3. Atoms and Molecules Science class 9 in English Medium ncert book solutions Chapter Review

Chapter Review:

• The smallest tiny particles of matter which cann't be divided further is called atom (Parmanu).
• The law of conservation of mass and the law of constant proportions these two laws of chemical combination were established by Lavoisier and Joseph L. Proust.
• Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
• The law of constant proportions which is also known as the law of definite proportions.
• In a chemical substance the elements are always present in definite proportions by mass, this law is called the law of constant proportions.
• Dalton's theory was based on the laws of chemical combination.
• According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.
• Dalton's second postulate states "Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction".
• Dalton's third postulate states "atoms of a given element are identical in mass and chemical properties".
• Dalton's third postulate states "atoms of different elements have different masses and chemical properties".
• Atomic radius is measured in nanometres.
• IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.
• The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase).
• Each element has a name and a unique chemical symbol.
• The atomic mass unit is writen as ‘u’ – unified mass.
• Carbon-12 isotope was chosen as the standard reference for measuring atomic masses.
• One atomic mass unit is a mass unit equal to exactly onetwelfth
  (1/12th) the mass of one atom of carbon-12.
• Atoms form molecules and ions.
• These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.
• 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.
• A molecule of oxygen consists of two atoms of oxygen and hence it
  is known as a diatomic molecule, O₂.
• 3 atoms of oxygen unite into a molecule these form ozone e.i O₃.
• The number of atoms constituting a molecule is known as its atomicity.
• Atoms of different elements join together in definite proportions to form molecules of compounds.
• The charged atoms are known as ions.
• An ion is a charged particle and can be negatively or positively
  charged.
• A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’.
• Ions may consist of a single charged atom or a group of atoms that have a net charge on them.
• A group of atoms carrying a charge is known as a polyatomic ion.
• The chemical formula of a compound is a symbolic representation of its composition.
• The combining power (or capacity) of an element is known as its valency, in other hand the number of valnce electron is known as valancy of an atom. 
• The quantity of a substance can be characterised by its mass or the number of molecules.
• One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.
• The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 × 10²³ ,  This number is called the Avogadro Constant or Avogadro Number.
• The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams.
• Mass of 1 mole of a substance is called its molar mass.
• The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12.