Atomic Physics Atomic Notation A X Z X – symbol of the atom Z – atomic number (no of protons) A – mass number 14 C 6 C – carbon Z = 6, the number of protons.

Atomic Physics

Atomic Notation

A

X 

Z

X – symbol of the atomZ – atomic number (no of protons)A – mass number

14

C 

6

C – carbonZ = 6 , the number of protons in the nucleusA = 14, the number or protons plus the number of neutrons in the nucleus

A is also known as the nucleon number, the number of particles in the nucleus

Atomic Physics

Atomic Notation

56

Fe 

26

Fe - ironthe number of protons = 26the number of neutrons = 30the number of electrons = 26

1.00794

H 

1

H - hydrogenthe number of protons = 1the number of electrons = 1the number of neutrons = 0.00794 ??

Atomic Physics

Isotopes

Isotopes are variations of the same elements. Their nuclei contain the same number of protons but a different number of neutrons

12

C 

6

13

C 

6

14

C 

6

Isotopes of carbon

Atomic Physics

Nuclear Reactions

4 Al + 3 O2 2 Al203

Al3+ loses 3 electronsO2- gains 2 electrons

In a chemical reaction it is the movement of electrons that brings about the formation of compounds

Atomic Physics

Nuclear Reactions

2

H + 

1

3

H  

1

4

He + 

2

1

n 

0

+ energy

A nuclear reaction involves nuclei only. Electrons don’t play a significant part

Atomic Physics

Atomic Notation

+ energy

Two small nuclei have collided to form a larger nuclei plus a neutron with the release of energy (fusion)

Atomic Physics

Radioactivity:

238

U +

1

n144

Ba  92

Kr 

31

n      +   +  92 0 56   36   0

A large nuclei is broken up into 2 smaller nuclei with the release of energy (fission)

  Before AfterNo of protons    Nucleon number    

This demonstrates two conservation laws:1. Conservation of charge: No. of protons before = no. protons

afterwards2. Conservation of nucleon number: total number of protons and

neutrons stays the same

Atomic Physics

Example

235

U +

1

na

Xe 94

Sr

21

n + + 92 0 54 b 0

Find the values of a and b.

Atomic Physics

Examples:

226

Ra222

Rn 4

He1. + 88 86 2

Radium 226 Radon 222 + α particle

  Radium Radonα -

particleprotons 88 86 2

neutrons 138 136 2

The radium nucleus emits an α particle (a helium nucleus) and changes into a radon nucleus.

Atomic Physics

14

C14

N 0

e2. + 6 7 -1

Carbon 14 Nitrogen 14 + β particle

  Carbon Nitrogenβ -

particleprotons 6 7 0

neutrons 8 7 01

n1

p  0

e    +  0 1   -1

A carbon nucleus emits an electron as a neutron is converted into a proton

Atomic Physics

60

Co60

Ni * 0

e3. + 27 28 -1

60

Ni*60

Ni

γ + 28 28

60

Ni* is a Ni nucleus in an excited state. It returns to ground state by emitting a photon

28

Atomic Physics

Radioactivity: The spontaneous and random emission of particles from the nucleus of an atom.

There are 3 types

α 2 protons and 2 neutronsβ 1 electronγ photon

217

At213

Bi

?1. +

85 83

234

Pa234

Bi

?2. +

91 92

Atomic Physics

In a nuclear reactor, is broken

down by a chain of α and β particles

238

U234

Th 92 90

α particle

238

U92

Atomic Physics

234

Th234

Pa 90 91

β particle

Atomic Physics

234

Pa234

U 91 92

β particle

Atomic Physics

234

U230

Th 92 90

α particle

Atomic Physics

230

Th226

Ra 90 88

α particle

Atomic Physics

226

Ra222

Rn 88 86

α particle

Atomic Physics

218

Po 84

then loses an α particle, then β, α, β, β, β then α

What does it end up as?

206

Pb 82

Atomic Physics

Deflection by a Magnetic Field

α – particle deflected slightly because it has the greatest mass

β – particle deflected more because it has less mass and in the opposite direction as it has the opposite charge

γ – radiation, no deflection because it has no charge and no mass

Atomic Physics

The radio active source is not a health risk as α – particles can travel only a short distance in air. As they are placed on the ceiling, they are well away from people.

Smoke Detector

The radioactive source emits α – particles that ionise the air, ie, they remove electrons forming positive and negative ions. This allows the current to flow. Smoke particles absorb the α – particles so the ionisation stops. The current stops that triggers the alarm

Atomic Physics

Calculating the Energy Released in a Nuclear Reaction

2

H + 

1

3

H  

1

4

He + 

2

1

n 

0

+ energy

The amount of energy released is given by the formulaE = mc2

Where mass is the mass that has been “lost”, and c is the speed of light, 3.00 x 108 ms-1

The mass of the nucleus of:Deuterium is 3.34330 x 10-27 kgTritium is 5.00784 x 10-27 kgHelium is 6.64591 x 10-27 kgA neutron has a mass of 1.67483 x 10-27 kgFind the energy released in this reaction

Deuterium 3.3433 x 10-27 Helium 6.64591 x 10-27

Tritium 5.00784 x 10-27 Neutron 1.67483 x 10-27

Total 8.35114 x 10-27 Total 8.32074 x 10-27

8.35114 x 10-27

8.32074 x 10-27

Difference 0.0304 x 10-27

Atomic Physics

Energy released = 0.0304 x 10-27 kg x (3.00 x 108 ms-1)2

= 2.736 x 10-12 J

Atomic Physics

+ β particle228

Ra228

Ac 88 89

The mass of the Radon nucleus is 3.81743 x 10-27 kgThe mass of the Actium nucleus is 3.68334 x 10-27 kgThe mass of a β particle is 0.00091 x 10-27 kg

The energy released is 1.20593 x 10-9 J