Saturday, 13 March 2021

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

Chapter: Atoms and Molecules (Important Definitions)

CBSE Class 9 - Chemistry

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



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.


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