Grade 9 Science Chapter Notes: Atoms and Molecules PDF

Grade 09 'S Science Chapter Notes JU BY Chapter Notes Atoms and 'S Molecules JU GRADE 09 BY 88  Topics to Be Covered 1. Atom 1.1 Introduction to atom 1.2 Element 1.3 Representation 2. Molecule of elements 1.4...

Grade 09 'S Science Chapter Notes JU BY Chapter Notes Atoms and 'S Molecules JU GRADE 09 BY 88  Topics to Be Covered 1. Atom 1.1 Introduction to atom 1.2 Element 1.3 Representation 2. Molecule of elements 1.4 Atomic mass 2.1 Introduction to molecule 2.2 Atomicity 3. Molecular Mass and Formula Unit Mass 3.1 Molecular mass 'S 4. Laws of Chemical Combination JU 3.2 Formula unit mass 4.1 Law of conservation of mass 4.2 Law of constant 5. Dalton’s BY proportions Atomic Theory 5.1 Postulates of Dalton’s atomic theory 6. Ion 6.1 Introduction to ion 7. Writing Chemical Formulae 6.2 Valency 7.1 Rules to write a chemical formula 8. Mole Concept 7.2 Formula of simple compounds 8.1 Mole 8.2 Relationship between mass, mole and Avogadro’s number 89  Mind Map Writing Chemical Representation formulae of elements Atomic mass Molecule Atom 'S Atomicity JU Atoms and Molecular mass Molecules BY Ion Mole Concept Valency Avogadro’s number Formula unit mass Laws of Chemical Molar mass Combination Important formulae Law of conservation of mass Law of constant proportions Dalton’s atomic theory 90  1. Atom 1.1 Introduction to Atom An atom is the smallest particle of matter and is considered as its building block. Atoms 'S Everything around us is made up of atoms. JU Atomic radius is measured in nanometres. 1 m = 109 nm BY 1.2 Element The simplest substance made up of one kind of atom that cannot be further broken down by chemical processes is known as an element. E.g., Oxygen, nitrogen, silver, gold, hydrogen, etc. 91  1. Atom 1.3 Representation of Elements Dalton was the first Hydrogen Carbon scientist to propose symbols for elements. Oxygen Sulphur Rules of Berzelius system: The symbols of the most common elements are Rule 1: 'S generally denoted by the first letter of its English name, written in uppercase. JU E.g., Hydrogen- H If the first letter of two elements are the same, then Rule 2: the first two letters are used for the symbols, where BY the first letter is in uppercase and the second one is in lowercase. E.g., Helium- He If the first two letters of two elements are the Rule 3: same, then the next prominent letter is used. E.g., Chlorine- Cl For elements whose names are derived from their Rule 4: Latin or Greek origin, above rules are followed but with their Latin and Greek name. E.g., Natrium- Na Nowadays, International Union of Pure and Applied Chemistry approves the names of elements. 92  1. Atom 1.4 Atomic Mass In 1961, the carbon-12 atom was chosen as the standard reference for measuring atomic masses for a universally accepted atomic mass unit. One atomic mass unit (amu or u) is defined as the mass unit equal to one-twelfth (1/12th) of the mass of one carbon-12 atom. 'S Atomic masses of a few elements: JU Atomic mass Elements (amu or u) Hydrogen (H) 1 BY Carbon (C) 12 Nitrogen (N) 14 Oxygen (O) 16 Magnesium (Mg) 24 Calcium (Ca) 40 Sodium (Na) 23 93  2. Molecule 2.1 Introduction to Molecule A molecule is a group of two or more atoms that are chemically bonded together and have an independent existence. It is defined as the smallest particle of an element or a compound that shows all the properties of that substance. H Atoms 'S JU Molecules of elements Molecules of compounds BY (Made up of same (Made up of different kind of atoms) kind of atoms) 2.2 Atomicity The total number of atoms that constitute a molecule is known as its atomicity. P P O P Ar N N P Argon Nitrogen Ozone Phosphorus Monoatomic Diatomic Triatomic Polyatomic 94  3. Molecular Mass and Formula Unit Mass 3.1 Molecular Mass The sum of the atomic masses of all the atoms in a molecule is called molecular mass. O