Atoms And Molecules Class 9 Notes For CBSE Board

Atom: Smallest particle of matter
Law of chemical combination:
- Law of conservation of mass: In a chemical reaction, mass can neither be created nor be destroyed.
- Law of constant proportions: In a chemical substance, all elements are present in definite proportion.
Postulates of Dalton’s Atomic Theory:
1. Matter is made up of small indivisible particles called atoms
2. Atoms are neither created nor destroyed, they are rearranged during the chemical reaction.
3. Atoms of same element have same mass, size and chemical properties.
4. Atoms of different element have different mass, size and chemical properties.
5. Atoms combines in whole number ratio to form compound.
6. Relative number and kind of atoms are constant in compounds.
Symbolic representation of elements:
- Dalton’s Representation: Symbols consist of circles containing, letter or different signs.
- Berzelius Representation: Symbols contains, first one or two letters from the name of element.
Nomenclature:
- Name of element derived from place of occurrence or from color of element.
- IUPAC approves the derived name.
- For symbols, first letter is in uppercase and second letter is in lowercase.
- Symbols are also derived from greek and latin names of elements.
Atomic mass: Mass of specific atom.
Atomic mass unit: A mass unit exactly equal to one-twelfth (1 / 12^th) mass of one atom of carbon – 12 isotope (¹²C). It is denoted by amu or u.
Relative atomic mass unit: Average mass of the atom, compared to 1/12th mass of one carbon-12 atom.
Molecule: Two or more atoms chemically combine with each other by attractive forces and forms molecule.
Molecules of elements: Contains atoms of similar type.
Atomicity: It is number of atoms in a molecule

Name of element	Atomicity	Number of atoms
Argon	Monoatomic	One
Sodium	Monoatomic	One
Nitrogen	Diatomic	Two
Hydrogen	Diatomic	Two
Phosphorous	Tetraatomic	Four

Molecules of compounds: Atoms of different elements are joined in different proportions.

Compound	combining elements	ratio by mass
Water	Hydrogen, Oxygen	1:8
Ammonia	Nitrogen, Hydrogen	14:3
Carbon Dioxide	Carbon, Oxygen	3:8

Chemical Formula: It is a symbolic representation of chemical compound.
Valency: Capacity of element to combine with other element.
Rules for writing chemical formula for binary compound:
- For binary compounds: Write elements and their valencies, crossover the valency and write it as a subscript.
- For ionic compounds: Write cation before anion, crossover the charges and write it as a subscript. If charges are same neglect the subscript.
- For more than one similar group: Arrange elements or ions, crossover their valency or charges, include ions occurring twice in brackets. If only one ion is present, neglect the brackets.
Molecular mass: It is sum of atomic masses of all atoms in a molecule.
Unit of molecular mass: atomic mass unit (u)
Example: Molecular mass of water = ( 2 × Atomic mass of hydrogen) + Atomic mass of oxygen = (2 × 1 u ) + 16 u= 2 u + 16 u= 18 u
Formula unit mass: It is the sum of masses of all atoms in formula unit or ions of given compound.
Unit of formula unit mass: atomic mass unit (u)
Example: Formula unit mass of NaCl = Atomic mass of Na+ + Atomic mass of Cl- = 1 × 22.9u + 1 × 35.45u= 22.9u + 35.45u= 58.35 u
One mole of substance: An amount of substance containing same number of particles, as that of atoms in 12g of carbon-12 isotopes. Particles can be atoms or molecules or ions. One mole of substance contains 6.022 × 10²³ particles.
Avogadro’s Number: One mole of substance contains 6.022 × 10²³ particles. This number is called Avogadro’s Number denoted by NA.
Molar Mass: Mass of one mole of substance
Relationship between Mole, Avogadro’s Number and Mass: