⚛️ Atomic Foundation of Matter – Class 9 Science Short Notes

Atomic Foundation of Matter is an important Class 9 Science chapter that introduces the basic concepts of atoms, molecules, ions, chemical formulae, atomic mass and molecular mass. These notes are designed for quick revision, conceptual clarity and exam preparation.

---

🔬 1. What is an Atom?

An atom is the smallest particle of an element that can take part in a chemical reaction.

• Atoms are extremely small and cannot be seen with ordinary optical microscopes.
• Advanced techniques such as scanning tunnelling microscopy help us observe atoms indirectly.
• The size of an atom is expressed in terms of its atomic radius.
• Atomic radius is generally measured in nanometres (nm).

📌 Remember: 1 nm = 10^-9 m

🧪 2. Symbols of Elements

A symbol is a short representation of an element. Modern symbols of elements were proposed using letters from the English or Latin names of elements.

• The first letter of an element's symbol is always written in capital.
• The second letter, if present, is always written in lowercase.

📚 Important Element Symbols

Element	Symbol	Element	Symbol
Hydrogen	H	Oxygen	O
Carbon	C	Nitrogen	N
Sodium	Na	Potassium	K
Iron	Fe	Copper	Cu
Silver	Ag	Gold	Au
Mercury	Hg	Lead	Pb
Tin	Sn	Calcium	Ca

💡 Tip: Pay special attention to symbols derived from Latin names, such as Na, K, Fe, Cu, Ag, Au, Hg and Pb.

⚖️ 3. Atomic Mass

The atomic mass of an element is expressed relative to the mass of a carbon-12 atom.

One atomic mass unit (1 u) is defined as exactly 1/12 of the mass of one carbon-12 atom.

📌 1 u = 1.66 × 10^-27 kg

📝 Some Common Atomic Masses

• Hydrogen (H) = 1 u
• Helium (He) = 4 u
• Carbon (C) = 12 u
• Nitrogen (N) = 14 u
• Oxygen (O) = 16 u
• Sodium (Na) = 23 u
• Magnesium (Mg) = 24 u
• Aluminium (Al) = 27 u
• Phosphorus (P) = 31 u
• Sulphur (S) = 32 u
• Chlorine (Cl) = 35.5 u
• Potassium (K) = 39 u
• Calcium (Ca) = 40 u
• Iron (Fe) = 56 u
• Copper (Cu) = 63.5 u

🧩 4. How Do Atoms Exist?

Atoms generally exist in different forms depending upon the element and its chemical nature.

• Atoms may combine to form molecules.
• Atoms may also exist as ions carrying electrical charges.
• Noble gases such as helium, neon and argon are chemically unreactive and can exist independently.

🔗 5. Molecules

A molecule is the smallest particle of a substance that can exist independently and shows all the properties of that substance.

A molecule may contain atoms of the same element or atoms of different elements.

🧬 Types of Molecules

1. Homoatomic Molecules

Molecules containing atoms of the same element are called homoatomic molecules.

Examples: H₂, N₂, O₂, Cl₂, P₄, S₈

2. Heteroatomic Molecules

Molecules containing atoms of different elements are called heteroatomic molecules.

Examples: H₂O, CO₂, NaCl, CaCO₃, Al₂O₃

🔢 6. Atomicity

The total number of atoms present in one molecule is called its atomicity.

Type	Example	Atomicity
Monatomic	He, Ne, Ar	1
Diatomic	H2, O2, N2	2
Tetraatomic	P4	4
Polyatomic	S8	8

⚡ 7. Ions

An ion is an atom or group of atoms carrying an electrical charge.

➕ Cations

Cations are positively charged ions generally formed by the loss of electrons.

Examples: Na⁺, K⁺, Ca²⁺, Al³⁺

➖ Anions

Anions are negatively charged ions generally formed by the gain of electrons.

Examples: Cl^-, O^2-, S^2-, OH^-

👥 Polyatomic Ions

A group of two or more atoms carrying an overall charge is called a polyatomic ion.

