Important points and formula of chemistry for class 9 chapter-Atom and Molecules

Important points and formula of chemistry for class 9 chapter-Atom and Molecules

May 13, 2022, 16:45 IST

Class-9 chapter-Atom and Molecules

Ancient thoughts about 'atom': The idea about the divisibility of matter was given by Indian and Greek philosophers around 500 BC. The smallest indivisible particle was called ‘parman’ or 'atom' in Greek).

Laws of chemical combination (based on experiments)

• Law of conservation of mass. Mass can neither be created nor destroyed or in any chemical reaction, the total mass of the reactants is equal to the total mass of the products

For example, for the reaction,

AgNO ₃ (aq)+NaCl(aq)→AgCl(s)+NaNO ₃ (aq)

Total mass of AgNO ₃ (aq) + NaCl(aq) = Total mass of AgCl(s) + NaNO ₃ (aq).

• Law of constant proportions. A chemical compound is always made up of two or more atoms of same or different elements combined together in the same fixed proportion by mass. For example, H ₂ O always contains H and O in the fixed ratio of 1:8 by mass.

Dalton's Atomic Theory: This theory was put forward by Dalton (1808) to explain the laws of chemical combination.

Properties of Atoms

• Atom : An atom is the smallest particle of an element, which may or may not be able to exist freely. It is, however, the smallest particle that takes part in a chemical reaction.
• Size of atoms : Atoms are so small in size that they cannot be seen even under a microscope. Their radii are of the order of 10 ^-10 m (1 nm-10 ^-9 m).
• How do atoms exist? Atoms of most of the elements exist in the combined state (as H ₂ , O2, etc. or H ₂ O, NH ₃ etc.) or as ions in the aqueous solution (as H ⁺ , Cu ²⁺ , Ag ⁺ , etc.).
• Symbols of atoms: Short hand method of representing full name of an element is called its symbol. It is first capital letter or first capital letter and another small letter from the full name. For example, carbon (C), cobalt (Co), chlorine (CI), copper (Cu, from Latin name, cuprum).
• Atomic mass unit (amu) or unified mass (u): It is 1/12 ^th of the mass of an atom of carbon-12 isotope
• Atomic mass: Atomic mass of an element is the average relative mass of its atoms as compared with mass of carbon-12 isotope taken as 12 units.
• Molecule: A molecule is the smallest particle of an element or a compound, which is capable of free existence.
• Atomicity: The number of atoms present in one molecule of the substance is called its atomicity. Thus, we have monoatomic, diatomic, triatomic, tetratomic or polyatomic molecules
• Some molecules of elements: Monoatomic = He, Ne, Na, Al, Fe etc. Diatomic = H ₂ , O ₂ , N ₂ etc. Triatomic = O ₃ . Tetratomic = P ₄ .
• Some molecules of compounds: Diatomic = HCI, CO etc. Triatomic = H ₂ O, SO ₂ , CO ₂ .

If you wants to learn the entire chapter of class 9 Chemistry check it out our class 9 Chemistry notes section for more details.Academic team of Physics Wallah prepared NCERT solutions do solve all questions given in exercise with the help of NCERT solutions for class 9 Science. If any students need to take the online test to check their concepts or undertstanding then they can visit Quiz for Atom and Molecule .

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