Problem 136 - A Complete Resource Book in Chemistry for JEE Main

Question

The values of \(\mathrm{IE}_{1}, \mathrm{IE}_{2}, \mathrm{IE}_{3}, \mathrm{IE}_{4}\) and \(\mathrm{IE}_{5}\) are \(7.1,14.3\), \(34.5,46.8\) and \(162.2 \mathrm{eV}\) respectively. Number of valence electrons in that element is __ .

Step-by-Step Solution

Answer

The element has 4 valence electrons.

Step 1: Understanding Ionization Energies

The ionization energy (IE) of an element is the energy required to remove an electron from the outer shell. The values \(\mathrm{IE_1}, \mathrm{IE_2}, \mathrm{IE_3}, \mathrm{IE_4}\), and \(\mathrm{IE_5}\) correspond to the energy needed to remove the first, second, third, fourth, and fifth electrons, respectively.

Step 2: Analyzing the Sequence of IEs

Identify any large jumps between consecutive ionization energies. A large increase in ionization energy suggests that an electron is being removed from a more stable, full shell, indicating that all valence electrons have been removed before this jump.

Step 3: Determining Large Jump Position

The ionization energies given are \(7.1, 14.3, 34.5, 46.8\), and \(162.2\ eV\). Observe the jump between \(\mathrm{IE_4} = 46.8\ eV\) and \(\mathrm{IE_5} = 162.2\ eV\) which is significantly larger than the preceding differences. This indicates that the first four electrons were valence electrons, and the fifth is in an inner shell.

Step 4: Counting Valence Electrons

Since the significant jump in ionization energy occurred after the fourth electron was removed, this suggests that the element has 4 valence electrons before reaching a noble gas configuration.

Key Concepts

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. These electrons are responsible for the chemical behavior of the element. Electrons in the outer shell engage in activities like bonding or chemical reactions.
It is crucial to understand that the number of valence electrons determines how an element interacts with others to form compounds.

• Valence electrons can vary from 1 to 8 in most cases.
• Elements with similar valence electron configurations often showcase similar chemical properties.
• The fewer valence electrons an atom has to complete its outer shell, the more reactive it typically is.

Recognizing Valence Electrons Through Ionization Energy The exercise provides a perfect illustration of how to deduce the number of valence electrons using ionization energies. When analyzing the jump in ionization energies, it signifies that we've moved past the valence electrons and into a more stable electronic configuration. In the given example, a large increase in IE after the fourth electron suggests the completion of its valence shell.

Electron Configuration

Electron configuration describes the distribution of electrons in an atom's orbitals. It follows the principle of filling lower energy levels first.
The electrons are filled according to the rules of quantum mechanics, and this configuration gives insight into the atom's structure and properties.

• The Aufbau principle says electrons occupy the lowest energy orbital available first.
• Pauli's exclusion principle states no two electrons in the same atom can have the same set of quantum numbers, leading to paired or unpaired electron spins.
• Hund's rule suggests that orbitals in the same subshell are singly occupied as far as possible, with electrons having parallel spins before pairing.

Linking Electron Configuration to Valence Electrons In the ionization energy exercise, understanding electron configurations helps predict which electrons are valence and which are core. The outermost electrons are the valence electrons, while those filling inner levels, or shells, are core electrons.

Periodic Table

The periodic table is a tabular arrangement of elements ordered by increasing atomic number, where elements with similar properties recur at intervals. It's an essential tool in identifying how elements will behave chemically and what kind of bonds they will form.
The organization of the periodic table helps us understand the electron configuration and the number of valence electrons.

• Groups or columns share common characteristics because they have the same number of valence electrons.
• Rows or periods have elements with the same number of atomic orbitals.
• The location on the table can help predict how atoms might bond together to form compounds.

Position on Periodic Table and Ionization Energies The transition in ionization energy in our example marks the shift from a valence electron space to an inner core electron. The position of an element on the periodic table will indicate how many such electrons an atom has, which is vital for predicting properties and reactions.