E-Content for Class IX (Science)�Chapter: Atoms and Molecules�BY

SANTOSH KUMAR CHAURASIA, PGT CHEMISTRY

JNV JHABUA-2 MP

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NAVODAYA VIDYALAYA SAMITI, NOIDA

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CHAPTER-3

ATOMS AND MOLECULES

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We come across different things around us like chair, table, etc. and all the things that surround us have mass and weight. They all constitute matter. Matter is anything that occupies space and has mass. Matter is made up of small particles called atoms.

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LEARNING OUTCOMES AND GOALS

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• LAW OF CONSERVATION OF MASS,
• LAW OF CONSTANT PROPORTIONS,
• POSTULATES OF DALTON’S ATOMIC THEORY,
• DEFINE AN ATOM,
• IDENTIFY THE SYMBOLS OF ELEMENTS,
• IDENTIFY THE ATOMIC MASS OF ELEMENTS,
• DEFINE MOLECULE,
• DEFINE AN ION,
• DEFINE VALENCY,
• RULES FOR WRITING A CHEMICAL FORMULA,
• MOLECULAR MASS,
• MOLE CONCEPT.

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The idea of divisibility by Indian philosophers-

• Maharishi Kanad – He postulated that if we keep on dividing the matter (called as ‘padarth’) we will get smaller and smaller particles.  And soon we will achieve the smallest of particles (called as ‘parmanu’) which may not divide further.

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Pakudha Katyayama – He said that these particles normally exist in a combined form which gives us various forms of matter.

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The idea of divisibility by Greek philosophers-

• Ancient Greek philosophers – Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms (meaning indivisible).

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We also know that a lot of chemical reactions take place in our day to day life like making of tea, changing milk to curd, making cheese from milk and lots more. All the chemical reactions taking place obey certain set of laws. Let us study these chemical reactions and the rules that they obey.�

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Chemical reaction

• The process by which some substances react to form a new substance. The substances which react are called reactants and the new ones formed are called products. For example:

A+B🡪C+D�In this, A and B are reactants and C and D are called products.

• For example: when we make tea, we add sugar, tea, milk and water. We mix them and heat the mixture. The result is that we get a new substance that is tea.

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Laws of Chemical Combination

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There are two main laws of Chemical Combination as established by Lavoisier and Joseph L. Proust-

�(a)Law of Conservation of Mass�(b) Law of constant or definite proportion

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Law of Conservation of Mass�

• For example:-�2H2 + O2 🡪 2 H2O�(Reactants) (Product)�=> 4 + 2 x 16=32 🡪 2 x 18�=> 36 g 🡪 36 g.

• It states that mass cannot be created or destroyed in chemical reaction. So mass of reactants is mass of the Product.

A + B -> C +D

• Mass of Reactants=Mass of (A+B)�Mass of Products = Mass of (C+D)�Mass of Reactants = Mass of Products

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Law of constant or definite proportion��

For example,

• Hydrogen and oxygen are present in water in a ratio of 1:8.

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So if we decompose 9g of water we will obtain 1g of hydrogen and 8g of oxygen.

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It states that elements combine in their definite proportion of mass to give compounds. or In a chemical substance the elements are always present in definite proportions by mass

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��The Dalton’s Atomic Theory���

John Dalton proposed an atomic theory which acted as an explanation of the above two laws. As per the theory, all matter whether it is an element, a compound or a mixture consists of tiny invisible particles called ‘atoms’.

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The postulates of the atomic theory by John Dalton -�

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• All matter is made of very tiny particles called atoms.
• Atoms are indivisible particles
• Atoms can neither be created nor be destroyed
• Atoms of same elements are similar.
• Atoms of different elements are different.
• 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.

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Drawbacks of Dalton’s Atomic Theory

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Drawbacks of Dalton’s Atomic Theory were as follows-

According to Dalton, an atom is indivisible but later on it was proved that atom can be subdivided into electrons, protons and neutrons.

