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 ionic bonds are formed through the transfer of electrons. It describes the formation of sodium and chloride ions, the formation of sodium chloride, the structure of ionic compounds and their important properties. Ionic bonding is responsible for the formation of many compounds used in daily life. 

Ionic Bond

An ionic bond is formed when one atom transfers one or more electrons to another atom. As a result, positively and negatively charged ions are formed, which are held together by electrostatic forces.

Definition

An ionic bond is the electrostatic force of attraction between oppositely charged ions formed due to the transfer of electrons.

Formation of Sodium Ion (Na⁺)

• Sodium has one valence electron.
• It loses one electron to achieve a stable electronic configuration.
• After losing one electron, it becomes a positively charged sodium ion (Na⁺).

Formation of Chloride Ion (Cl⁻)

• Chlorine has seven valence electrons.
• It gains one electron to complete its octet.
• After gaining one electron, it becomes a negatively charged chloride ion (Cl⁻).

Formation of Sodium Chloride (NaCl)

When sodium transfers one electron to chlorine, both atoms attain stable electronic configurations. The oppositely charged ions attract each other and form an ionic bond, producing sodium chloride (NaCl). 

Characteristics of Ionic Bonds

• Electron Transfer – Electrons are transferred from one atom to another.
• Ion Formation – Positively charged cations and negatively charged anions are formed.
• Electrostatic Attraction – Opposite charges hold the ions together.
• Stable Electronic Configuration – Both ions attain stable outermost shells.

Properties of Ionic Compounds

• Hard and Crystalline – Ionic compounds usually form hard crystalline solids.
• High Melting and Boiling Points – Strong electrostatic forces require more energy to break.
• Conduct Electricity in Molten or Aqueous State – Free ions carry electric current.
• Usually Soluble in Water – Many ionic compounds dissolve easily in water.
• Do Not Conduct Electricity in Solid State – Ions remain fixed in their positions.

Difference Between Covalent Bond and Ionic Bond

Applications of Ionic Compounds

• Common Salt (NaCl) – Used in cooking and food preservation.
• Calcium Compounds – Used in construction and cement manufacturing.
• Fertilisers – Many fertilisers are ionic compounds.
• Medicines – Several medicinal salts are ionic in nature.
• Industrial Chemicals – Used in chemical industries and laboratories.

ATP Education Concept Builder

Remember that in ionic bonding, electrons are transferred, whereas in covalent bonding, electrons are shared. Sodium becomes a positive ion by losing an electron, while chlorine becomes a negative ion by gaining that electron. The attraction between these oppositely charged ions forms the ionic bond.

ATP Education Exam Booster

• Remember the definition of an ionic bond.
• Learn the formation of Na⁺, Cl⁻ and NaCl.
• Differentiate between covalent and ionic bonds.
• Remember the important properties of ionic compounds.
• Practise electron-transfer diagrams for sodium chloride.
• CBSE competency-based questions often ask students to explain ionic bond formation and compare ionic and covalent compounds.