Atoms and Molecules CBSE Class 9 Revision

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the main premise of the Law of Conservation of Mass?

Mass of products is always equal to the mass of reactants.

Mass of reactants is always greater than the mass of products.

Mass can neither be created nor destroyed in a chemical reaction. (correct)

Mass can be created and destroyed in a chemical reaction.

Which of the following philosophers proposed the idea of indivisible particles called atoms?

Maharishi Kanad

Antoine Lavoisier

Democritus and Leucippus (correct)

Pakudha Katyayama

According to the Law of Definite Proportion, what can be inferred about a chemical substance?

The elements can be present in any proportion.

Elements are always in definite proportions by mass. (correct)

The mass of the elements can change during a reaction.

Chemical substances do not have a fixed composition.

What does the term 'parmanu' refer to in the context of Indian philosophy?

The smallest particle that cannot be divided further Signup and view all the answers

In the composition of water, what ratio by mass do hydrogen and oxygen exist?

1:8 Signup and view all the answers

Which philosopher suggested that various forms of matter exist due to combinations of particles?

Pakudha Katyayama Signup and view all the answers

What is the significance of Antoine Lavoisier's work in the context of chemical laws?

He formulated the Law of Conservation of Mass and the Law of Definite Proportion. Signup and view all the answers

What idea did Greek philosophers put forth about the division of matter?

There exists a limit to how far matter can be divided into atoms. Signup and view all the answers

What does John Dalton's atomic theory state about atoms?

Atoms are the building blocks of matter. Signup and view all the answers

What is the atomic mass unit based on?

The mass of a carbon-12 atom. Signup and view all the answers

Which statement correctly represents Dalton's view on chemical reactions?

Atoms are neither created nor destroyed during chemical reactions. Signup and view all the answers

What is the appropriate size measure of an atom?

Nanometers Signup and view all the answers

According to Dalton's theory, how do atoms combine to form compounds?

Only in whole number ratios. Signup and view all the answers

Which of the following is NOT a postulate of Dalton's atomic theory?

Atoms can be transformed into different elements. Signup and view all the answers

Which of the following accurately describes atomic radius?

Measured in nanometers Signup and view all the answers

What is a defining characteristic of atoms from different elements according to Dalton?

They differ in mass and properties. Signup and view all the answers

Study Notes

Philosophers on Divisibility

Maharishi Kanad proposed that continuous division of matter leads to ultimate particles called ‘parmanu,’ which cannot be divided further.
Pakudha Katyayama believed that varied forms of matter arise from particles existing in combinations.
Greek philosophers Democritus and Leucippus posited that matter can only be divided to a certain extent, yielding indivisible particles known as atoms.

Laws of Chemical Combination

Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction.
Law of Constant Proportion asserts that elements in a chemical substance are always in fixed mass ratios, exemplified by water (H₂O) containing hydrogen and oxygen in a ratio of 1:8.

John Dalton’s Atomic Theory

Matter comprises tiny, indivisible particles known as atoms.
Atoms are conserved during chemical reactions, not formed or destroyed.
Atoms of a given element have identical properties (size, mass, nature), whereas atoms of different elements differ.
Atoms group in whole number ratios to form compounds, which maintain a constant number and kind of atoms.

Atoms as Building Blocks

Atoms are the fundamental components of matter, analogous to bricks in a building.
The size of atoms is measured in nanometers (1 nm = 1/10^9 m), indicating their minuscule scale.
Relative sizes of particles range widely, from hydrogen atoms (~10^-10 m) to macroscopic objects like watermelons (~10^-1 m).

Symbols for Atoms

Chemical symbols for elements are derived from the first letters of their names; the first letter is capitalized, and the second is lowercase (e.g., H for hydrogen, O for oxygen).

Atomic Mass

Dalton’s Atomic Theory introduced the concept that each element has a specific atomic mass.
Atomic masses are evaluated relative to a standard; carbon-12 is now the standard (12 atomic mass units).
One atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom, providing a basis for comparing other elements.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Revise essential concepts from CBSE Class 9 Science Chapter 3 on Atoms and Molecules. These expert-created notes include key points, diagrams, and tables to aid your understanding and memorization. Perfect for quick revisions leading up to your exams, these notes are accessible for all students.