Atoms of an element can somehow differ from each other.

So, these drawbacks led to the failure of Dalton’s theory of an atom.

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What is an Atom

• Matter is made up of Atoms
• Atom are the smallest particle of elements
• Atoms are very small, they are smaller than anything that we can imagine or compare with
• The size of the atoms is measured by the Atomic Radius .Atomic radius is measured in nanometres ( 1 nm=10−9 m)
• Atomic Radius of the Hydrogen is 10−10 m And it is the smallest of all

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Atomic radius is measured in nanometers�1/10⁹ = 1nm�1m  = 10⁹ nm

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Symbols of Atoms or Elements�

Dalton proposed the below symbols for the atoms

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IUPAC and Atomic symbols�

IUPAC - International Union Of Pure & Applied Chemistry

• According to IUPAC-

• Symbol of an element is either the first letter of the name of the element or the first two letters of the name of an element.
• In case there are two letters, then the first letter is in a capital case and the second letter is in small case.

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Sometimes, the symbol is derived from the Latin name of an element.

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The Atomic Mass�

• The Dalton’s Atomic Theory suggested that each element has a distinguishing atomic mass. With this theory, the law of constant proportions could be explained easily.
• But it is indeed difficult to evaluate the mass of an atom since the size of an atom is relatively small.

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• Therefore scientists started evaluating the mass of an atom by comparing it with the mass of a standard atom.

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Earlier 1/16 of the mass of an oxygen atom was used as a standard for calculating the mass of other elements. Now, carbon - 12 is considered a standard atom for calculating the mass. Its atomic mass is 12u (12 atomic mass units).

• We define One atomic mass unit is a mass unit equal to exactly one-twelth (1/12th) the mass of one atom of carbon-12. relative atomic masses of all elements have been found with respect to an atom of carbon-12.
• So, Atomic mass of atom is measured in amu. amu is written as 'u

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•  In simple language, atomic mass is the number of times an atom of an element is heavier than 1/12th of a Carbon atom.

�For example: If we say that the atomic mass of Sodium is 23, it means that sodium is 23 times heavier than 1/12th of a carbon atom.

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Can atoms exist independently?�

• Atoms cannot survive independently. So, atoms join together and form molecules or ions. 

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Molecule�

• A molecule is a collection of various atoms that combine chemically with each other.
• These atoms are bound together by certain forces of attraction.
• Atoms of the same elements or different elements can bind together to form molecules.
• Therefore, a molecule is the smallest particle of a substance that can exist independently and shows all the properties of that substance

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Molecules of Elements -Generally atoms of most of the elements exists as molecules Like Oxygen exists as O2, Hydrogen exists as H2. �

�Molecules of Compounds- The molecule of compounds contains two or more different atoms chemical bonded together.�Example- HCl ,H20

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Atomicity – the number of atoms in a molecule of an element is called its atomicity. For example, helium is monoatomic and oxygen is diatomic

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• Monoatomic – when an element comprises of a single atom. Example – all metals
• Diatomic – when an element comprises of two atoms. Example – all gases

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• Triatomic – when an element comprises of three atoms
• Tetra-atomic – when an element comprises of four atoms
• Poly-atomic – when an element comprises of more than two atoms

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Molecules of Compounds�

• Molecules of compounds constitute atoms of different elements that combine together in a fixed proportion. For example, water comprises of two atoms of hydrogen and one atom of oxygen.

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Ions�

• 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.

