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Physics(14 - 16)

Physics(14 - 16)

Nuclear Radiation - 1Introduction

Nuclear Radiation - 1Introduction

In 1806, the elements were defined by Dalton as being substances composed of onlyone type of atom.

We now define an element as a substance composed of atoms,each of which has the samenumber of protons.

John DaltonJohn

Dalton

John Dalton 1766 - 1844

He also introduced the ideathat atoms had a ‘solid ball’ structure.

In 1897, J. J. Thompson showed that an atom contained small negatively charged particles.

He called these particles electrons.

He proposed that the atom mustbe made of a positively charged sphere which contained the negatively charged electrons.

This became known as the ‘plum pudding’ model.

J. J. Thompson

J. J. Thompson

Thompson knew that the atom wasneutral (had no electrical charge).

J. J. Thompson 1856 - 1940 Positively charged

sphere

Negatively chargedelectron

In 1910, Earnest Rutherford’s team carried out experiments in which particles were fired at a thin sheet of gold.

Earnest RutherfordEarnest Rutherford

Rutherford expected all of the particles to pass straight through...E. Rutherford 1871 - 1937

In 1910, Earnest Rutherford’s team carried out experiments in which particles were fired at a thin sheet of gold.

Rutherford expected all of the particles to pass straight through...E. Rutherford 1871 - 1937

Earnest RutherfordEarnest Rutherford

Rutherford also predicted the existence of neutrons in the nucleus, and named the three types of nuclear radiation alpha (), beta () and gamma ().

Press ‘Start’ to take a closer look at Rutherford’s experiment...

Earnest RutherfordEarnest Rutherford

The Structure of the Atom

An atom contains a central nucleus around which electrons orbit in different levels.

The nucleus is composed of neutrons and protons.

The nucleus is often shown as a static body, whose sub-atomic particles fill the space of the nucleus.

The Nucleus

The nucleus of an atom istiny compared to the volumeoccupied by the electronsof the atom.

In fact, the nuclear particlesare in constant motion.

If an atom could be enlarged tothe size of a football stadium, thenucleus of the atom would be about thesize of a golf ball, somewhere above the centre spot.

The Nucleus

An electron has an incredibly small mass, which is about 1/2000the mass of a neutron or a proton.

Therefore, electrons arenot considered to contributeanything to the massof an atom.

Hence, the vast majority of themass of an atom is found in the nucleus.

In this model the electrons are shown orbiting around a central nucleus.

However, electrons also possess properties typical of waves.

The Structure of the AtomThe Structure of the Atom

There are many different models used to describe the structure of an atom.

Each shell has a maximum number of electrons that it can hold.

Electrons orbit so fast that they effectively form shells arranged around the nucleus.

Electrons

The electrons fill the shells starting from the one nearest to the nucleus.

The charges on the proton and electron are opposite in sign but equal in magnitude, so they cancel each other out exactly.

Protons, neutrons, and electrons are types of sub-atomic particle.

Protons are positively charged

Electrons are negatively charged

Neutrons have no charge

Sub-Atomic ParticlesSub-Atomic Particles

Protons and electrons are electrically charged, which affects the structure and stability of the atom.

The number of protons in an atom is called the atomic number. This is the factor which decides the identity of an atom - which element it belongs to, e.g.

All atoms are electrically neutral, so they always have an equal number of protons and electrons.

All lithium atoms have three protons

All substances on Earth are built from atoms containing these three sub-atomic particles.

Sub-Atomic ParticlesSub-Atomic Particles

Elements differ from each other in that their atoms contain different numbers of sub-atomic particles.

All uranium atoms have ninety two protons

All carbon atoms have six protons

The mass of an atom is mainly in the nucleus.

Carbon atoms that have six protons and six neutrons,have an atomic mass oftwelve atomic mass units.

Sub-Atomic ParticlesSub-Atomic Particles

Electrons have negligible mass. Therefore, the mass of an atomis equal to the sum of the number of protons and neutrons.This number is called the atomic mass number of the atom.

Protons and neutrons have an approximately equal mass of one atomic mass unit.

Example:

Atomic Number

AtomicMass Number 12

6

AtomicSymbol

IsotopesIsotopes

Within a sample of oxygen, some atoms can have 8, 9 or 10 neutrons - these are the different isotopes of the oxygen atom...

All atoms of the same element have the same number of protons. However, atoms may have different numbers of neutrons.

