Atomic Structure

HISTORY OF THE ATOMHISTORY OF THE ATOM

460 BC Democritus develops the idea of atoms

he pounded up materials in his pestle and

mortar until he had reduced them to

smaller and smaller particles which he

called

ATOMAATOMA

(greek for indivisible)


1808 John Dalton

suggested that all matter was made up of

tiny spheres that were able to bounce

around with perfect elasticity and called

them

ATOMSATOMS


1898 Joseph John Thompson

found that atoms could sometimes eject a

far smaller negative particle which he

called an

ELECTRONELECTRON


Thompson develops the idea that an atom was made up of

electrons scattered unevenly within an elastic sphere

surrounded by a soup of positive charge to balance the

electron's charge

1904


1910 Ernest Rutherford

oversaw Geiger and Marsden carrying out

his famous experiment.

they fired Helium nuclei at a piece of gold

foil which was only a few atoms thick.

they found that although most of them

passed through. About 1 in 10,000 hit


gold foil

helium nuclei

They found that while most of the helium nuclei passed

through the foil, a small number were deflected and, to

their surprise, some helium nuclei bounced straight back.

helium nuclei


Rutherford’s new evidence allowed him to propose a

more detailed model with a central nucleus.

He suggested that the positive charge was all in a

central nucleus. With this holding the electrons in place

by electrical attraction

However, this was not the end of the story.


1913 Niels Bohr

studied under Rutherford at the Victoria

University in Manchester.

Bohr refined Rutherford's idea by

adding that the electrons were in

orbits. Rather like planets orbiting the

sun. With each orbit only able to

contain a set number of electrons.

Bohr’s Atom

electrons in orbits

nucleus

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

What do these particles consist of?

ATOMIC STRUCTUREATOMIC STRUCTURE

Particle

proton

neutron

electron

Charge

+ ve charge

-ve charge

No charge

1

1

nil

Mass


the number of protons in an atom

the number of protons and neutrons in an atom

HeHe22

44 Atomic mass

Atomic number

number of electrons = number of protons


Electrons are arranged in Energy Levels

or Shells around the nucleus of an atom.

• first shell a maximum of 2 electrons

• second shell a maximum of 8

electrons

• third shell a maximum of 8

electrons


There are two ways to represent the atomic

structure of an element or compound;

1. Electronic Configuration

2. Dot & Cross Diagrams

ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION

With electronic configuration elements are

represented numerically by the number of

electrons in their shells and number of shells. For

example;

N

Nitrogen

7

14

2 in 1st shell

5 in 2nd shell

configuration = 2 , 5

2 + 5 = 7

DOT & CROSS DIAGRAMSDOT & CROSS DIAGRAMS

With Dot & Cross diagrams elements and

compounds are represented by Dots or Crosses to

show electrons, and circles to show the shells. For

example;

Nitrogen N XX X

X

XX

X

N7

14

SUMMARYSUMMARY

1. The Atomic Number of an atom = number of

protons in the nucleus.

2. The Atomic Mass of an atom = number of

Protons + Neutrons in the nucleus.

3. The number of Protons = Number of Electrons.

4. Electrons orbit the nucleus in shells.

5. Each shell can only carry a set number of electrons.

MASS DEFECT

EXPERIMENTAL MASS OF A NUCLEUS > SUM OF THE MASSES OF THE CONSTITUENT PARTICLES.

MASS DEFECT=ZMp+(A-Z)Mn-MA WHERE DOES THIS MASS GONE????

MASS VANISHED TO GIVE THE REQUIRED ENERGY TO BIND THE NUCLEONS TOGETHER.

BINDING ENERGY=MASS DEFECT x C2

NUCLEAR STABILITY

For small Z (A-Z)/Z is almost equal to one where as for higher Z value it increases and at Z=80 it is about 1.5 , About 270 stable nuclides resides in this region. Exceptional stability for magic nuclei with magic number ofNeutron or proton or both(double magic)Magic No. 2, 8, 20,50,82 for proton 2, 8, 20,50,82,126 for neutron (equivalent to complete filling of shell by electron for inert elements. Inert elements are chemically stable where as nucleus with magic number of nucleon are having higher nuclear stability )

A-Z

Z

-emitter

+emitter

CLASSIFICATION OF NUCLEAR REACTIONS:

1. Absorption 2. Scattering

Radiative Fission Nuclear Elastic In-elastic

Capture Transmutation.

