Problem 110 - A Complete Resource Book in Chemistry for JEE Main

Question

The statement that is not correct for periodic classification of elements is (a) the properties of elements are a periodic function of their atomic numbers (b) non-metallic elements are less in number than metallic elements (c) the first ionization energies of elements along a period do not vary in a regular manner with in crease in atomic number (d) for transition elements, the d-subshells are filled with electrons monotonically with increase in atomic number

Step-by-Step Solution

Answer

Option (d) is incorrect because d-subshells do not fill monotonically in transition elements.

Step 1: Understanding Periodic Properties

Periodic properties such as ionization energy and metallic character vary systematically across the periodic table because they are periodic functions of atomic numbers. Elements are arranged according to their atomic numbers, supporting statement (a) as correct.

Step 2: Comparing Metallic and Non-metallic Elements

There are fewer non-metals compared to metals on the periodic table. Metals occupy the majority of the table, supporting statement (b) as correct.

Step 3: Analyzing Ionization Energies

Generally, ionization energies increase across a period, but they do not always increase smoothly due to subshell arrangements. The non-uniformity is particularly noticeable when moving from s to p orbitals, making statement (c) correct.

Step 4: Electrons in Transition Elements

For transition elements, their electron configuration is less straightforward. The d-subshells do not always fill in a straightforward manner due to electron repulsion and stability of half-filled subshells. Thus, statement (d) is the incorrect statement.

Key Concepts

Atomic Number

The atomic number of an element is a fundamental property representing the number of protons in an atom's nucleus. This value is unique for each element and essentially defines the element. Periodic classification is based on arranging elements in order of their atomic numbers, leading to the periodic table, which groups elements with similar chemical properties together.

• The atomic number increases steadily across a period and down a group.
• This sequential arrangement helps in predicting and explaining the chemical behavior of the elements
• It forms the basis for periodic trends such as electronegativity, atomic radius, and ionization energy.

Thus, understanding atomic numbers is key to grasping how the periodic table classifies elements.

Ionization Energy

Ionization energy refers to the energy required to remove an electron from an atom in its gaseous state. This property is influenced by an element's electron configuration and atomic size. Generally, ionization energy increases across a period due to the increased nuclear charge attracting electrons more strongly.

However, this trend isn't always linear. For example:

• There can be dips in ionization energy when moving from s to p orbitals.
• Poorly shielded electrons in different orbitals can cause irregularities.
• As in the case of elements like oxygen and nitrogen, where half-filled p subshells provide a special stability, causing nitrogen to have a higher ionization energy than oxygen.

These nuances explain some exceptions to the general trends in ionization energy along the periodic table.

Metallic and Non-metallic Elements

In the periodic table, elements are broadly categorized as metals or non-metals. Metals are more abundant and make up the majority of the elements. They are typically found on the left and middle parts of the table.

• Metals generally have a shiny appearance, conduct electricity, and are malleable.
• Non-metals appear on the right side of the periodic table, and include elements like oxygen, nitrogen, and chlorine.
• Non-metals are generally more varied in their properties than metals; they can be gases, liquids, or brittle solids.

Therefore, the periodic classification showcases a clear predominance of metallic elements over non-metals.

Transition Elements

Transition elements are found in the d-block of the periodic table and include metals like iron, copper, and gold. They are characterized by having partially filled d-subshells in their electron configuration.

These elements display unique properties due to their electron arrangements, such as:

• Variable oxidation states
• Colored compounds
• Good conductivity and high melting points
• Complex magnetism

The electron filling order in transition elements doesn’t follow a simple pattern due to the effects of electron repulsion and stability achieved by half-filled or fully filled d-orbitals. Understanding these elements requires a deep dive into quantum mechanics and electron configurations.

Electron Configuration

Electron configuration describes the distribution of electrons in an atom's energy levels and orbitals. This configuration underpins an element's chemical properties and placement in the periodic table. Electrons fill orbitals in a way prescribed by Aufbau principle, utilizing the order of increasing energy.

Key details include:

• Electrons fill from lower to higher energy orbitals.
• Hund’s rule states each orbital is singly filled before any is doubly filled.
• This configuration influences how elements interact and bond with each other.

Transition elements exhibit deviations in electron filling order owing to similar energies of d and s orbitals, thus providing room for intriguing chemical behaviors.