16

88 neutrons8 protons

9 neutrons8 protons

10 neutrons 8 protons

Atoms of the same element with different numbers of neutrons are called isotopes of that element.

17

8

18

8

Carbon IsotopesCarbon Isotopes

The most common isotope of carbon (12C) - “carbon-12” canbe represented as follows:

Atomic Number

AtomicMass Number 12

6

AtomicSymbol

A carbon-12 atom has:

6 protons

6 neutrons

6 electrons

However, another isotope of carbon, 14C can be represented as follows:

Carbon IsotopesCarbon Isotopes

Atomic Number

AtomicMass Number 14

6

AtomicSymbol

A carbon-14 atom has:

6 protons

8 neutrons

6 electrons

Henri BecquerelHenri Becquerel

In 1896, Henri Becquerel discovered radioactivity by accident.

Becquerel was investigating the phosphorescent properties of uranium. As he was unable to continue his experiments, he placed the photographic paper and the uranium salt, with a key in between them, in a drawer.

Four days later Becquerel looked at his photographic paper and found it had an image of the key on it.

Following further experiments, Becquerel discovered that uranium produced radioactivity, for which he was awarded a Nobel Prize.

Image formed by Becquerel on photographic paper

H. Becquerel1852 - 1908

Marie Curie and her husband Pierre, spent

many years studying the radioactive uranium

ore, pitchblende.

Pitchblende had the unusual property of being more radioactivethan uranium. The Curies deduced that pitchblende must contain another radioactive element.

Eventually after refining tonnes of pitchblende, Marie and Pierreidentified two new elements, which they called polonium and radium.

Marie & Pierre CurieMarie & Pierre Curie

Pierre Curie 1859 - 1906 Marie Curie 1867 - 1934

In 1903 Marie Curie, Pierre Curie andHenri Becquerel were all jointly awarded the Nobel Prize for Physics.

Marie & Pierre CurieMarie & Pierre Curie

Marie Curie working in her laboratory

In 1911 Marie Curie was also awarded the Nobel Prize for Chemistry and thus became one of only two people to be awarded a Nobel Prize in two different subjects.

Marie & Pierre CurieMarie & Pierre Curie

Marie died in 1934 of a medical condition which was almost certainly caused by working with pitchblende.

The medical condition was aplastic anaemia which is known to be caused by exposure to radiation.

Marie Curie’s radioactive sources

RadioactivityRadioactivity

Isotopes such as 14C are not stable and will emit atomic particles and/or energy - this is radioactivity.

The emission of a particle and/or energy is called radioactive decay, and the change that occurs is known as a decay reaction.

A decay reaction will leave behind a more stable nucleus.

Heat is always given out.

RadioactivityRadioactivity

For an individual atom, the decay process is spontaneous and totally random. However, for a mass of radioactive material, the rate of decay is predictable.

Press the buttons to show the three types of radioactivity...

Types of RadioactivityTypes of Radioactivity

An unstable isotope is called a radioisotope.

Parent & Daughter NuclidesParent & Daughter Nuclides

The original radioisotope is always referred to as the parent nuclide (the unstable nucleus), and the resulting isotope as the daughter nuclide.

Many radioactive elements decay as part of a decay chain, in which the original parent nuclide forms an unstable daughter nuclide, which also decays...

When a radioactive element decays, it may become...

A different isotope of the same element

A new element

Parent & Daughter NuclidesParent & Daughter Nuclides

Th234

U238 234

Pa U234

There are another 11 stages to this chain, ending in...

Pb206

Click to see the complete decay chain

Checking the atomic number figures...

TransmutationTransmutation

A transmutation is where one element changes into another, dueto a nuclear reaction.

Checking the atomic mass figures...

+He2

4+N

14

7

17

O8

H1

1

14 + 4 = 17 + 1

7 + 2 = 8 + 1

When alpha particles are fired at nitrogen-atom nuclei, the nitrogen atoms are changed into oxygen and hydrogen atoms.

Example:

Half-LifeHalf-Life

Half-life is the time it takes for half the number of radioactive nuclei within a sample to decay.

Another way of saying this is...

Half-life is the time it takes for the count rate from a radioactive sample to fall to half its level.

Half-LifeHalf-Life

Half-life is the time it takes for half the number of radioactive nuclei within a sample to decay.

Another way of saying this is...

Half-life is the time it takes for the count rate from a radioactive sample to fall to half its level.

What is the half-life of the above radioactive source? 10 years

End of ShowEnd of Show

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