3 Atoms and Molecules

8/18/2019 3 Atoms and Molecules

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CHAPTER - 3

ATOMS AND MOLECULES

CLASS :- IX

MADE BY :- MANAS MAHAJAN SCHOOL :- K.V. GANESHKHIND PUNE-7


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1) La! "# $%&'($a $"'*(+a,("+ :-i) Law of conservation of mass :- states that ‘Mass can neitherbe created nor destroyed in a chemical reaction’.

This means that during a chemical reaction the sum of the masses ofthe reactants and products remain unchanged.

ctivity :-

Ta!e some sodium sulphate solution in a conical flas! and some

barium chloride solution in an ignition tube. "ang the ignition tube in

the flas! by a thread and pot a cor! on the flas!. #ind the mass of theflas! on a balance. Then tilt the flas!. chemical reaction ta!es place

and sodium chloride and barium sulphate are formed. Then find the

mass of the flas! again. $t will be seen that the sum of the masses of the

reactants and products remain unchanged.

%a&l'

solution

%a(* white


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ii) Law of constant proportions :- states that ‘ $n a chemicalcompound the elements are always present in a definite proportion by

mass’.

+g :- ,ater "') always contains two elements hydrogen and oygencombined together in the same ratio of ':/0 or /:1 by mass. $f 2 g of

water is decomposed we get / g of hydrogen and 1 g of oygen.

mmonia 3"4) always contains two elements nitrogen and

hydrogen combined together in the same ratio of /*:4 by mass.


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) Da,"+! a,"'($ ,%&"/0 :-

i) Matter is made up of tiny particles called atoms.

ii) toms are indivisible and cannot be created or destroyedin a chemical reaction

iii) toms of a given element are similar in mass and

properties.

iv) toms of different elements have different masses and

properties.

v) toms combine in small whole number ratios to form

compounds.

vi) $n a given compound the relative number and !ind of

atoms are constant.


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3) A,"' :- n atom is the smallest particle of an element that may or

may not eist independently and retains all its chemical

properties. toms are very small in si5e and smaller than anything

we can imagine or compare with.

tomic radius is measured in nanometres nm)

/ nanometer 6 /7 -2 m or / meter 6 /72 nm

+g :- The atomic radius of an atom of hydrogen is /7-/7 m.

The radius of a molecule of water is /7-2 m.


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) S0'*"! "# a,"'! "# 2(##&/&+, &&'&+,! :- The symbols of elements are represented by letters.

The symbols of some elements are represented by one letter and the

symbols of some elements are represented by two letters. $f the symbol has only one letter it should be written as capital letter

and if the symbol has two letters then the first letter should be capital

letter and the second letter should be small letter.

(ymbols of some common elements :-

+lement (ymbol +lement (ymbol +lement (ymbol

luminium l &opper&u

3itrogen 3

rgon r #luorine#

ygen

%arium %a 8old u 9otassium

%oron % "ydrogen " (ilicon (i

%romine %r $odine $ (ilver g

&alcium &a $ron #e (odium 3a

&arbon & Lead 9b (ulphur (&hlorine &l Magnesium Mg ;ranium ;


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) A,"'($ 'a!! :- (ince atoms are very small in si5e its mass is very small anddetermining its mass is very difficult. (o the mass of an atom is

compared with the mass of a standard atom.

The atom which is considered as a standard atom for comparing the

masses of other atoms is carbon = /' atom whose atomic mass is

/' u atomic mass unit).

ne atomic mass unit u) is the mass of />/'th the mass of a carbon =

/' atom.

The atomic mass of an element is defined as the average mass of one

atom of the element compared with />/'th the mass of a carbon = /'

atom.

tomic masses of some elements :-

+lement tomic mass u) +lement tomic mass u)

"ydrogen / Magnesium '*

&arbon /' luminium '?

3itrogen /* (ulphur 4'

ygen /0 &hlorine 4@.@

(odium '4 &alcium *7


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4) M"&$5& :- molecule is the smallest particle of an element or

compound which eists independently and shows all the

properties of that substance. molecule is a group of two or more elements that are

held together by attractive forces.

toms of the same element or different elements can Aoin

together to form molecules.


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i) Molecule of elements :- Molecule of an element contains atoms of the same element.

