NEET Notes: Chemical Periodicity

Introduction

Chemical periodicity refers to the recurring trends observed in the properties of elements as they are arranged in the periodic table. The periodic table is a systematic arrangement of elements based on their atomic number, electron configuration, and chemical properties. Understanding chemical periodicity helps predict the behavior of elements and their compounds, making it an essential topic for NEET aspirants.

Periodic Table and Periodicity

1. Development of the Periodic Table

The modern periodic table evolved from the earlier classifications of elements, such as Dobereiner’s Triads, Newlands’ Law of Octaves, and Mendeleev’s Periodic Table. The periodic law, proposed by Moseley, states that the properties of elements are a periodic function of their atomic numbers.

2. Periodic Law

According to the modern periodic law, the physical and chemical properties of elements repeat at regular intervals when they are arranged in increasing order of atomic number.

3. Classification of Elements in the Periodic Table

The elements in the periodic table are classified into:

• Groups: Vertical columns containing elements with similar chemical properties.

• Periods: Horizontal rows where properties change progressively.

• Blocks: Elements are divided into s-block, p-block, d-block, and f-block based on their valence electron configuration.

Periodic Trends in Atomic Properties

1. Atomic Radius

Atomic radius is the distance from the nucleus to the outermost electron. It varies across the periodic table:

• Decreases across a period due to increasing nuclear charge.

• Increases down a group due to the addition of electron shells.

2. Ionic Radius

Ionic radius depends on whether an atom gains or loses electrons:

• Cations (positive ions) are smaller than their parent atoms due to the loss of electrons and increased nuclear attraction.

• Anions (negative ions) are larger than their parent atoms due to electron gain and increased electron repulsion.

3. Ionization Energy

Ionization energy is the energy required to remove an electron from an atom in its gaseous state.

• Increases across a period due to higher nuclear charge.

• Decreases down a group as atomic size increases and electrons are held less tightly.

Electron Affinity and Electronegativity

1. Electron Affinity

Electron affinity is the energy released when an atom gains an electron.

• Increases across a period as nuclear charge increases.

• Decreases down a group due to increased atomic size and electron shielding.

2. Electronegativity

Electronegativity is the tendency of an atom to attract shared electrons in a chemical bond.

• Increases across a period due to greater nuclear charge.

• Decreases down a group due to increased atomic radius.

Chemical Reactivity Trends

1. Reactivity of Metals

Metals tend to lose electrons and form cations.

• Reactivity decreases across a period as ionization energy increases.

• Reactivity increases down a group as atomic size increases and ionization energy decreases.

2. Reactivity of Non-Metals

Non-metals tend to gain electrons and form anions.

• Reactivity increases across a period due to higher electronegativity.

• Reactivity decreases down a group as atomic size increases and electron affinity decreases.

Metallic and Non-Metallic Character

1. Metallic Character

Metallic character refers to the ability of an element to lose electrons and form positive ions.

• Decreases across a period due to increasing ionization energy.

• Increases down a group due to lower ionization energy.

2. Non-Metallic Character

Non-metallic character is the tendency of an element to gain electrons.

• Increases across a period due to higher electron affinity.

• Decreases down a group as atomic size increases.

Periodicity in Oxidation States

1. Variation of Oxidation States Across a Period

• Elements show multiple oxidation states, especially in transition metals.

• The oxidation state increases positively across a period due to increasing valence electrons.

2. Variation of Oxidation States Down a Group

• In groups like halogens, the most common oxidation state remains the same.

• Some elements exhibit variable oxidation states due to the involvement of d and f orbitals.

Periodic Trends in Chemical Properties

1. Acidic and Basic Nature of Oxides

• Metal oxides are basic and react with acids.

• Non-metal oxides are acidic and react with bases.

• Amphoteric oxides (e.g., Al₂O₃) show both acidic and basic behavior.

2. Nature of Hydrides

• Ionic hydrides are formed by alkali and alkaline earth metals.

• Covalent hydrides are formed by non-metals like carbon, nitrogen, and oxygen.

• Metallic hydrides are found in d-block and f-block elements.

Importance of Chemical Periodicity

1. Predicting Chemical Behavior

Periodic trends help predict the reactivity and bonding tendencies of elements, essential for understanding chemical reactions.

2. Industrial and Biological Applications

• Elements like alkali metals and halogens are widely used in industries.

• Transition metals play a crucial role in catalysis and biological processes.

3. Basis for the Development of New Materials

Understanding periodicity aids in the development of new materials, such as semiconductors and superconductors.

Conclusion

Chemical periodicity is the foundation of modern chemistry, providing a systematic approach to understanding the properties of elements. The periodic trends in atomic size, ionization energy, electron affinity, and reactivity help in predicting the behavior of elements in various chemical reactions. Mastering these concepts is essential for NEET aspirants to solve problems related to periodic properties and their applications.