CBSE – 9th Standard Science

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Class 9 chapter 3 – Atoms & Molecules

LAWS OF CHEMICAL COMBINATION

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According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

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  • The relative atomic mass of the atom of an element is defined as the average mass of the atom.
  • The number of atoms constituting a molecule is known as its atomicity.

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  • A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.
  • Atoms of the same element or of different elements can join together to form molecules.

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Compounds composed of metals and non-metals contain charged species. The charged species are known as ions

MOLECULAR MASS

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CHEMICAL FORMULAE

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  • It is therefore the relative mass of a molecule expressed in atomic mass units (u).
  • The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 x 1023. This number is called the Avogadro Constant or Avogadro Number.
  • Avogadro Number is named in honour of the Italian scientist, Amedeo Avogadro.
  • The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams.
  • Molar mass of atoms is also known as gram atomic mass.

MOLE CONCEPT

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ISOTOPES

The atoms of same element, having same atomic number but different mass numbers. e.g. hydrogen 11H, deuterium 1 2H and tritium 1 3H