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2.5 Atomic & Nuclear Physics. 3 Credits - Internal. Rutherford’s Model of the Atom. Planetary Model. Ernest Rutherford (1871 – 1937). Rutherford’s Model of the Atom. Protons have a positive charge which is equal to the negative charge of the electron. The neutron has no charge.

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2 5 atomic nuclear physics
2.5 Atomic & Nuclear Physics

3 Credits - Internal


Rutherford s model of the atom
Rutherford’s Model of the Atom

Planetary Model

Ernest Rutherford

(1871 – 1937)


Rutherford s model of the atom1
Rutherford’s Model of the Atom

Protons have a positive charge which is equal to the negative charge of the electron.

The neutron has no charge.

Protons and neutrons are nuclear particles called nucleons.


Rutherford s model of the atom2
Rutherford’s Model of the Atom

In a stable nucleus, nucleons are bound together by very strong balanced forces.

Attraction: Nuclear Force

Repulsion: Electric Force


Rutherford s model of the atom3
Rutherford’s Model of the Atom

Splitting a nucleus frees up a large amount of energy (nuclear Power)

Adding or removing electrons within their orbits (ionisation) also involves energy but much less than nuclear energy.

(chemical energy)


2 5 atomic nuclear physics

Composition of the Atom - Nuclides

the number of nucleons in the nucleus

Mass

Number

A

A = Z + N

Chemical

Symbol

Atomic

Number

Z

the number of protons in the nucleus

(determines the type of element)

The neutron number Nis the number of neutrons


2 5 atomic nuclear physics

Composition of the Atom

Mass

Number

A

Examples of Nuclides:

4

20

Chemical

Symbol

He

Ne

2

10

Atomic

Number

Z

A nuclide is a symbolic way of showing mass number and atomic number

A = Z + N



Complete the following data table
Complete the following data table

isotopes

4He

14N

24Mg

32S

40Ar

38Ar

2

7

18

18

12

16

4

24

32

38

14

40

18

18

2

7

12

16

20

2

7

12

16

22

7

12

16

18

2

18

A

A = Z + N

Ch

Z


2 5 atomic nuclear physics

atoms of the same element that have a different number of neutrons


2 5 atomic nuclear physics

Nuclei of three naturally occurring isotopes of the element hydrogen

1

2

3

H

H

H

1

1

1


2 5 atomic nuclear physics

Electrons are arranged in orbital shells (energy levels)

The innermost shell can take up to

2 electrons.

The next two shells can take up to

8 electrons each

23

Na

11

The centripetal force holding the electrons in circular orbit is the electric force




Radioactivity
Radioactivity

Some elements or isotopes are less stable than others and can spontaneously emit particle or wave radiations from their nuclei

4

240

236

+

He

Pu

U

2

94

92

Nuclear Equation


Radioactivity1
Radioactivity

Conservation of Mass Number (A)

Conservation of Atomic Number (Z)

4

238

234

+

+ γ

He

U

Th

2

92

90

heavy

nucleus

new

element

alpha

particle

gamma

radiation


Radioactivity2
Radioactivity

Complete the following Nuclear Reaction Equations

214

4

218

+

He

Alpha particle

Po

Pb

2

84

82

99

99

0

+

Beta particle

Tc

Ru

β

43

44

-1


Radioactivity3
Radioactivity

Complete the following Nuclear Reaction Equations

6

1

4

9

1

+

+

He

Li

Alpha particle

Be

H

2

4

1

3

42

42

0

Beta particle

+

K

Ca

β

20

19

-1


3 types of radiations
3 Types of Radiations

4

Alpha particle α

a high speed helium nucleus

Beta particle β

a high energy electron formed when a neutron splits into a proton and an electron

Gamma waveγ

a very short wavelength (high frequency) electromagnetic wave.

He

2

0

β

-1


Ionization and radioactivity
Ionization and Radioactivity

These radiations have the ability to ionise atoms (knock out electrons from their orbits) to produce ions

Alpha particleα

strong ionisers (heavy and slow) but can be stopped by paper

Beta particle β

less ionising, more penetrating (lighter, faster) can be stopped by metal foil

Gamma particle γ

least ionising but travel quickly.

Dense materials such as concrete or lead can stop them

To prevent radiation, shielding of varying thickness is used


Ionising radiation
Ionising Radiation

Sources

These radiations are charged particles that can cause the atoms they encounter to become charged


2 5 atomic nuclear physics

can travel a few cm in air, absorbed by paper

particulate radiation

4

He

High

2

up to 1 m in air, absorbed by aluminum sheet

0

particulate radiation

Med

e

-1

hardly affected by air, partially absorbed by concrete, lead

γ

electromagnetic radiation

Low


Half life
Half Life

The half life of a radioactive element is the time it takes for half of the atoms in a sample to decay

0 – all nuclei are intact. Sample is most active

1 – after 1 half life (8 days), one half of the nuclei have decayed (16 g) leaving the other half intact. Radiation emitted is now half its initial level.

2 – after 2 half lives (16 days) three quarters of the nuclei have decayed (24g) leaving a quarter intact.

e.g. Consider a 32g sample of iodine – 131 with a half life of 8 days.


Half life1
Half Life

Result is an exponential decay curve

0 – all nulei are intact. Sample is most active (32g)

1 – after 1 half life (8 days), one half of the nuclei have decayed (16 g) leaving the other half intact. Radiation emitted is now half its initial level.

2 – after 2 half lives (16 days) three quarters of the nuclei have decayed (24g) leaving a quarter intact.

8 days


Nuclear fission
Nuclear Fission

The process of splitting an atomic nucleus.

Can be achieved by bombarding a nucleus with a high speed particle

e.g. an alpha particle collides with a nitrogen nucleus to produce an oxygen atom & hydrogen atom

1

4

17

14

+

+

He

O

H

N

8

7

2

1


Nuclear fission1
Nuclear Fission

e.g. a chain reaction where the neutrons produced in the first fission produce further fissions

92

1

141

1

235

+

+

+ 3

n

Ba

Kr

n

U

56

92

0

0

36


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