7.1 7.1 Atomic Theory and Atomic Theory and Radioactive DecayRadioactive Decay

Natural Natural background radiationbackground radiation exists all exists all around us.around us.

Radioactivity Radioactivity is the release of high energy is the release of high energy particles or waves that we call particles or waves that we call radiationradiation– When atoms lose When atoms lose high energy particleshigh energy particles and and

waves, new atoms can be formed.waves, new atoms can be formed.– Radiation can be good or bad:Radiation can be good or bad:

X-rays, radiation therapy and electricity X-rays, radiation therapy and electricity generation are beneficial.generation are beneficial.

High energy particles and waves can do damage High energy particles and waves can do damage to DNA in our cells.to DNA in our cells.

See pages 286 - 287

The Electromagnetic Spectrum

Early Discoveries of Early Discoveries of RadiationRadiation

– Roentgen named X rays with an “X” 100 years ago because Roentgen named X rays with an “X” 100 years ago because they were previously unknown.they were previously unknown.

– Becquerel realized uranium emitted seemingly invisible Becquerel realized uranium emitted seemingly invisible energy as well.energy as well.

– Marie Curie and her husband Pierre named this energy Marie Curie and her husband Pierre named this energy radioactivity.radioactivity. Early discoveries of radiation reliedEarly discoveries of radiation relied

on photographic equipment.on photographic equipment.– Later, more sophisticated devicesLater, more sophisticated devices

such as the Geiger-Müller countersuch as the Geiger-Müller counter Radium salts, after being Radium salts, after being placed on a photographic placed on a photographic plate, leave behind the plate, leave behind the dark traces of radiation. dark traces of radiation.

IsotopesIsotopes

are atoms of the same element, with a are atoms of the same element, with a different number of neutronsdifferent number of neutrons..– changing the # of neutrons changes the mass changing the # of neutrons changes the mass

numbernumber Remember: mass # = # protons + # neutronsRemember: mass # = # protons + # neutrons

– isotopes still have theisotopes still have the same number of protons same number of protons and the same element symboland the same element symbol

Atomic Mass (the decimal #’s)Atomic Mass (the decimal #’s) Atomic mass = average of the mass Atomic mass = average of the mass

numbers for all isotopes of an element.numbers for all isotopes of an element.

39 40 4119 19 19K, K, K

Representing IsotopesRepresenting Isotopes Isotopes are written two waysIsotopes are written two ways

– with the mass number at the end with the mass number at the end Ex. Potassium – 40Ex. Potassium – 40– With its chemical symbol With its chemical symbol Ex. Ex. 40

19 K

Mass number

Atomic number

39 40 4119 19 19K, K, K

19 19 19

19 19 19

20 21 22

Radioactive DecayRadioactive Decay

Can result in new atoms forming.Can result in new atoms forming.– Radioactivity results from having an Radioactivity results from having an

unstable nucleus.unstable nucleus.– Radioactive decayRadioactive decay = when nuclei break = when nuclei break

apart + release energy from the nucleus.apart + release energy from the nucleus. Radioactive decay continues until a stable Radioactive decay continues until a stable

element forms.element forms. An element may have isotopes that are An element may have isotopes that are

radioactive called radioactive called radioisotopes radioisotopes – Ex. carbon-12, carbon-13 and carbon-14 Ex. carbon-12, carbon-13 and carbon-14

(only C-14 is radioactive)(only C-14 is radioactive)

Uranium goes through many decay Uranium goes through many decay steps before it becomes stable:steps before it becomes stable:

Rutherford identified three types of Rutherford identified three types of radiation using an electric field.radiation using an electric field.– Positive Positive alpha particlesalpha particles were attracted to the were attracted to the

negative plate.negative plate.– Negative Negative beta particlesbeta particles were attracted to the positive were attracted to the positive

plate.plate.– Neutral Neutral gamma particlesgamma particles did not move towards any did not move towards any

plate.plate.

Alpha Radiation:Alpha Radiation: is a stream of alpha particles, (shown is a stream of alpha particles, (shown

as as ))– positively chargedpositively charged– weighs the mostweighs the most– are the same as a helium nucleusare the same as a helium nucleus– Alpha particles are represented by the Alpha particles are represented by the

symbolssymbols

2 protons and 2 neutrons make a mass 2 protons and 2 neutrons make a mass number of 4number of 4

it has a charge of 2+ because of the protonsit has a charge of 2+ because of the protons

4 42 2 or He

Alpha particles are big and slow. A sheet of Alpha particles are big and slow. A sheet of paper will stop an alpha particle.paper will stop an alpha particle.

Example: the Example: the alpha decayalpha decay of Radium - 226 of Radium - 226

226 222 4 226 222 488 86 2 88 86 2oRa Rn + Ra Rn + er H

Beta Radiation:Beta Radiation:

A Beta particle, A Beta particle, , is a high energy , is a high energy electron.electron.– negatively charged, and weigh less than alpha negatively charged, and weigh less than alpha

particles.particles.– Beta particles are represented by the symbols Beta particles are represented by the symbols

electrons are very tiny, so beta particles are electrons are very tiny, so beta particles are assigned a mass of 0.assigned a mass of 0.

one electron gives a beta particle has a charge of 1–.one electron gives a beta particle has a charge of 1–.

0 0-1 -1 or e

Beta decay occurs when a Beta decay occurs when a neutron changes neutron changes into a proton + an electroninto a proton + an electron..– The proton stays in the nucleus, and the The proton stays in the nucleus, and the

electron is released.electron is released.

Example: The Example: The beta decaybeta decay of iodine - 131 of iodine - 131

It takes a thin sheet of aluminum foil to stop It takes a thin sheet of aluminum foil to stop a beta particle.a beta particle.

131 131 053 54 –1

131 131 053 54 –1

I Xe +

I Xe +

o

e

r

Gamma Radiation:Gamma Radiation: Gamma radiation, Gamma radiation, , is a ray of high energy, , is a ray of high energy,

short-wavelength radiation.short-wavelength radiation.– has no charge and no mass.has no charge and no mass.– is the highest energy form of electromagnetic is the highest energy form of electromagnetic

radiation.radiation.– It takes thick blocks of lead or concrete to stop It takes thick blocks of lead or concrete to stop

gamma rays.gamma rays.– Gamma decay results from energy being Gamma decay results from energy being

released from a high-energy nucleus.released from a high-energy nucleus.60 60 028 28 0Ni Ni + *

Shows unstable nucleus for gamma decay

Often, other kinds of radioactive decay will Often, other kinds of radioactive decay will also release gamma radiation.also release gamma radiation.

– Uranium-238 decays into an alpha particle and Uranium-238 decays into an alpha particle and also releases gamma rays.also releases gamma rays.

238 234 492 90 2U Th + He + 2

Nuclear Equations:Nuclear Equations: are written like chemical equations, but are written like chemical equations, but

represent changes in the nucleus of represent changes in the nucleus of atoms.atoms.– Chemical equations represent changes in Chemical equations represent changes in

the position of atoms, not changes to the the position of atoms, not changes to the atoms themselves.atoms themselves.

Remember:Remember:1.1.The sum of the mass numbers on each The sum of the mass numbers on each

side of the equation should equal.side of the equation should equal.

2.2.The sum of the charges on each side of The sum of the charges on each side of the equation should equal.the equation should equal.

Take the Section 7.1 Quiz

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