Chapter-3: Atoms and Molecules

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1 1 Titbits: Chapter-3: Atoms and Molecules Smallest unit of any matter is called an atom Molecule is a group of atoms that are held together by a chemical bond. Compound is a form of matter formed when elements combine chemically in fixed ratio of their mass A compound having two different elements is called a binary compound. Charged atom or group of atoms is class an ion. An element can be represented by a letter or letters. The abbreviation is called chemical symbol. The abbreviation that shows how many atoms of each element are present in one molecule of compound is called chemical formula. A mole = 6.022X particles (in number) of anything 6.022X is called Avogadro number Mass of 1 mole of a particular substance is fixed. The molar mass of any element is same as its atomic mass but unit is changed from u to g (gram) Molar mass of an atom is also known as gram atomic mass The molar mass of a particular substance in fixed. The relative atomic mass of the atom of an element is the average mass of the atom as compared to 1/12th the mass of one carbon-12 atom. The number of moles (N) = Given Mass (m) / Molar Mass (M) OR = Given number of particles(n) / Avogodro number (N0) Number of particles present in any given mass of element or molecule can be given by The number of atoms or molecules (N) = Avogodro number (N0)/given number of partcles (N) Revision Notes: # Law of Conservation of Mass: Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. This law was given by Lavoisier. This means in a chemical reaction, total mass of resultant products is equal to the total mass of reactants. # Law of Constant Proportions: This law was given by Proust. This says that In a chemical substance the elements are always present in definite proportions by mass. This means, a chemical compound always consists of the same elements combined together in the same proportion by mass. # Dalton s Atomic Theory: All matter, whether an element, a compound or a mixture is composed of small particles called atoms. The postulates of this theory are as follows: All matter is made of very tiny particles called atoms.

2 2 (iv) (v) (vi) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. Atoms of a given element are identical in mass and chemical properties. Atoms of different elements have different masses and chemical properties. Atoms combine in the ratio of small whole numbers to form compounds. The relative number and kinds of atoms are constant in a given compound. # Naming Elements: In the beginning, the names of elements were derived from the name of the place where they were found for the first time Some names were taken from specific colours. For example, gold was taken from the English word meaning yellow. Now-a-days, IUPAC (International Union of Pure and Applied Chemistry) approves names of elements. # Symbols of Elements: Many of the symbols are the first one or two letters of the element s name in English. The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase). Ex: Hydrogen (H), Aluminium (Al) Other symbols have been taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from its Latin name ferrum. Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name. Example - chlorine (Cl) # Atomic Mass: One atomic mass unit (1u) is defined as exactly one-twelfth the mass of Carbon-12 atom. The atomic masses of other elements are determined comparing the mass their atom with the mass of Carbon-12 atom. Carbon-12 atom has been assigned an atomic mass of 12 amu. Atomic mass unit is abbreviated as amu or nowadays as u. One atomic mass unit is = the mass of carbon-12 atom. # The relative atomic mass of the atom of an element is the average mass of the atom as compared to 1/12th the mass of one carbon-12 atom. # Molecule is a group of atoms that are held together by a chemical bond.

3 3 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. # The molecules of an element are constituted by the same type of atoms. Ex: Oxygen and hydrogen-diatomic, Sodium & Copper mono atomic # Atoms of different elements join together in definite proportions to form molecules of compounds. Ex: Water is a compound of Hydrogen and Oxygen in ration of mass of 1:8. # Compounds composed of metals and non-metals contain charged species. The charged species 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. Take, for example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na + ) and negatively charged chloride ions (Cl - ). # The rules that you have to follow while writing a chemical formula are as follows: the valencies or charges on the ion must balance. when a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left. in compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. # Molecular Mass: The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u). Ex: The molecular mass of HNO3 = the atomic mass of H + the atomic mass of N+ 3 the atomic mass of O = = 63 u # Formula Unit Mass: The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular mass. Formula unit mass of NaCl is calculated as = 58.5 u

4 # Mole Concept: Mole is a link between the mass of atoms or molecules and the number of atoms or molecules. A number of 6.

4 4 # Mole Concept: Mole is a link between the mass of atoms or molecules and the number of atoms or molecules. A number of particles (atoms/molecules/ions) of a substance is called a mole of that substance. 1 mole of oxygen atoms = atoms 1 mole of Oxygen (O2) molecules = Oxygen molecules 1 mole (of anything) = in number The number which represents a mole is known as Avogadro Constant or Avogadro Number (represented by No) The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams. For example, molecular mass of water (H2O) is 18 u. 18 u water has only 1 molecule of water, 18 g water has 1 mole molecules of water, that is, molecules of water. Similarly, 16 u oxygen has only 1 atom of oxygen, 16 g oxygen. # Try to solve the following: Q.1 Calculate the mass of 0.5 mole of CO2 Q.2 Calculate the no. of particles in 8 g of oxygen molecules. ***

5 # Formulae of some Compounds: Name of compound Hydrogen Chloride Hydrogen Sulphide Sodium Nitrate Calcium Hydroxide Sodium Carbonate Magnesium chloride Aluminium Oxide Calcium Oxide Ammonium

5 5 # Formulae of some Compounds: Name of compound Hydrogen Chloride Hydrogen Sulphide Sodium Nitrate Calcium Hydroxide Sodium Carbonate Magnesium chloride Aluminium Oxide Calcium Oxide Ammonium Sulphate Sodium Oxide Aluminium Odide Magnesium Hydroxide Aluminium Sulphate Calcium Chloride Potassium Sulphate Potassium Nitrate Calcium Carbonate Formula HCl H2S NaNO3 Ca(OH)2 Na2CO3 MgCl2 Al2O3 CaO (NH4)2SO4 Na2O Al2O3 Mg(OH)2 Al2(SO4)2 CaCl2 K2SO4 KNO3 CaCO3 ***

CHAPTER - 3 Atoms and Molecules

CHAPTER - 3 Atoms and Molecules CONCEPT DETAILS KEY CONCEPTS : [ *rating as per the significance of concept] 1. Laws of Chemical Combination *** 2. John Daltons Atomic Theory ** 3. Atoms, ions & Chemical