Topic 1Atomic Structure

Bohr’s Model

Theories Of Theories Of Atomic StructureAtomic Structure

Some important Atomic Theories put forth in last two centuries.

• Dalton’s Theory• Thomson Model of Atom• Rutherford model of atom• Bohr Theory• Wave mechanical theory• String Theory

Some Old Models Of Some Old Models Of Atomic StructureAtomic Structure

Dalton ModelAtoms as hard spheres (1808)

Thomson ModelPlum-Pudding Model (1897)

Rutherford ModelNuclear Model (1911)

Bohr ModelDiscrete shells of electron (1913)

• An atomic model should be able to explain the phenomena considered to be related to atomic structure.

Expectations Expectations From An Atomic ModelFrom An Atomic Model

Phenomena Observed By Phenomena Observed By 1910 AD1910 AD

1) Emission of cathode rays by many elements

2) Scattering of (α) alpha particles by thin foil of gold

3) Stability Of An AtomStability Of An Atom• The atom as a whole is electrically neutral with electrons revolving around the

nucleus in various orbits. The centripetal force provided the electrostatic attraction between electrons and the nucleus.

• In spite of Rutherford's atomic model successfully explaining the large angle scattering of a particles and justifying the classifications of elements according to their atomic number in the periodic table, there were certain limitations in this model.

• As the radiating energy of a revolving electron (according to classical electromagnetic theory) continuously decreases the electron should follow a spiral path and fall into the nucleus. So this atomic model could not account for the stability of the atom. The observed spectra of elements cannot be accounted for by the atom model.

4) Emission Spectra Of Elements4) Emission Spectra Of Elements

BohrBohr’’s Postulatess Postulates

i) Electrons revolve in stable discrete orbits E1,E2,E3,E4,…

• Inner orbit corresponds to lower energy• Outer orbits corresponds to higher energy

Cont…

ii) An electron gains energy when it jumps from lower energy orbit to higher energy orbit.

iii) Electron loses energy when it jumps from an outer orbit to inner orbit.

Success Of BohrSuccess Of Bohr’’s Models Model

How does Bohr model explain stability of atom and emission spectra of elements?

Stability of atomic structure:By postulate of Bohr theory electrons occupying

specific allowed, discrete orbits do not lose energy though in the state of accelerated motion. Therefore the question of collapse of atomic structure does not arise.

Cont…

Electronic Excitation Of AtomElectronic Excitation Of Atom

Emission spectrum of an element

Atom of an element is excited by supplying some kind of energy. During this step electron gains energy and is promoted from lower energy level to higher energy level.

The excited atom soon get de-excited. The electron from higher energy level gives away energy and comes down to lower energy level.

E2

E1

Higher energy level

Lower energy level

∆E absorbed

Excitation of atom

E2

E1

Higher energy level

Lower energy level

∆E given out

De-excitation of atom

Additional Features Of Atomic Additional Features Of Atomic Structure Useful In ChemistryStructure Useful In Chemistry

1) Electronic configuration

Electronic orbits have definite capacity to hold electrons given by expression 2n2

(n is the orbit number)

There is a limit on capacity of outermost orbit octet/duplet

2) 32) 3--Dimensional Nature Of Electron Dimensional Nature Of Electron OrbitsOrbits

• Electronic orbits are not planar like planetary orbits. Better word to describe position of electrons around nucleus is ‘shell'. The shells have 3-dimensional shape.

3) Finer Structure Of Shells3) Finer Structure Of Shells

• Electronic shells are made up of subshells which in turn are made up of orbitals.There are different types of orbital depending upon the subshell these are named as s,p,d,f,…

• At this stage it is enough to know thatDifferent orbital have different 3-d shapesEach orbital can accommodate maximum two electrons.Energy of an electron depends upon both the shell number and subshell type.

ValencyValency

• Valency indicates the number of bonds an atom can form.