O + 2 H = H H 16 u 2×1u 'S 18 u JU BY 3.2 Formula Unit Mass The sum of atomic masses of all atoms in a formula unit of a compound is called formula unit mass. Na + Cl = Na + Cl - 23 u 35.5 u 58.5 u 95  4. Laws of Chemical Combination 4.1 Law of Conservation of Mass Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. Total mass of the reactants = Total mass of the products 'S JU 4.2 Law of Constant Proportions BY Law of constant proportions states that in a chemical substance, the elements are always present in definite proportions by mass. E.g., O C O Mass of one carbon atom 12 u 12 3 = = = Mass of two oxygen atoms 2 × 16 u 32 8 96  5. Dalton’s Atomic Theory 5.1 Postulates of Dalton’s Atomic Theory According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. The postulates are as follows: (i) All matter is made of very tiny particles called atoms, 'S which participate in chemical reactions. JU (ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. (iii) Atoms of a given element are identical in mass and BY chemical properties. (iv) Atoms of different elements have different masses and chemical properties. (v) Atoms combine in the ratio of small whole numbers to form compounds. (vi) The relative number and kinds of atoms are constant in a given compound. 97  6. Ion 6.1 Introduction to Ion Compounds composed of metals and non-metals contain charged species. The charged species are known as ions. A single atom carrying a charge is known as a 2+ - monoatomic ion. E.g., Mg , Br A group of atoms carrying a charge is known as a + - polyatomic ion. E.g., NH4 , NO3 'S An ion can be negatively or positively charged: JU The positively charged ion is called a cation. Cation + 2+ E.g., NH4 , Mg The negatively charged ion is called an anion. BY Anion - - E.g., Br , NO3 6.2 Valency The combining capacity of an atom/ion of an element to form a compound is called valency. Sodium ion Oxide ion Aluminium ion Na + O2- Al 3+ Valency: 1 Valency: 2 Valency: 3 98  7. Writing Chemical Formulae 7.1 Rules to Write a Chemical Formula The valencies or charges on the ions Rule 1 must balance. O2- K+ K+ 'S Potassium oxide K2O JU When a compound consists of a metal Rule 2 and a non-metal, the name or symbol of the metal is written first. BY - Na+ Cl Sodium chloride NaCl For polyatomic ions, the formula of ion is enclosed in a bracket and the number of ions Rule 3 written outside the bracket. If the number of polyatomic ions is 1, the bracket is not required. Mg2+ - OH- OH- Na+ OH Magnesium hydroxide Sodium hydroxide Mg(OH)2 NaOH 99  7. Writing Chemical Formulae 7.2 Formula of Simple Compounds While writing the chemical formulae for compounds, we write the constituent elements and their valencies. Then we must crossover the valencies of the combining atoms. Sodium nitrate Symbol: Na NO3 Valency: 'S 1 1 JU Formula: NaNO3 BY Calcium oxide Symbol: Ca O Valency: 2 2 Formula: CaO The valencies of the elements are the same, so we may arrive at the formula Ca2O2. But upon simplifying, the formula becomes CaO. 100  8. Mole Concept 8.1 Mole One mole of any species (atoms, molecules, ions or particles) contains a fixed number of particles which 23 is 6.022 × 10. This number is called Avogadro’s constant or Avogadro’s number (represented by NA). 'S 1 mole of atoms = 6.022 × 10 23 atoms JU 23 1 mole of molecules = 6.022 × 10 molecules 23 1 mole of ions = 6.022 × 10 ions BY The mass of 1 mole of a substance is equal to its atomic or molecular mass in grams. The molar mass of atoms is also known as gram atomic mass. For example, the atomic mass of hydrogen = 1 u. So, the gram atomic mass of hydrogen = 1 g. 101  8. Mole Concept 8.2 Relationship between Mass, Mole and Avogadro’s Number Given mass Number of moles = Molar mass m ⇒ n = M 'S JU Given number of particles Number of moles = Avogadro’s number BY N ⇒ n = NA Mass = Molar mass × Number of moles ⇒ m= M × n 102