Summary -Class 9- Chemistry –CH3 – Atoms and Molecules

Summary -Class 9- Chemistry –Chapter 3 – Atoms and Molecules

Atoms and Molecules:

Atoms: Atoms are the basic units of matter, consisting of a nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons. Atoms are incredibly small, with diameters typically ranging from 0.1 to 0.5 nanometers.

Molecules: Molecules are formed when two or more atoms chemically bond together. These bonds can be covalent, where atoms share electrons, or ionic, where one atom transfers electrons to another. Molecules can be of elements (like O2 for oxygen) or compounds (like H2O for water).

Laws of Chemical Combination:

Law of Conservation of Mass: This law states that in a chemical reaction, matter cannot be created or destroyed; it only changes form. This means that the total mass of reactants must be equal to the total mass of products.

Law of Constant Proportions: According to this law, a pure chemical compound always contains the same elements in the same proportion by mass, regardless of its source or how it was prepared.

Dalton’s Atomic Theory:

Indivisibility of Atoms: Dalton proposed that atoms are indivisible and indestructible particles.

Identical Properties: Atoms of the same element are identical in mass and properties, while atoms of different elements have different masses and properties.

Combination in Fixed Ratios: Atoms combine in simple, whole-number ratios to form compounds.

Conservation of Atoms: Atoms are neither created nor destroyed in chemical reactions; they are rearranged to form new compounds.

Atomic Mass and Molecular Mass:

Atomic Mass: Atomic mass is the mass of an atom, which includes the masses of its protons, neutrons, and electrons. It is typically expressed in atomic mass units (amu).

Molecular Mass: Molecular mass is the sum of the atomic masses of all the atoms in a molecule. It is calculated by adding up the atomic masses of the constituent atoms.

Mole Concept and Avogadro’s Number:

 Mole Concept: The mole is a unit of measurement used in chemistry to express the amount of a substance. One mole of a substance contains Avogadro’s number (6.022×10^23) of particles, such as atoms, molecules, or ions.

Avogadro’s Number: Avogadro’s number represents the number of entities (atoms, molecules, ions) in one mole of a substance. It allows chemists to relate mass to the number of particles present in a sample.

Molar Mass:

Definition: Molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol).

Calculation: To calculate the molar mass of a compound, one adds up the atomic masses of all the atoms present in the compound according to its chemical formula.

Structure of Atom and Atomic Valency:

Atomic Structure: Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons arranged in energy levels or orbitals.

Atomic Valency: Valency refers to the combining capacity of an atom, which is determined by the number of electrons it gains, loses, or shares to achieve a stable electron configuration.

Writing Chemical Formulae and Ionic Compounds:

Chemical Formulas: Chemical formulas represent the composition of compounds using symbols for elements and subscripts to indicate the number of atoms present.

Ionic Compounds: Ionic compounds are formed through the transfer of electrons between atoms, resulting in the formation of positively charged cations and negatively charged anions. These ions combine to form compounds with neutral charges.