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Types of ion-�1-Cation 2- Anion

• Cation: they are positively charged ions. They are formed when an atom loses a negative charge. Therefore, they always have negative charges less than normal atoms.
• Example: Na +1 positive charge 🡪Na+ (cation)�

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• Anion: they are negatively charged ions and are formed when an atom gains a negative charge. Therefore, they have negative charges more than normal atoms.�Example: Cl -1 negative charges 🡪 Cl- (anion)

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Valency�

• The combining power (or capacity) of an element is known as its valency. For ions, the charge indicates the valency of the ions

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Rules of writing a Chemical Formula�

• Valencies of on the ions must balance.
• In a case where both metal and non-metal substances are present in a compound, the name of the metal is always written first in the chemical formula. For example, Sodium Chloride is written as NaCl
• In case of polyatomic ions, the ion is written in brackets before writing the number of ions associated to it. In case of a single ion, there is no need to mention the ion in brackets

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Writing the Formulae of Simple Compounds�

Binary compounds – compounds that consist of two different elements

How to write a Formula of a Compound

• Write the symbols of the corresponding elements of the compound as explained above
• Write the valencies of the elements of the compound
• Crossover the valencies of the elements

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Here are a few examples of writing the chemical formula��

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Molecular Mass�

• Molecular Mass – summation of all the atomic masses in a molecule
• Molecular mass is expressed in atomic mass units (amu).
• For example, the molecular mass of HNO3 can be calculated as:
• Atomic mass of H =1u
• Atomic mass of N =14u
• Atomic mass of O =16u
• Molecular mass of HNO3 = 1 + 14 + (16*3) = 63u

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Formula Unit Mass�

• The sum of atomic masses of all atoms in a formula unit of a compound is called as its formula unit mass.  The formula unit mass is used in case of substances that constitute ions. For example, formula unit mass of Sodium Chloride (NaCl) can be calculated as:    (1*23) + (1*35.5) = 58.5u

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Mole Concept�How do we interpret a chemical equation?�

2C + O2 = 2CO2

• We say that two molecules of carbon combine with one molecule of oxygen to form two molecules of carbon dioxide.
• We can also say that 24u of Carbon molecules combine with 32u of oxygen molecules to form 56u of carbon dioxide molecules.
• Therefore, we can characterize the quantity of a substance by its mass or by its number of molecules.
• A chemical equation directly indicates the number of molecules participating in the reaction. Thus, it is convenient for us to refer to the number of substances in a chemical reaction as numbers of molecules or atoms.
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Mole�

• Mole is a numerical quantity that has a mass equal to the atomic or molecular mass of species (atoms, molecules, ions or particles).
• 1 mole of any substance = 6.022 X 1023 number of particles (atoms, ions or molecules)
• This is called the Avogadro number or Avogadro Constant which is represented as N0
• The mass of 1 mole of a substance is the same as that its atomic mass or molecular mass expressed in grams.

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• Gram atomic mass of a substance – the atomic mass of a substance when expressed in grams is known as its gram atomic mass.
• Gram molecular mass of a substance – the molecular mass of a substance when expressed in grams is known as its gram molecular mass.
• For example, the atomic mass of Sulphur is 32u. Gram atomic mass of Sulphur is 32g.
• Also, 32u of Sulphur has 1 atom of Sulphur. 32g of Sulphur has 1 mole atoms, that is, 6.022 X 1023 atoms of Sulphur.

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Formula

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A quick review of how mole, Avogadro number and Mass are related to each other –

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RECAPITULATION

During a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is known as the Law of Conservation of Mass.

• In a pure chemical compound, elements are always present in a definite proportion by mass. This is known as the Law of Definite Proportions.

• An atom is the smallest particle of the element that cannot usually exist independently and retain all its chemical properties.

• A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance.

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HOME ASSIGNMENTS�

Q 1.What is meant by the term chemical formula?�Q 2.State the postulates of Dalton’s atomic theory which indicates the law of constant proportions?�Q3.Name two scientists who established the laws of chemical combination?�Q 4.Give two drawbacks of Dalton’s atomic theory?�Q 5.What is meant by a molecule? Give examples?�Q 6.What formed the basis for John Dalton to formulate Dalton’s atomic theory?�Q7.State the law of conservation of mass. Is this law applicable to the chemical reactions? Elaborate your answer with the help of an example?

Q8.How to write a chemical formula of a binary compund

Q9. Define mole/ Avogadro number.

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