Monday August 20

Monday August 20Monday August 20

Describe subatomic Describe subatomic particles.particles.

Models of the AtomModels of the Atom

• John Dalton—pictured the atom as a hard sphere that was the same throughout

• William Crookes—CRT One electrode, called the anode, has a positive charge

JJ ThompsonJJ Thompson

• He concluded that the CRT produced particles that were negatively charged.

• These particles later became know as the ELECTRON.

• Perhaps one of the biggest surprises that came from Thompson’s experiments was the evidence that particles smaller than the atom did exist. (subatomic particles)

Thompson’s ModelThompson’s Model

• Solid core with the solid part being positively charged and the ball bearing that were spread evenly throughout being the negative charges.

Rutherford’sRutherford’s experiment experiment

• Questioned Thompson’s model, Was it correct.

• In Rutherford’s experiment, alpha particles bombarded the gold foil or veered slightly from a straight path but

then some particles bounced right back. (409)

continuedcontinued

• It was proposed that there must be another particle in the NUCLEUS of the atom.

• The particles is the neutron. It is electrically neutral.

Niel BohrNiel Bohr

• Physicist even calculated the energy levels in the nucleus.

• Scientist discovered that the electrons did not move in an orbit as suggested by Bohr. They were more random.

• Electrons travel in a region surrounding the NUCLEUS, which is called the electron cloud.

Model with a NUCLEUSModel with a NUCLEUS

• Due to the unexpected results from Rutherford’s model he proposes a new model for the atom.

• From the new information, he concluded that the atom the dense center of positive charges known as the NUCLEUS.

• There was a problem

Important VocabularyImportant Vocabulary

• Atomic Number —the number of protons in the nucleus

•

These three kinds of carbon atoms are called isotopes. Isotopes are atoms of the same element that have different numbers

of neutrons

Mass NumberMass Number —The number of protons PLUS the —The number of protons PLUS the

number of neutrons.number of neutrons.

You can find the number of neutrons in an isotope by subtracting the atomic number from the mass number.

Radioactive Decay of the NucleusRadioactive Decay of the Nucleus

• Radioactive decay—the release of nuclear particles and energy.

• Transmutation—the changing of one element into another by radioactive decay.

• Alpha particle—consist of TWO protons and TWO neutrons. Together the energy and particles are called Nuclear Radiation.

Radioactive Decay• Transmutation is occurring in most of your

homes right now. • A smoke detector makes use of radioactive

decay.

The NucleusThe Nucleus

• This device contains americium-241 (a muh RIH shee um), which undergoes transmutation by ejecting energy and an alpha particle.

Radioactive Decay• In the smoke detector, the fast-moving

alpha particles enable the air to conduct an electric current.

• As long as the electric current is flowing, the smoke detector is silent.


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Radioactive Decay


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• The alarm is triggered when the flow of electric current is interrupted by smoke entering the detector.

When americium expels an alpha particle, it’s no longer americium.

Changed Identity

Changed Identity

• After the transmutation, it becomes the element that has 93 protons, neptunium.


Loss of Beta Particles

• Some elements undergo transmutations through a different process.

• Their nuclei emit an electron called a beta particle.


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• A beta particle is a high-energy electron that comes from the nucleus, not from the electron cloud.


• During this kind of transmutation, a neutron becomes unstable and splits into an electron and a proton.

• The electron, or beta particle, is released with a large amount of energy.


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• The proton, however, remains in the nucleus.


• Because a neutron has been changed into a proton, the nucleus of the element has an additional proton.


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• Unlike the process of alpha decay, in beta decay the atomic number of the element that results is greater by one.


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Rate of Decay

• Radioactive decay is random.

• The rate of decay of a nucleus is measured by its half-life.


• The half-life of a radioactive isotope is the amount of time it takes for half of a sample of the element to decay.


• Some elements undergo transmutations through a different process.

• Their nuclei emit an electron called a beta particle.


• A beta particle is a high-energy electron that comes from the nucleus, not from the electron cloud.

Calculating Half-Life Decay

• Iodine-131 has a half-life of eight days.


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• If you start with a sample of 4 g of iodine-131, after eight days you wouldhave only 2 g of iodine-131 remaining.



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• After 16 days, or two half-lives, half of the2 g would have decayed and you would have only 1 g left.


• The radioactive decay of unstable atoms goes on at a steady pace, unaffected by conditions such as weather, pressure, magnetic or electric fields, and even chemical reactions.


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Carbon DatingCarbon Dating

• Carbon-14 is used to determine the age of dead animals, plants, and humans.

Carbon Dating• When archaeologists

find an ancient item, they can find out how much carbon-14 it has and compare it with the amount of carbon-14 the animal would have had when it was alive.


• Knowing the half-life of carbon-14, they can then calculate when the animal lived.

Carbon Dating• When geologists want to determine the age

of rocks, they cannot use carbon dating.


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• Instead, geologists examine the decay of uranium.

• Uranium-238 decays to lead-206 with a half-life of 4.5 billion years.

• By comparing the amount of uranium to lead, the scientist can determine the age of a rock.

Uses of Radioactive Isotopes

• Tracer elements are used to diagnose disease and to study environmental conditions.


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• The radioactive isotope is introduced into a living system such as a person, animal, or plant.

• It then is followed by a device that detects radiation while it decays.

Medical UsesMedical Uses

• The isotope iodine-131 has been used to diagnose problems with the thyroid, a gland located at the base of the neck.

• Other radioactive isotopes are used to detect cancer, digestion problems, and circulation difficulties.

Environmental UsesEnvironmental Uses

• Radioisotopes also can be placed in pesticides and followed to see what impact the pesticide has as it moves through an ecosystem.

Calculating Half-lifeCalculating Half-life

• Read the problem

• Write down the information known

• What do you want to find out?

• Determine the number of half-lives

• Final weight= initial weight/2 (number of half-lives)

• Refer to page 419

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Monday August 20