• Valency is related to number of electrons in outermost shell.

e.g.H2, HCl ,HBr, NaH, LiH, H2O, H2S, Na2O, Na2S

Cont…

ElementElement Electronic Electronic ConfigurationConfiguration

ValencyValency CompoundsCompounds

Li 2,1 1 LiCl,LiH

Na 2,8,1 1 NaCl,NaH,Na2O

Mg 2,8,2 2 MgCl2,Mg(OH)2

Be 2,2 2 BeCl2

Al 2,8,3 3 AlCl3

B 2,3 3 BH3

C 2,4 4 CCl4,,CH4

Si 2,8,4 4 SiCl4, Si(CH3)4

N 2,5 3 NH3,N(CH3),N2O3

P 2,8,5 3 PH3,PCl3

O 2,6 2 H2O,BeO,CO2

S 2,8,6 2 H2S

ElementElement Electronic Electronic ConfigurationConfiguration

ValencyValency CompoundsCompounds

F 2,7 1 HF

Cl 2,8,7 1 HCl

He 2 0 -

Ne 2,8 0 -

Ar 2,8,8 0 -

Topic 2 Modern Periodic TableTopic 2 Modern Periodic Table

• Four blocks s,p,d,f originate from the subshell being filled.

• s,p-blocks : s & p subshell of valence shell (last shell) gets filled.

• d block : subshell of second last shell gets filled.• f block : f subshell of third last shell gets filled

Characteristics Of PeriodsCharacteristics Of Periods

• Period number indicates last shell in electronic configuration.

• 1st Period : 1H, 2 He(1) (2)

• 2nd period :3Li, 4Be, 5B, 6C, 7N, 8O, 9F, 10Ne(2,1) (2,2) (2,3) (2,4) (2,5) (2,6) (2,7) (2,8)

• 3rd Period :

11Na, 12Mg, 13Al, 14Si, 15P, 16S, 17Cl, 18Ar(2,8,1) (2,8,2) (2,8,3) (2,8,4) (2,8,5) (2,8,6) (2,8,7) (2,8,8)

Gradual Variation In Properties Along Gradual Variation In Properties Along Second PeriodSecond Period

3Li 4Be 5B 6C 7N 8O 9F 10Ne

Metal/Nonmetal character

Metal Metal Metalloid Nonmetal Nonmetal Nonmetal Nonmetal Nonmetal

ElectropositiveChracter

Elctronegativity Increases

Atomic Radius

Molecular formula of compound

LiH BeH2 BH3 CH4 NH3 H2O HF -

Valency 1 2 3 4 3 2 1 0

Decreases

Decreases

Characteristics Of GroupsCharacteristics Of Groups

• Group number and valency (for s - and p block elements)

1IA

2IIA

13IIIA

14IVA

15VA

16VIA

17VIIA

18Zero

3Li 4Be 5B 6C 7N 8O 9F 10Ne

Valency 1 2 3 4 3 2 1 0

Electronic Configura

tion

2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8

Similarities and Gradation in Similarities and Gradation in Properties of Elements in a GroupProperties of Elements in a Group

