Following the discovery of radioactivity by Henri Becquerel in 1896 many scientists were keen to find out more about it and understand where it came from In a radioactive atom the nucleus is unstable and so it emits particles or waves of radiation to form a more stable atom ID: 930873
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Slide1
Slide2Slide3How is radioactivity related to atomic structure?
Following the discovery of radioactivity by Henri Becquerel in 1896, many scientists were keen to find out more about it and understand where it came from.
In a radioactive atom, the nucleus is unstable and so it emits particles or waves of radiation to form a more stable atom.
This process is called
radioactivity or radioactive decay.
This is why early experiments with radioactivity lead to important discoveries about the structure of the atom.
As a result of this work, we now know that radiation comes from radioactive atoms.
Slide4What are atoms?
It is now known that all matter is made of
atoms.
In some substances, all the atoms are the same, which means that the substance is called an element.
For example, gold is an element made up of only gold atoms
.It is only relatively recently that we have had microscopes powerful enough to ‘see’ individual atoms. Before that, the idea that atoms existed was only a theory.
The first person to suggest the idea of atoms was the Greek philosopher Democritus, in 450
BC
.
Slide5What did Dalton think atoms were like?
Ideas about atomic structure have changed over time.
In 1803, John Dalton reintroduced the idea that everything is made of atoms. He said atoms were solid spheres of matter that could not be split.
Dalton also suggested that each element contained identical atoms.
Slide6How did electrons spoil Dalton’s model?
In 1897, whilst studying cathode rays, JJ Thomson discovered tiny particles with a negative charge.
His discovery did not fit with Dalton’s model of the atom, and so Thomson had to propose a new model.
These negative particles were given out by atoms and were much smaller than atoms.
Thomson had discovered the existence of
electrons.
Slide7What is the plum pudding model?
Thomson’s model became known as the
plum pudding model, because the electrons in the atom were thought to be like raisins in a plum pudding.
Based on his discovery, Thomson adapted Dalton’s model of the atom.
Electrons had been proved to exist but there were doubts about this model.
He suggested that an atom is a positively-charged sphere with negative electrons distributed throughout it.
Slide8What was Rutherford’s involvement?
Slide9What did Geiger and Marsden do?
Slide10The results of Geiger and Marsden’s experiment were:
What were Geiger and Marsden’s results?
The experiment was carried out in a vacuum, so deflection of the alpha particles must have been due to the gold foil.
1. Most alpha particles went straight through the gold foil, without any deflection.
2.
Some alpha particles
were slightly deflected
by the gold foil.
3.
A few alpha particles were bounced back from the gold foil.
How can these results be explained in terms of atoms?
Slide11Rutherford had expected all the alpha radiation to pass through the gold foil. He was surprised that some alpha particles were deflected slightly or bounced back.
The ‘plum pudding’ model could not explain these results, so Rutherford proposed his
‘nuclear’ model of the atom.
How did Rutherford interpret the results?
He suggested that an atom is mostly empty space with its positive charge and most of its mass in a tiny central nucleus.
Electrons orbited this nucleus at a distance, like planets around the Sun.
Slide12How did Rutherford explain the results?
Slide13Which model of the atom?
Slide14The
electrons
orbit the nucleus in layers called shells.
The nucleus is where most of the mass of the atom is found. It contains protons and neutrons.
Experiments showed that Rutherford’s atomic model (a tiny, positively-charged nucleus orbited by electrons) was correct.What is the modern model of the atom?
Further developments in understanding about atomic structure followed, but Rutherford’s nuclear model still forms the basis of the modern model of the atom.
Slide15Atoms are made of three basic building blocks called
protons, neutrons
and electrons. In any atom, the number of electrons is equal to the number of protons and so the overall charge of an atom is zero.
What are atoms made of?
There are two properties of protons, neutrons and electrons that are especially important: mass and charge.
-1
almost 0
0
1
+1
1
electron
neutron
proton
Charge
Mass
Particle
Slide16Particles in the modern model
Slide17What makes a carbon atom carbon?
The atoms of any particular element always contains the same number of protons.
In the periodic table, there are two numbers found with each element. What do these numbers represent?
Carbon atoms always have six protons. Atoms with different numbers of protons must be other elements. For example:
all atoms with 1 proton are hydrogen atoms;
all atoms with 17 protons are chlorine atoms.
Atomic number
(or proton number) is the number of protons
Mass number
is the number of protons + the number of neutrons.
Slide18All carbon atoms have the same number of protons, but not all carbon atoms are identical.
mass number is different
atomic number is the same
Although atoms of the same element
always
have the same number of protons, they can have different numbers of neutrons. Atoms that differ in this way are called isotopes. What are isotopes?
For example, carbon exists as three different isotopes: carbon-12, carbon-13 and carbon-14:
Potassium is another element that exists as three different isotopes: potassium-39, potassium-40 and potassium-41.