Molecules of some elements contain only one atom and molecules of

some elements contain two or more atoms.

tomicity of an element :- is the number of atoms present inone molecule of the element.

tomicity of some elements :-

Type of element 3ame tomicity

3on metal rgon r / = Monoatomic3on metal "elium "e / = Monoatomic

3on metal ygen ' ' = Biatomic

3on metal "ydrogen "' ' = Biatomic

3on metal 3itrogen 3' ' = Biatomic

3on metal &hlorine &l' ' = Biatomic

3pn metal 9hosphorus 9* * = 9hosphorus

3on metal (ulphur (1 9oly atomic

Metal (odium 3a / = Monoatomic

Metal $ron #e / = Monoatomic

Metal luminium l / = Monoatomic


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ii) Molecule of compounds :-

Molecule of a compound contains atoms of two or more different

types of elements.

Molecules of some compounds :-

&ompound &ombining elements 3umber of atoms of eachelements

,ater = "'"ydrogenC ygen ' - "ydrogenC / - ygen

mmonia = 3"4 3itrogenC "ydrogen / - 3itrogenC 4 - "ydrogen

&arbon dioide

&'

&arbonC ygen $ - &arbonC ' - ygen

"ydrochloric acid

"&l

"ydrogenC &hlorine / - "ydrogenC / - &hlorine

3itric acid

"34

"ydrogenC 3itrogenC ygen / - "ydrogenC / - 3itrogenC

4 - ygen

(ulphuric acid

"'(*

"ydrogenC (ulphurC ygen ' - "ydrogenC / - (ulphurC

* - ygen


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7) I"+! :- &ompounds containing metal and non metal elements containcharged particles called ions.

n ion is a charged particle having positive or negative charge.

positively charged ion is called ‘cation’ and a negatively charged ion iscalled ‘anion’.

Dalency :- is the combining capacity of an element.

(ome common ions and their valencies :-Dalency 3ame of ion (ymbol 3ame of ion (ymbol 3ame of ion (ymbol

/ (odium 3aE "ydrogen " E mmonium 3" *-

/ 9otassium E "ydride " - "ydroide " -

/ (ilver gE &hloride &l - 3itrate 34-

/ &opper $) &uE %romide %r - "ydrogen carbonate "&4-

' Magnesium Mg 'E $odide $ -

' &alcium &a 'E ide '-

&arbonate &4'-

'


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6) /(,(+8 $%&'($a #"/'5a& :- i) ,rite the symbols > formula of the elements or ions so that the

symbol of the metal or positive ion is on the left and symbol > formula

of the non metal or negative ion is on the right. ii) ,rite the valencies of the elements or ions below the elements or

ions.

iii) &ross over the valencies of the combining ions.

iv) 9olyatomic ions should be enclosed in brac!et before writing the

formula.+amples :-

i) #ormula of hydrogen chloride ii) #ormula of hydrogen sulphide

(ymbol " &l (ymbol " (

Dalency / / Dalency / '#ormula "&l #ormula "'(

iii) #ormula of Magnesium chloride iv) #ormula of &arbon tetrachloride

(ymbol Mg &l (ymbol & &l

Dalency ' / Dalency * / #ormula Mg&l' #ormula &&l*


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v) #ormula of &alcium oide vi) #ormula of luminium oide

(ymbol &a (ymbol l

Dalency ' ' Dalency 4 '

#ormula &a'' 6 &a #ormula l'4

vii) #ormula of (odium nitrate viii) #ormula of &alcium hydroide(ymbol> 3a 34 (ymbol> &a "

#ormula #ormula

Dalency / / Dalency ' /

#ormula 3a34 &a")'

i) #ormula of (odium carbonate ) #ormula of mmonium sulphate

(ymbol> 3a &4 (ymbol> 3"* (*

#ormula #ormula


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9) M"&$5a/ 'a!! ;"/'5a 5+(, 'a!!:-

The molecular mass of a substance is the sum of theatomic masses of all the atoms in a molecule of the

substance. Molecular mass is epressed in atomic mass units u).

+g:- Molecular mass of water = "'

tomic mass of " 6 $ u

tomic mass of 6 /0 u

Molecular mass of "' 6 /'E/7 6 'E/0 6 /1 u

Molecular mass of 3itric acid = "34

tomic mass of " 6 / u

tomic mass of 3 6 /* u

tomic mass of 6 /0 u

Molecular mass of "34

6 /E/*E/04 6 /E/*E*1 6 04 u


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