at attracting electrons.


• Halogens tend to form salts when they react with metals, such as sodium chloride (table salt), where sodium (a metal) has combined with chlorine.

Recognizing the properties of halogens helps in understanding their behavior and how they interact with other elements, particularly in forming various compounds.