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CBSE Class 9 Science Notes Chapter 3 Atoms And Molecules
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Introduction:
• Everything around us is made up of atoms and molecules, which are the building blocks of matter.
• In this chapter, we will learn about the composition of matter, the laws of chemical combination, and the concept of atoms and molecules.
• Laws of Chemical Combination:
• The law of conservation of mass states that the total mass of the reactants in a chemical reaction is equal to the total mass of the products.
• The law of constant proportions states that the elements in a compound are always present in a fixed ratio by mass.
• The law of multiple proportions states that when two elements combine to form different compounds, the ratio of the masses of one element that combine with a fixed mass of the other element can be expressed in small whole numbers.
• Dalton’s Atomic Theory:
• John Dalton proposed the atomic theory which had the following postulates:
• All matter is made up of tiny particles called atoms.
• Atoms of the same element are identical in all respects.
• Atoms of different elements are different.
• Atoms can neither be created nor destroyed.
• In a chemical reaction, atoms combine in small whole-number ratios to form compounds.
• Atomic and Molecular Masses:
• The atomic mass of an element is the mass of one atom of that element compared to the mass of an atom of carbon-12 (which is assigned a mass of exactly 12 units).
• The molecular mass of a compound is the sum of the atomic masses of all the atoms in the molecule.
• The formula unit mass of an ionic compound is the sum of the atomic masses of all the atoms in the formula unit of the compound.
• Mole Concept:
• A mole is defined as the amount of a substance that contains the same number of particles as there are atoms in 12 g of carbon-12.
• One mole of any substance contains Avogadro’s number (6.022 × 10²³) of particles.
• The mass of one mole of a substance is called its molar mass.
• Chemical Formulae and Equations:
• A chemical formula is a shorthand notation that represents the composition of a compound in terms of its constituent elements and the number of atoms of each element.
• A chemical equation represents a chemical reaction by using symbols and formulae of the reactants and products.
• The law of conservation of mass applies to chemical equations.
• Atoms are the basic units of chemical elements, while molecules are formed when two or more atoms chemically combine.
• Atoms are made up of subatomic particles - protons, neutrons, and electrons. The number of protons in the nucleus determines the identity of the element.
• The electrons in an atom are arranged in energy levels or shells. The first shell can hold up to 2 electrons, while the second and third shells can hold up to 8 electrons each.
• The valence electrons in the outermost shell of an atom are involved in chemical bonding and determine the chemical properties of the element.
• Chemical bonds can be ionic, covalent, or metallic. In ionic bonding, electrons are transferred from one atom to another, while in covalent bonding, atoms share electrons.
• The properties of molecules are determined by the type and arrangement of atoms within them. Isomers are molecules that have the same chemical formula but different structural arrangements.
• Chemical reactions involve the breaking and formation of chemical bonds. The reactants are the starting materials, while the products are the end materials.
• The stoichiometry of a chemical reaction refers to the relationship between the quantities of reactants and products. This is determined by balancing the chemical equation.
• The study of atoms and molecules has many applications in various fields such as materials science, biochemistry, and pharmaceuticals.
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