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Class 9 Chemistry Atoms and MoleculesTopics

Introduction

Laws of chemical combination

Laws of conservation of mass

Laws of constant proportion

Atoms

Dalton atomic theory

Naming atoms

Atomic mass

Molecules

Atomicity

Molecules of compounds

Ions

Valency

Writing chemical formula

Molecular mass

Formula unit mass

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Molar massIntroductionThe entire Universe is composed of matter and energy. Energy is the aptitude to perform a work. On the other hand matter has mass and occupies space. Maharishi kannad postulated that if we go on dividing matter we will derive smaller and smaller particles which on further division gives off further smaller particles finally leading to the derivation of the smallest particle that cannot be further divided. He named these particles to be parmanu or atom. Matters are composed of basic elements that cannot be broken down to substances with different chemical or physical properties.

Laws of chemical combinationTo understand the chemistry of the compounds, it is essential to devise a theory that accounts for both qualitative and quantitative interpretations during chemical changes.

The laws were laid down by Antoine L. Lavoisier.

Laws of conservation of massThe law states that mass can neither be created nor destroyed in a chemical reaction i.e. Total masses of reactants is equal to the sum of masses of products and the masses of unreacted reactants. For instance, the reaction of carbon with oxygen to produce carbon-dioxide involves conservation of mass in the following way.

C (12g) + O2 (32g) = CO2 (44g)

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Here we used 12 g of carbon and 32 g of oxygen. After reaction the amount of carbon –dioxide produced will be 44g i.e. the sum total of the amount of carbon and oxygen used.

Laws of constant proportionThe law states that in a chemical substances the elements are always present in a definite proportion by mass. It is also termed as law of definite proportion.

For instance, the ratio of hydrogen and oxygen in water obtained from any source is 1:8 i.e. 1g of hydrogen and 8g of oxygen together make up 9g of water.

Similarly the ration of nitrogen and hydrogen in ammonia is 14:3.

AtomsAccording to John dalton all elements, compounds or mixtures are composed of some small particles which cannot be further divided into smaller particles. They are known as atoms. They are referred to as the basic building blocks of an element, compound or mixture. They are very small and their radius can be measured in nanometers.

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

Example Radii (in m)

Atom of hydrogen10-10

Molecule of water10-9

Molecule of haemoglobin10-8

Grain of sand10-4

Ant 10-2


Watermelon10-1

Dalton’s atomic theory

John Dalton

The postulates of this theory may be stated as follows:

o Matters are composed of very tiny particles called atoms.o These particles are too small to see and cannot be created or destroyed in a

chemical reaction.o Atoms of a specified element are identical in mass and chemical properties.o Atoms of different elements have different masses and chemical properties.o Atoms combine in the ratio of small whole numbers to form compounds.o The relative number and kinds of atoms are constant in a given compound.Naming atomsSuppose there are ten students in your class but they do not have their names. Now if you want to call anyone of your friend. How would you call them without any specification that denotes them particularly????

At that time calling through their names makes it easier to call them. Similarly atoms too possess their specific names so that they can be denoted by their names. Initially Dalton used symbols to denote the elements. Using one symbol meant one atom of that element.


Symbols of some commonly known elements as anticipated by Dalton

The names of the elements were named in accordance with the place from where they were derived. For instance, copper was excavated from Cyprus.

Then later on Berzilius insisted of using the first single or the first two alphabets to denote the element. But now-a-days IUPAC (International Union of Pure and Applied Chemistry) assigns the names of the elements.

Symbols of atoms of different elements

o There are 115 elements present on earth till now.o Symbols of elements are either the first letter or first two letters of the name of

the elements.o First letter of the symbols is always capital. For instance, H for hydrogen, Al not

AL for aluminum, Co for cobalt.o Some of the names involves the first letter of the element name and the letter

that appears later in it. For instance, Cl for chlorine, Zn for zinc etc.o Some of the elements have their symbols derived from their names in latin or

greek. For instance, Na (natrium) for sodium, Fe (Ferrous) for iron, K (kalium) for potassium.


Elements with modern symbols

Atomic massAtomic mass refers to the mass of an atom. It depicts how many times an atom of an element is heavier than one-twelth (1/12th) the mass of one atom of carbon-12 of mass of one carbon atom. The relative atomic masses of all elements have been established with reverence to an atom of carbon-12. It is measured in unit called amu (atomic mass unit). 1/12 of the mass of one atom of carbon is termed as relative atomic mass. It doesnot possess a unit.Mathematically,

mass of one atom of an element = atomic mass X (1/12th) of the mass of one atom of carbon. Or

atomic mass = mass of one atom of an element / (1/12th) of the mass of one atom of carbon.Easy way to calculate atomic mass of oxygen atom.

The Proton number Z = 8The Neutron number N = 8The atomic mass of the oxygen atom is given by 15.9994 amu i.e. nearly 16.

MoleculesWe all know that bricks are the basic building blocks of a building. The bricks cannot exist independently. The bricks are used to make small rooms that together constitute the entire building. The rooms can be regarded as molecules that are composed of bricks and together build up the entire building.

Similarly in most of the elements atoms do not exist independently. They exist as molecules that aggregates in large numbers to form the matter.

Molecules refers to the cluster of two or more atoms chemically bonded together and tightly held together by attractive forces. It can be defined as the tiniest particle of an element or compound proficient of an independent existence and shows all the properties of that substance.

AtomicityThe number of atoms present in a single molecule is termed as its atomicity. It can be classified into three types.


o The molecules of many elements are composed of only a single atom of that element. They are termed as monoatomic. For instance, Ar (Argon), He(Helium). So their atomicity is one.

Fig. Argon moleculeo On the other hand the molecules of many elements are composed of two atoms

of that element. For instance, oxygen can never exist independently and exist as O2 molecule known as diatomic So its atomicity is 2.