• Similarity : Due to same number of valence electrons

• Gradation : Due to increased atomic radius

Alkali metalsIAIA Atomic Atomic

RadiusRadiusMolecular Molecular FormulaFormulaChloridesChlorides

ElectropositivElectropositive charactere character

Vigour of Vigour of reaction reaction

with waterwith water

3Li LiCl

11Na NaCl

39K KCl

87Rb RbCl

55Cs CsCl

Increases Increases Increases

2M + 2H2O 2MOH + H2 +Energy

HalogensHalogensVIIAVIIA Atomic Atomic

RadiusRadiusMolecular Molecular FormulaFormula

Compound Compound with with

HydrogenHydrogen

ElectronegatiElectronegativityvity

Physical Physical StateState

9F HF

17Cl HCl

35Br HBr

53I HI

85At

Gas

Gas

Liquid

Solid

Increases Decreases

Topic 3Percent Composition Of Elements

In A Compound(1) Theoretical Exercise

Name Symbol Total Mass % Composition

H2O (2 x1) +16 =18

2H 2 x1 = 2 2/18 x 100 = 11.11%

O 16 16/18 x 100 = 88.89%

CO2 12+(2 x 16)=44

C 12 12/44 x 100 = 27.27 %

2O 2 x 16 =32 32/44 x 100 = 72.73%

Carbondioxide

Carbon

Oxygen

Water

Hydrogen

Oxygen

Name Symbol Total mass % composition

CH4 12 + (4 x1)=16

C 12 72/16 x 100 = 75%

4H 4x1=4 4/16 x 100 = 25%

H2S (2 x 1)+32 =34

2H 2 x 1 =2 2/34 x 100 = 5.89%

S 32 32/34 x 100 = 94.11%

Hydrogen Sulphide

Hydrogen

Sulphur

Methane

Carbon

Hydrogen

Name Symbol Total mass % composition

Benzene C6H6 (6 x 12) +(6 x 1) = 78

Hydrogen 6H 6 x 1 = 6 6/78 x 100 = 7.69%

Carbon 6C 6 x 12 = 72 72/78 x100 = 92.30%

2) Applications Of Experimental Values 2) Applications Of Experimental Values Of %Composition Of Elements In Of %Composition Of Elements In

CompoundsCompounds

(a) To find out empirical formula of compound

Hydrocarbon 1 C Hydrocarbon 1 C HH

% composition 75% 25%

Atomic mass 12 1

Relative number of atoms in one molecule

75/12 = 6.25 25/1=25

Proportion of Atoms 6.25/6.25 = 1 25/6.25 = 4

Empirical Formula CH4

Hydrocarbon 2Hydrocarbon 2

Hydrocarbon 2 C H

% composition 92.30% 7.69%

Atomic mass 12 1

Relative number of atoms in one molecule

92.30/12 = 7.69 7.69/1 = 7.69

Proportion of Atoms 7.69/7.69 = 1 7.69/7.69 = 1

Empirical Formula CH

b) To Find Out Molecular Formulab) To Find Out Molecular Formula(If molecular mass is determined by some (If molecular mass is determined by some

method)method)Illustration 1

Consider the hydrocarbon -1 in the above illustration with empirical formula CH4.

• Its molecular mass is determined by some method and found to be 16. Now, empirical formula:CH4Therefore,

• Empirical formula mass =12+ (4 x 1)=16 but, molecular mass = 16

= empirical formula mass. Therefore, Molecular formula = CH4

Illustration 2

Now consider Hydrocarbon-2 having empirical formula CH. The hydrocarbon - 2 has molecular mass = 78.

• Now, empirical formula :CHTherefore,

Empirical formula mass = 12+1 = 13But, molecular mass = 78

= 6 x13= 6 x empirical formula mass

Therefore, molecular formula = 6 x empirical formula= 6 x CH= C6H6

c) To Find Molecular Massc) To Find Molecular Mass(If number of atoms of one of the constituent elements in (If number of atoms of one of the constituent elements in

the molecule is known by some method)the molecule is known by some method)

Compound C Total

A hydrocarbon containing two carbon atoms in the molecule

92.3% 100%

Atomic mass 12 u

Number of atoms in Mass 100 u

92.3/12 = 7.69

Mass containing two carbon atoms

100 x 2/ 7.69 = 26.07 u

Molecular mass 26 u

C2H2 (H C C H)

≅

Compound S Total

An Organic compound containing one sulphur

atom

29.1% 100%

Atomic mass 32

Number of atoms in Mass 100 u

29.1/32 = 0.9093

Mass containing one sulphur atoms

100 x 1/0.909 = 110.02 u

Molecular mass 110 u≅

C6H6S