Monday August 20 Describe subatomic particles.

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

Rutherford’s 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)

continued 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 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 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 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 Number —The number of protons PLUS the 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 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 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. The Nucleus 2 2

Radioactive Decay The Nucleus 2 2 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

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

Loss of Beta Particles Some elements undergo transmutations through a different process. Their nuclei emit an electron called a beta particle. The Nucleus 2 2 A beta particle is a high-energy electron that comes from the nucleus, not from the electron cloud.

Loss of Beta Particles 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. The Nucleus 2 2 The proton, however, remains in the nucleus.

Loss of Beta Particles Because a neutron has been changed into a proton, the nucleus of the element has an additional proton. The Nucleus 2 2

Loss of Beta Particles Unlike the process of alpha decay, in beta decay the atomic number of the element that results is greater by one. The Nucleus 2 2

Rate of Decay Radioactive decay is random. The rate of decay of a nucleus is measured by its half-life. The Nucleus The half-life of a radioactive isotope is the amount of time it takes for half of a sample of the element to decay.

Loss of Beta Particles Some elements undergo transmutations through a different process. Their nuclei emit an electron called a beta particle. The Nucleus 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. The Nucleus 2 2 If you start with a sample of 4 g of iodine- 131, after eight days you would have only 2 g of iodine-131 remaining.

Calculating Half-Life Decay The Nucleus 2 2 After 16 days, or two half-lives, half of the 2 g would have decayed and you would have only 1 g left.

Calculating Half-Life Decay 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. The Nucleus 2 2

Carbon 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. The Nucleus 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. The Nucleus 2 2 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. The Nucleus 2 2 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 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 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-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