9th Class Chemistry Notes of Chapter 1

Chemistry is study of composition and properties of matter. It has different branches.

Substances are classified into elements and compounds.

Elements consist of only one type of atoms.

Compounds are formed by chemical combination of atoms of the elements in a fixed ratio.

Mixtures are formed by mixing up elements or compounds in any ratio. They are classified as homogeneous and heterogeneous mixtures.

Each atom of an element has a specific atomic number (Z) and a mass number or atomic mass (A).

Atomic mass of an atom is measured relative to a standard mass of C-12.

Relative atomic mass of an element is the mass of an element compared with 1/12 mass of an atom of C-12 isotope.

Atomic mass unit is 1/12 of the mass of one atom of C-12, lamu = 1.66 x l0-24g

Empirical formula is the simplest type of chemical formula, which shows the relative number of atoms of each element in a compound.

Molecular formula gives the actual number of atoms of each element in a molecule.

Formula mass is the sum of atomic masses of all the atoms in one formula unit of a substance.

An atom or group of atoms having a charge on it is called an ion. If it has positive charge it is called a cation and if it has negative charge it is called an anion.

There are different types of molecules: monoatomic, polyatomic, homoatomic and heteroatomic.

The number of particles in one mole of a substance is called Avogadro’s number. The value of this number is 6.02 x 1023 It is represented as NA

The amount of a substance having 6.02 x 1023 particles is called a mole. The quantitative definition of mole is atomic mass, molecular mass or formula mass expressed in grams.

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