Fig. Oxygen moleculeo The molecules of many elements are composed of three atoms of that element.

They are termed as triatomic For instance, O3, CO2. So their atomicity is 3.

Fig. Molecule of ozone (left) and carbon-dioxide (right)


o The molecules of many elements are composed of four atoms of that element. They are termed as tetra atomic For instance, P4, SO3. So their atomicity is 4.

Fig. Molecule of sulphur trioxideMolecules of compoundsAtoms of different elements associate in definite proportions to form molecules of different compounds.

For instance,

o In sodium chloride NaCl is 1:1.o In water (H2O) ratio of hydrogen and oxygen is 1:8.

IonsMetals and non-metals leads to the formation of Compounds that possess charged species. These charged species are called ions.


These charged species can be either positively charged called cation or negatively charged called anion. For instance, In sodium chloride (NaCl), Na exist as cation Na+ whereas Cl exist as anion Cl-. Valency It is the ability of an atom to gain or lose electron in order to achieve the noble gas configuration. It refers to the ability of an element to combine with other element. It is obtained by determining the number of electrons in the outermost shell (also called valence shell) of each atom of an element. For instance, sodium has 1 electron in its outermost shell and hence valency of sodium is 1.

Fig. Atomic number of sodium is 11. So the electronic configuration stands out to be 2,8,1 i.e. there is one electron in the outermost shell. In order to gain inert gas configuration it is better for sodium to lose one electron and achieve the nearest noble gas configuration of neon with atomic number 10. On the other hand atomic number of chlorine is 17. So electronic configuration stands out to be 2,8,7. In order to achieve noble gas configuration to become stable it requires one electron then it will acquire the configuration of neon (noble gas). Therefore valency of chlorine is1.


That means the every element tries attain stability by acquiring noble gas configuration for which it tries to either gain electron or donate electron. Na donates 1 electron in its outermost shell to attain noble gas configuration whereas chlorine acquires 1 electron in its outermost shell to acquire noble gas configuration.

Metals are electropositive because they have tendency to lose electrons. E.g. Na+ whereas non-metals are electronegative since they have a tendency to gain electrons. E.g. Cl-.We can find the valency of an element through its atomic number and its electronic configuration. For instance,

o In Boron with atomic number 5, the configuration stands out to be 2,3 and it has 3 electrons in its outermost shell. Being a metal Boron has a tendency to lose its electrons to gain noble gas configuration and shows its valency 3.

o Whereas in Fluorine with atomic number 9, the configuration stands out to be 2,7 and it has 7 electrons in its valence shell and needs to gain 1 more electron to achieve noble gas configuration and hence its valency is 1.


They can be further classified into following two types:

Ions possessing only one atom are termed as monoatomic ions. For instance, Na+, K+ etc.Ions possessing more than one atom are termed as polyatomic ions. For example, CO3

2-, NO3

- etc.Writing chemical formulao RULE I: Cross multiply the valencies of the elements to form the formula of the

respective compound.


In the above formulas Rule I is followed i.e. cross multiply the valencies of the elements to form the formula of the respective compound.

o RULE II: If a compound consists of both metallic as well non-metallic elements then name or symbol of the metal is considered first.

Where Mg (magnesium) is a metal and Cl (chlorine) is a non-metal.

o RULE III: In case compounds are formed from polyatomic ions then the ion is enclosed in a bracket before writing the number to indicate the ratio. But in case the number of polyatomic ion is one then there is no need for bracket.

Since the number of polyatomic ion is one therefore no brackets are required.


The formula for calcium hydroxide is Ca(OH)2. The brackets around oh with subscript 2 implies the presence of two hydroxyl group joined to one calcium atom.Molecular massMolecular mass of a substance is defined as the sum o the atomic masses of all the atoms present in a molecule of a substance. Therefore the relative mass of a molecule is expressed as atomic mass units (amu).

Let us calculate the molecular mass of H2O. we will find it out by following process.o We know the atomic mass of hydrogen is 1u and that of oxygen is 16u.o Molecular mass of H2O containing two hydrogen atoms and single oxygen atom

is2 X 1 + 1 X 16 = 18u

Formula unit massFormula unit mass of a substance is the sum of the atomic masses of all the atoms present in the formula of a respective compound.

Let us calculate the formula unit mass of NaCl. we will find it out by following process

o We know atomic mass of Na is 23 and that of Cl is 35.5o Formula unit mass of NaCl is 1 X 23 + 1 X 35.5 = 58.5 uMoleThe word mole was coined by Wilhelm Ostwald from a Latin word moles meaning heap or piles. One mole of molecule or ion refers to the quantity in number possessing a mass equal to its atomic or molecular mass in grams. Number of particles present in one mole of any substance is 6.022 X 1023. This value is called Avogadro number.So we get 1 mole = 6.022 X 1023 in number.Mass of one mole of a molecule or ion is equivalent to its relative atomic or molecular mass in grams.

Let us calculate the number of moles in 48g of He. we will find it out by following process

o Atomic mass of He =4uo Molar mass of He = 4go Therefore the number of moles (n) = given mass (m) / molar mass (M)


n = 48/4 =12

Molar massIn order to calculate the molar mass of a respective molecule keep the numerical value same as molecular mass but alter the units from u to g.

For instance, molecular mass of water is 18 u.

We know the atomic mass of hydrogen is 1u and that of oxygen is 16u.

o Molecular mass of H2O containing two hydrogen atoms and single oxygen atom is

2 X 1 + 1 X 16 = 18u

i.e. 18 u of water has 1 molecule of water.

18u water = 18 g water

18g water = 1 mole molecules of water

Or 18g water = 6.022 X 1023 molecules of water


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