Science Exploration Notes Class 9 English Medium 2026-27

Science Exploration Chapter 9. Atomic Foundations of Matter

CBSE Notes New Syllabus: Class 9

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Chapter 9. Atomic Foundations of Matter

This section explains how atoms combine by sharing electrons to form covalent bonds. It also describes the formation of simple covalent molecules such as hydrogen, chlorine and oxygen. Covalent bonding helps atoms achieve a stable electronic configuration by completing their outermost shell. 

How Do Atoms Combine?

Atoms combine with one another to achieve a stable electronic configuration. They may combine by sharing electrons or by transferring electrons.

Main Methods of Combination

• Sharing of Electrons – Forms a covalent bond.
• Transfer of Electrons – Forms an ionic bond.

Covalent Bond

A covalent bond is formed when two atoms share one or more pairs of electrons to complete their outermost shell.

Definition

A covalent bond is a chemical bond formed by the mutual sharing of electrons between atoms. 

Characteristics of Covalent Bonds

• Electron Sharing – Electrons are shared between atoms.
• No Ion Formation – Atoms remain electrically neutral.
• Stable Molecules – Sharing helps atoms attain a stable electronic configuration.
• Usually Formed Between Non-metals – Most covalent compounds are formed by non-metallic elements.

Formation of Hydrogen Molecule (H₂)

• Each hydrogen atom has one electron.
• Both hydrogen atoms share one pair of electrons.
• Each atom attains a stable duplet configuration.
• A single covalent bond is formed between the two hydrogen atoms.

Formation of Chlorine Molecule (Cl₂)

• Each chlorine atom has seven valence electrons.
• Each atom shares one electron with the other.
• Both chlorine atoms complete their octet.
• A single covalent bond is formed.

Formation of Oxygen Molecule (O₂)

• Each oxygen atom has six valence electrons.
• Each atom shares two electrons.
• Both oxygen atoms complete their octet.
• A double covalent bond is formed.

Types of Covalent Bonds

Importance of Covalent Bonding

• Helps atoms achieve stable electronic configurations.
• Leads to the formation of molecules.
• Forms many important compounds found in living organisms.
• Explains the structure of many non-metallic substances.

Formation of Covalent Compounds

Covalent compounds are formed when atoms of different non-metals share electrons to achieve stable electronic configurations.

Definition

A covalent compound is a compound in which atoms are held together by the mutual sharing of electrons.

Formation of Hydrogen Chloride (HCl)

• Hydrogen has one valence electron.
• Chlorine has seven valence electrons.
• Both atoms share one pair of electrons.
• Hydrogen attains a duplet, while chlorine completes its octet.
• A single covalent bond is formed, producing a hydrogen chloride molecule. 

Formation of Water (H₂O)

• Oxygen has six valence electrons.
• Each hydrogen atom contributes one electron for sharing.
• Oxygen shares one electron with each hydrogen atom.
• Oxygen completes its octet and each hydrogen completes its duplet.
• Two single covalent bonds are formed, producing one water molecule. 

Characteristics of Covalent Compounds

• Electron Sharing – Atoms are joined by sharing electrons.
• Molecular Nature – They exist as molecules.
• No Free Ions – They do not contain positively or negatively charged ions.
• Stable Structure – Atoms attain stable electronic configurations after bonding.

Naming of Covalent Compounds

Covalent compounds are named by using standard prefixes that indicate the number of atoms of each element present in a molecule. :contentReference[oaicite:3]{index=3}

Common Prefixes

Examples of Covalent Compounds

Importance of Covalent Compounds

• They form many substances essential for life.
• They are commonly found in air, water and living organisms.
• They help explain the structure and properties of molecular substances.
• They are important in chemistry, biology and environmental science.

ATP Education Concept Builder

In covalent compounds, atoms become stable by sharing electrons, not by gaining or losing them. Count the valence electrons carefully to understand how many electrons must be shared to complete the duplet or octet.

ATP Education Exam Booster

• Learn the formation of HCl and H₂O using electron sharing.
• Remember the common prefixes used in naming covalent compounds.
• Practise drawing electron-dot structures of simple molecules.
• Differentiate between molecules formed by the same element (H₂, O₂) and different elements (HCl, H₂O).
• CBSE competency-based questions often ask students to explain covalent bond formation using electron-dot diagrams.