All absorption and inelastic scattering takes place in 2 stages.

Stage1:

The neutron is captured by the target nucleus and a compound nucleus is formed. The compound nucleus is in excited state.

Stage 2:

Emission of -particle (or) emission of -ray (or) breaking up into two more or less equal parts.

ELASTIC SCATTERING:

In elastic scattering, there is no change in the internal energy of the system and kinetic energy is also conserved - “billiard ball” type collision.

1. Potential scattering: It occurs in the case of nuclei of low mass number with neutrons

having energy up to few MeV. • No compound nucleus formation.

2. Resonance scattering: Compound nucleus is formed.

The compound nucleus then expels a neutron leaving the target nucleus in its ground state.

INELASIC SCATERING OF NEUTRONS

zXA + on1 [zX

A+1]* zXA + on1 +

A compound nucleus, after its formation by neutron capture, will have excess energy , supplied to it by the kinetic energy and the bindingenergy of the capture neutron. It may dispose of this surplus energy by emitting a neutron and a gama-ray. The net result of the reaction is that a slower neutron is obtained while the target nucleus remains intact.So an effective slowing down of the neutron occurs. The reaction hasa threshold energy, i.e. it will not occur with neutron below a certain minimum energy, which is determined by the minimum energy differences between the nuclear shells of the in compound nucleus.

Example:

92U238 + on1 ( 92U239 )* 92U238+ on1 +

ENERGY TRANSITIONS IN INELASTIC SCATTERING:

EXCITED COMPOUND NUCLEUS

E2 EMITTED NEUTRON

INCIDENT E1 NEUTRON

E GAMMA RAY

E1= E2 + E

TARGET NUCLEUS

RADIATIVE CAPTURE:

zXA + on1 [zX

A+1]* zXA +

This is the most common reaction with neutrons and can be used to produced isotope of any element. It has no threshold energy and requires only thermal neutrons.

The compound nucleus zXA+1 may not have excess energy to

emit a particle but it emit - ray to return to the ground state.

zXA+1 may or may not be radioactive. If it is radioactive it will

be most likely a - emitter. Example:

1H2 + on1 1H3+

92U238 + on1 92U239 + 92U239 93Np239+ o-1

93Np239 94Pu239+ o-1

NUCLEAR TRANSMUTATION:

Compound nucleus may emit a charged particle which results in the formation of an entirely new element. This reaction requires threshold energy of the projectile particle.

• With slow neutrons:

5B10 + on1 [5B11]* 3Li7 + 2He4 (The reaction is used in BF3 or boron coated counters)

3Li6 + on1 [3Li7]* 1H3 + 2He4 Here, 1H3 is a - emitter.

2. With fast neutrons:

8O16 + on1 [8O17]* 7N16 + 1H1 A threshold energy of 10 MeV is

required. 7N16 is a - emitter with half-life of 7.35 secs . In addition, it emits a

-ray of energy about 6 MeV and 7 MeV.

(n,) reactions

NUCLEAR FISSION:

Splitting of heavier nucleus into two roughly equal parts.

f = 550 barns(thermal neutrons)

92U235 + on1 [92U235]* X + Y

The yield of fission fragments show 2 peaks one at A ~ 96 and A ~ 140. So symmetric fission is rare in U235 with slow neutrons.

Energy released ~ 2 120 (nucleons) 8.5 (MeV/nucleon)per fission - 240 (nucleons) 7.6

(MeV/nucleon) ~ 200 MeV

Approximate Energy Distribution Per Fission:

pJ MeV

Kinetic energy of fission fragments

Instantaneous gamma-ray energy

Kinetic energy of fission neutrons

Beta particles from fission products

Gamma rays from fission products

Neutrinos

26.9

1.1

0.8

1.1

1.0

1.6

168

7

5

7

6

10

Total fission energy ~ 32 ~ 200

Atomic Structure

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