CBSE Class 9 - Chemistry - Chapter: Atoms and Molecules (Important Definitions) (#class9Chemistry)(#cbsenotes)(#eduvictors)(#class9Science)

Chapter: Atoms and Molecules (Important Definitions)

CBSE Class 9 - Chemistry

1. Law of Conservation of Mass: 

Mass can neither be created nor can it be destroyed in a chemical reaction. In a chemical reaction,

   Total mass of reactants = Total mass of products

2. Law of Constant Proportions: 

In a pure substance same elements are always present in a definite proportion by weight.

3. Atom: 

Smallest unit of an element, which may or may not exist independently, but always takes part in a chemical reaction.

4. Molecule: 

Smallest unit of an element or a compound which always exists independently and retains the complete physical and chemical properties of the element or the compound.

5. Atomicity of a molecule of an element: 

It is the number of atoms which constitute one molecule of an element.

6. Molecular formula: 

The symbolic representation of the kind and the actual number of atoms in one molecule of a pure substance, may be an element or a compound.

7. Atomic mass unit: 

The mass of 1/12 part of C-12 (isotope of carbon) is equivalent to one atomic mass unit.

8. Gram-atomic mass: 

The atomic mass of an element expressed in terms of grams.

9. Atomic mass : 

The number of times an atom of an element is heavier than 1/12 part of C-12 (isotope of carbon).

10. Molecular mass: 

The number of times a molecule of a pure substance is heavier than 1/12 part of C-12 (isotope of carbon).

11. Gram-molecular mass: 

The molecular mass of a pure substance expressed in grams.

12. Mole: 

A group of 6.022 × 10²³ particles (atoms, molecules, ions, electrons, protons, neutrons, etc.) of a substance is called mole.

13. Ion: 

It is an electrically charged atom or group of atoms. It is formed by the loss or gain of one or more electrons by an atom.

14. Valency: 

It is defined by the combining power (or capacity) of an element.

15. Formula unit mass:

It is the sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is used for those substances whose constituent particles are ions.

👉See Also:

How to learn naming Chemical Formula
Ch 3 - Atoms and Molecules
Ch 3 - Atoms and Molecules (Worksheet)
Ch3 - Atoms and Molecules (MCQs) 
Ch 3 - Atoms, Molecules and Ions (Study Points)
Ch 3 - Atoms and Molecules (MCQs-2)
Ch 3 - Atoms and Molecules - Atomicity of Elements (Worksheet)
All about Names of the Chemical Elements