Slide19Atomic structure – key words
Slide20Slide21Types of radioactive decay
Slide22An
alpha particle
consists of two protons and two neutrons. It is the same as a helium nucleus.
When an atom’s nucleus decays and releases an alpha particle, it loses two protons and two neutrons.
atomic number
decreases by 2
mass number decreases by 4
What happens during alpha decay?
The number of protons has changed, so the decayed atom has changed into a
new element
.
238
92
234
90
2
4
U
Th
+
α
Slide23An
beta particle
consists of a high energy electron, which is emitted by the nucleus of the decaying atom.
When an atom’s nucleus decays and releases a beta particle, a neutron turns into a proton, which stays in the nucleus, and a high energy electron, which is emitted.
What happens during beta decay?
The decayed atom has gained a proton and so has changed into a
new element
.
atomic number
increases by 1
mass number remains the same
14
6
14
7
C
N
+
β
Slide24Gamma radiation
is a form of electromagnetic radiation, not a type of particle.
When an atom’s nucleus decays and emits gamma radiation, it releases energy in the form of electromagnetic radiation.
What happens during gamma decay?
Gamma rays are usually emitted with alpha or beta particles. For example, cobalt-60 decays releasing a beta particle. The nickel formed is still not stable and so emits gamma radiation.
60
28
Ni*
60
28
Ni
+
The nickel
does not
change into a new element.
There is no change to the make up of the nucleus and so a new element is
not
formed.
60
27
Co
β
+
Slide25Radioactive decay – true or false?
Slide26Slide27Radioactivity cannot be seen, it has no smell and does not make any sound so how can it be detected?
Radioactivity can be detected with a
Geiger counter
, which is a Geiger-Müller (GM) tube connected to a ratemeter.
The ratemeter gives a reading in ‘counts per second’ and a loudspeaker ‘clicks’ for each particle, or burst of radiation, detected by the GM tube.How can radioactivity be measured?
GM tube
ratemeter
It can also be used to measure the amount of radiation.
Slide28What happens to radioactivity?
Slide29Radioactive decay is a spontaneous process that cannot be controlled and is not affected by temperature.
What is half-life?
The
half-life
of a radioactive element is the
time
that it takes
half the atoms in a sample to decay.
For example, the half-life of the isotope iodine-131 is
8 days
.
However, each radioactive element has its own particular
decay rate
, which is called the
half-life
.
This means that after 8 days half the nuclei in a sample of iodine-131 have decayed. 8 days later half the remaining nuclei have decayed and so on.
Slide30How is half-life calculated?
Slide31Half-lives range from millionths of a second to millions of years.
Uranium-235, which is used in nuclear reactors and nuclear weapons, has a half-life of 710 million years. Why is the use of uranium-235 considered controversial?
Xenon-133 is a radioactive isotope used for studying lung function. Why does its half-life of 5.2 days make it suitable for this use?
How long are half-lives?
Some types of nuclei are more unstable than others and decay at a faster rate.
Radioisotope
Half-life
boron-12
uranium-235
radium-226
0.02 seconds
1602 years
710 million years
Slide32What is the half-life of carbon-14?
Slide33How does carbon dating work?
Slide34What are the problems of using carbon dating?
What are some of the problems with using carbon dating to predict the age of a sample?
The half-life of carbon-14 is 5,700 years. If the sample is older than 60,000 years, the amount of carbon-14 is too small to measure accurately. Instead, radioactive isotopes with longer half-lives, such as uranium-235 with a half-life of 710 million years, can be used to date older samples.
Carbon dating anything that died after the 1940s, when nuclear bombs, nuclear reactors and open-air nuclear tests began, is harder to date precisely due to contamination from this increased background radioactivity.
Samples can become contaminated with materials of a different age which may confuse the readings of carbon-14.
Slide35Using half-life to date a sample
Half-life can be used to do many useful calculations.
For example, the half-life of carbon-14 is 5,700 years. If a fossil bone has a count of 25
, and a piece of bone from a living body has a count of 200, how old is the fossil?
After one half-life, the count will decrease by half to 100.
Three half-lives of carbon-14 have passed,
so 3 x 5,700 years makes the fossil
17,100 years old
.
After the second half-life, the count decreases by half again to 50.
After the third half-life, the count decreases to 25.
Slide36Using half-life in calculations
Slide37Slide38Glossary
atomic number –
The number of protons in the nucleus
of an atom, which is the same for all isotopes of an element.
half-life – The time taken for the number of radioactive atoms in a sample, or the count rate, to decrease by half. isotopes – Different forms of the same element, with the same number of protons but different numbers of neutrons.
mass number – The total number of protons and neutrons in the nucleus of an atom, which differs for each isotope of an element.radioactive decay –
The breakdown of unstable radioactive nuclei by releasing radiation.radioisotope – A radioactive isotope of an element, which may be naturally occurring or artificially created.
Slide39Anagrams
Slide40Multiple-choice quiz