Chapter 10 Nuclear Changes. What is Radioactivity? Certain isotopes of many elements undergo a...

Chapter 10Nuclear Changes

What is Radioactivity?

• Certain isotopes of many elements undergo a process called radioactive decay

• During radioactive decay, the unstable nuclei of these isotopes emit particles, or release energy, to become stable isotopes– The released energy and matter is referred to as

nuclear radiation– After radioactive decay, the element changes into a

different isotope of the same element or into an entirely different element

• Recall that isotopes of an element are atoms that have the same number of protons but different numbers of neutrons in their nuclei– Different elements are distinguished by

having different numbers of protons

• Four types of nuclear radiation– Alpha particles– Beta particles– Gamma rays– Neutrons

• Alpha particles– Made of 2 protons and two neutrons

• Same as the helium nuclei

– Positively charged– More massive than any other type of nuclear

radiation– Do not travel far through materials

• Barely pass through a sheet of paper

– Can ionize other atoms thus losing energy

• Beta particles– Often fast-moving electrons but may also be

positively charged particles called positrons• Positrons have the same mass as electrons

– Neutrons, which are not charged, decay to form a proton and an electron

• The electron, which has a very small mass, is then ejected at a high speed from the nucleus as a beta particle

– Pass through a piece of paper , but most are stopped by 3 mm of aluminum or 10 mm of wood

– Not as massive as alpha particles– Can ionize other atoms thus losing energy

• Gamma rays– Not made up of matter and do not have an electric

charge– Form of electromagnetic energy– Consist of energy packets called photons

• Like light and X-rays

– Have more energy than light or X-rays– Can ionize and cause damage in matter– Can penetrate up to 60 cm of aluminum or 7 cm of lead – Greater damage to health

• Neutron radiation– No charge

• Therefore, do not ionize matter– Therefore, they are able to travel farther through matter

than alpha and beta particles

» A block of lead about 15 cm thick is required to stop most fast neutrons

• Anytime that an unstable nucleus emits alpha or beta particles, the number of protons and neutrons change– Example: radium-226 (an isotope of radium

with the mass number 226)changess to radon-222 by emitting an alpha particle

• Gamma decay changes the energy of the nucleus– No change in the atomic number of the element

• Beta Decay:– Loses electron – Atomic number changes (increase of one)– Mass number before and after the decay does not change– The atomic number of the product nucleus increases by one (neutron

changes into a proton) – Example: carbon-14 changes into nitrogen-14

• Alpha Decay:– Loses 2 protons and 2 neutrons (helium nuclei)

– Both atomic mass and number change’

• What do the colors mean?• Zone 1 counties have a predicted

average indoor radon screening level greater than 4 pCi/L (picocuries per liter) (red zones)Highest Potential

• Zone 2 counties have a predicted average indoor radon screening level between 2 and 4 pCi/L (orange zones)Moderate Potential

• Zone 3 counties have a predicted average indoor radon screening level less than 2 pCi/L (yellow zones)Low Potential

PHILADELPHIA COUNTY

MONTGOMERY COUNTY

BUCKS COUNTY

• Radioactive Decay Rates– Half-Life: time in which half of a radioactive

substance decays– Last from a nanosecond to billions of years– Used to diagnose medical problems

• Detector follows the element as it moves through the patient’s body

– Predict the age of fossils/rocks

• Radium-226 has a half-life of 1,599 years. How long will 7/8 of a sample of radium-226 take o decay?– Fraction remaining 1/8– Amount of sample remaining after 3 half-lives

• ½ x ½ x ½ = 1/8

– 3 half-lives X 1,599 years/half-life = 4,797 years

• Radioactive decay is exponential decay

• Example: Carbon-14

Nuclear Change

• The stability of a nucleus depends on the nuclear forces that hold the nucleus together.

– These forces act between the protons and the neutrons

• Like charges repel, so how can so many positively charged protons fit into an atomic nucleus without flying apart?

– Strong nuclear forces cause neutrons and protons to attract one another—this attraction is much stronger than the electric repulsion between protons

Nuclear Change

• Nuclei with more than 83 protons are always unstable– These nuclei will always decay and, in the

process, release large amounts of energy and nuclear radiation

Nuclear Change

• Nuclear fission: process of splitting heavier nuclei into lighter nuclei– Neutrons and energy are released– Some mass will be changed into energy– The equivalence of mass and energy is

explained by the Theory of Relativity (Einstein)• Energy= mass X (speed of light)2 = mc2

– Under ordinary conditions this change does not happen spontaneously

Nuclear Change

• Neutrons released by fission can start a chain reaction (one reaction triggers another and so on)– Continuous series of

nuclear fission reactions

Nuclear Change

• Critical mass: the minimum mass of a fissionable isotope that provides the number of neutrons needed to sustain a chain reaction– In nature the concentration is

• Chain reactions can be controlled to be used to generate electricity– Control rods regulate fission

by slowing the chain reaction

Nuclear Change

Nuclear Change• Nuclear Fusion: energy can be obtained

when very light nuclei are combined to form heavier nuclei

– Example:• Sun: energy is formed

primarily when hydrogen nuclei combine releasing energy

– Large amount of energy is needed to start a fusion reaction

• Reason is that all nuclei are positively charged and repel one another with electric force

– Energy needed to overcome this force (Stars: the extreme temperature overcomes this force)

Nuclear Change

Nuclear Radiation Today

• We are continually exposed to radiation from natural sources (sun, soil, rocks, plants)(80%)– Human-made sources

(computer monitors, smoke detectors, X-rays, etc.)(20%)

– Measured in units called rems (1rem = 1,000 millirems)

• Dental X-ray= 1 millirem• Safe limit for workers is

about 5,000 rems per year

Nuclear Radiation Today

• Exposure varies from location to location, activities (jobs/recreational), habits (smoking), home to home (smoke detectors: emit alpha particles)

Chapter 10 Nuclear Changes. What is Radioactivity? Certain isotopes of many elements undergo a...

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