Chapter – 3

Atoms and Molecules

In this post we have given the detailed notes of class 9 Science Chapter 3 (Atoms and Molecules) in English. These notes are useful for the students who are going to appear in class 9 board exams.

Chapter 3: Atoms and Molecules

Introduction 📖

• The idea of the divisibility of matter came up long ago in India, around 500 BC. 🇮🇳
• Maharishi Kanad, an Indian philosopher, proposed that dividing matter continuously would result in smaller and smaller particles, eventually reaching a point where further division is impossible. 🇨🇮
• He called these particles Parmanu. 🇨🇮
• Around the same time, Greek philosophers Democritus and Leucippus also explored the idea of matter’s divisibility. 🇬🇷
• Democritus called the indivisible particles atoms, meaning indivisible. 🇬🇷

Laws of Chemical Combination 📜

• Antoine L. Lavoisier established two important laws of chemical combination: the Law of Conservation of Mass and the Law of Constant Proportions. 👨‍🔬  

Law of Conservation of Mass ⚖️

• This law states that mass cannot be created or destroyed in a chemical reaction. 🔄
• In a chemical reaction, the total mass of the reactants is equal to the total mass of the products. 🔄

Law of Constant Proportions 🧪

• This law, also known as the Law of Definite Proportions, states that in a chemical substance, the elements are always present in definite proportions by mass. ♾️  
• For example, in water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, regardless of the source of the water. 💧

Dalton’s Atomic Theory 👨‍🏫

• John Dalton proposed the atomic theory, which explained the laws of chemical combination. 📚

Postulates of Dalton’s Atomic Theory:

1. All matter is made of tiny particles called atoms, which participate in chemical reactions. ⚛️
2. Atoms are indivisible particles that cannot be created or destroyed in a chemical reaction. 🚫
3. Atoms of a given element are identical in mass and chemical properties. 🅰️
4. Atoms of different elements have different masses and chemical properties. 🅱️
5. Atoms combine in the ratio of small whole numbers to form compounds. 🔢
6. The relative number and kinds of atoms are constant in a given compound. 🔄

What is an Atom? 🤔

• Atoms are the building blocks of all matter. 🧱
• They are incredibly small, much smaller than anything we can imagine. 🔬
• Modern techniques allow us to produce magnified images of surfaces showing atoms. 🔬

Modern Day Symbols of Atoms 🔤

• Dalton was the first scientist to use symbols for elements in a specific sense. 👨‍🔬
• Berzelius suggested using one or two letters from the element’s name as the symbol. 🅰️
• The first letter of a symbol is always capitalized, and the second letter is lowercase. 🇦
• Some symbols are derived from the element’s Latin, German, or Greek names. 🇬🇷

Atomic Mass ⚖️

• Dalton’s atomic theory proposed the concept of atomic mass, stating that each element has a characteristic atomic mass. ⚛️
• Relative atomic masses were determined using the laws of chemical combinations and the compounds formed. 🧪
• In 1961, the carbon-12 isotope was chosen as the standard reference for measuring atomic masses. 🧪
• One atomic mass unit (u) is equal to one-twelfth the mass of one atom of carbon-12. ⚛️
• The relative atomic mass of an element is defined as the average mass of the atom compared to 1/12th the mass of one carbon-12 atom. ⚛️

How do Atoms Exist? ⚛️

• Atoms of most elements cannot exist independently. 🔄
• They form molecules and ions, which aggregate to form the matter we can see, feel, or touch. 🖐️

What is a Molecule? 🤔

• A molecule is a group of two or more atoms chemically bonded together. ⚛️
• It is the smallest particle of an element or a compound that can exist independently and exhibit the properties of that substance. 🧪

Molecules of Elements ⚛️

• Molecules of an element are made up of the same type of atoms. 🅰️
• Some elements, like argon (Ar) and helium (He), have molecules consisting of only one atom. 🅰️
• Non-metals usually exist as diatomic molecules, like oxygen (O2​). 🧪
• The number of atoms in a molecule is called its atomicity. 🔢

Molecules of Compounds ⚛️

• Atoms of different elements combine in definite proportions to form molecules of compounds. 🧪

What is an Ion? 🤔

• Compounds made of metals and non-metals contain charged species called ions. ⚛️
• Ions can be single charged atoms or groups of atoms with a net charge. ⚛️
• Negatively charged ions are called anions, and positively charged ions are called cations. ⚛️
• A group of atoms carrying a charge is a polyatomic ion. ⚛️

Writing Chemical Formulae ✍️

• The chemical formula of a compound represents its composition symbolically. 🧪
• The combining power of an element is its valency. 🧪
• Valency helps determine how atoms of one element combine with atoms of another element to form a compound. ⚛️

Rules for Writing Chemical Formulae:

• The valencies or charges on the ions must balance. ⚛️
• In a metal-non-metal compound, the metal’s name or symbol is written first. 🧪
• In compounds with polyatomic ions, the number of ions is indicated by enclosing the ion’s formula in brackets and writing the number outside. ⚛️

Formulae of Simple Compounds 🧪

• The simplest compounds, made of two different elements, are binary compounds. 🧪
• Valencies are used to write the formulae for compounds. ⚛️

Molecular Mass ⚖️

Molecular Mass

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

Formula Unit Mass

• The formula unit mass is the sum of atomic masses of all atoms in a formula unit of a compound. ⚛️
• It is calculated similarly to molecular mass, but the term “formula unit” is used for substances whose constituent particles are ions. ⚛️

Conclusion 🎓

• Atoms and molecules are the fundamental building blocks of matter. ⚛️
• Understanding the laws of chemical combination and Dalton’s atomic theory is crucial for comprehending the nature of matter. 🧪
• Writing chemical formulae requires knowledge of valencies and the rules for combining elements. ✍️
• Molecular mass and formula unit mass are essential concepts for quantifying the masses of molecules and compounds. ⚖️

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