Examples: NH₄⁺, SO₄^2-, CO₃^2-, NO₃^-, OH^-

🧾 8. Chemical Formula

A chemical formula is the symbolic representation of the composition of a compound.

✍️ Rules for Writing Chemical Formulae

1. Write the symbols of the elements or ions present in the compound.
2. Write their respective valencies.
3. Cross-multiply the valencies to obtain the subscripts.
4. Ignore the sign of the charge while writing the final formula.
5. Use brackets when a polyatomic ion occurs more than once.

🧪 Important Examples

• Hydrogen chloride → HCl
• Hydrogen sulphide → H₂S
• Carbon tetrachloride → CCl₄
• Magnesium chloride → MgCl₂
• Aluminium oxide → Al₂O₃
• Calcium oxide → CaO
• Sodium nitrate → NaNO₃
• Calcium hydroxide → Ca(OH)₂
• Sodium carbonate → Na₂CO₃
• Ammonium sulphate → (NH₄)₂SO₄

🧮 9. Molecular Mass

The molecular mass of a substance is the sum of the atomic masses of all the atoms present in one molecule.

Example: For H₂O:

Molecular mass = (2 × 1) + (1 × 16) = 18 u

Similarly, CO₂ = 12 + (2 × 16) = 44 u

⚖️ 10. Formula Unit Mass

The formula unit mass is the sum of the atomic masses of all the ions and atoms represented in the formula unit of an ionic compound.

Example: NaCl

Formula unit mass = 23 + 35.5 = 58.5 u

📊 11. Percentage Composition

The percentage of an element present in a compound can be calculated using:

Percentage of element = (Mass of element ÷ Mass of compound) × 100

🎯 Important Exam-Focused Questions

The following questions are based on the concepts covered in the provided chapter notes and are useful for Class 9 revision and examination practice.

1. What is an atom? Define atomic radius and state its unit.
2. What is meant by the symbol of an element?
3. State the rules for writing the symbols of elements.
4. Why are symbols of some elements derived from their Latin names? Give examples.
5. Define atomic mass. What is one atomic mass unit?
6. What is a molecule? Give two examples.
7. Differentiate between homoatomic and heteroatomic molecules.
8. What is atomicity? Give examples of monoatomic, diatomic and polyatomic molecules.
9. What are ions? Differentiate between cations and anions.
10. What are polyatomic ions? Give two examples.
11. Write the chemical formulae of MgCl₂, Al₂O₃, Ca(OH)₂ and (NH₄)₂SO₄.
12. Calculate the molecular mass of H₂O and CO₂.
13. Calculate the formula unit mass of NaCl.
14. Explain the rules for writing a chemical formula with suitable examples.
15. Write the formula used to calculate the percentage composition of an element in a compound.

🚀 Quick Revision

• ⚛️ Atom → Smallest particle of an element taking part in a chemical reaction.
• 📏 Atomic radius → Size of an atom.
• 🔤 Symbol → Short representation of an element.
• ⚖️ 1 u → 1/12 of the mass of a carbon-12 atom.
• 🧬 Molecule → Smallest independently existing particle of a substance.
• 🔢 Atomicity → Number of atoms present in one molecule.
• ➕ Cation → Positively charged ion.
• ➖ Anion → Negatively charged ion.
• 👥 Polyatomic ion → Group of atoms carrying an overall charge.
• 🧾 Chemical formula → Symbolic representation of the composition of a compound.
• 🧮 Molecular mass → Sum of atomic masses of atoms in a molecule.
• ⚖️ Formula unit mass → Sum of atomic masses represented by an ionic compound's formula unit.

🌟 Exam Tip

Focus especially on element symbols, atomic mass, atomicity, ions, valencies, chemical formula writing, molecular mass and formula unit mass. Practising formula-based questions regularly can significantly improve conceptual understanding and numerical accuracy.