Class 9 Science Atoms & Molecules

Important Notes

1. Law of conservation of mass : Mass can neither be created nor can it be destroyed in a chemical reaction.
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. Mole number : It is a number which states, how many times one molecular mass comes in certain mass of a substance in grams.

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