what is radioactivity? radioactive decay. nucleus contains protons and neutrons electron circles the...

What is Radioactivity?

Radioactive Decay

• Nucleus contains protons and neutrons

• Electron circles the nucleus in orbits

• Proton: +1 charge, mass number (MN) of 1

• Neutron: 0 charge, MN of 1

• Electron: -1 charge, MN nearly zero

Quick Review

Standard Atomic Notation (SAN)

B95• SAN tells us all

we need to know about an atom

• For boron, # of protons = 5# of electrons = 5

• Mass number = 9# neutrons = 4

Atomic Symbol

Atomic Number (number of protons)

Mass Number

Use your periodic table to state the following for each atom:

Atomic number # of protons # of electrons rounded atomic mass # of neutrons Standard Atomic Notation

1) barium 2) uranium 3) chlorine

Try these…

Barium Uranium Chlorine

Atomic number

56 92 17

# of protons 56 92 17

# of electrons 56 92 17

Mass number 137 238 35

# of neutrons 81 146 18

Standard Atomic Notation

Ba137

56 U238

92 Cl35

17

Answers

O168 O178and

O188and

All three are forms of the oxygen atom found in nature but each have different numbers of neutrons. has 8 protons and 8 neutrons

has 8 protons but 9 neutrons

has 8 protons and 10 neutrons

O168

O178

O188

Isotopes can also be written as Oxygen-18 for

O188

What about these?

• Isotopes are the same atoms with different mass numbers

• The different masses in the three oxygen isotopes are from extra neutrons in the nucleus

• Isotopes appear in all the same compounds as the element

• Isotopes have different but similar properties, e.g., ordinary water (water with atoms) boils at 1000 C but heavy water (water with atoms) boils at 101.420 C

They are isotopes

• Isotopes mean that the nucleus has changed (added neutrons)

• This is new because in chemical reactions the nuclei of the reactants and products remain unchanged

• A reaction that changes the nucleus of an atom is called a nuclear reaction

A new type of reaction

• Elements with atomic number <83 [except for technectium (43) and promethium (61)] have one or more stable isotopes

• All elements above atomic number 83 have isotopes with unstable nuclei

• The nuclei of these elements spontaneously disintegrate

• The spontaneous disintegration of a nucleus is called radioactivity

• The unstable nucleus that disintegrates is said to be radioactive

Unstable nuclei

• When a nucleus disintegrates it can emit:• alpha particles • beta particles (electrons or positrons)• gamma rays• energy

• The new nucleus that remains may be radioactive or stable

Nuclear disintegration

• Given the Greek symbol for alpha, (first letter of the Greek alphabet)

• Fast moving, high energy• Relatively heavy, loses energy quickly• Stopped by a piece of paper• Travel 2.5 cm in air• Penetrate skin only 0.3 mm• Consist of 2 protons and 2 neutrons• Rutherford found an alpha particle to be a helium

atom,

Alpha particles

• When an unstable nucleus emits an alpha particle it undergoes alpha decay

• The resulting new nucleus is an isotope

Example: Uranium-238 undergoes alpha decay

Uranium-238 decays to make a new isotope - thorium and an alpha particle

Nuclear Equation

Alpha Decay

* Notice the conservation of mass holds (no particles are destroyed).

Alpha particle

U238

92 Th234

90 He4

2

• Given the Greek symbol for beta, β (second letter of the Greek alphabet)

• Very fast • Very light• Stopped by 0.1 mm of lead• Can travel 4.5 m in air• Penetrate skin 17 mm• Beta particles may be positively or negatively charged

Beta particles

Two types of beta decay are known:

β – Decay• Neutron in the nucleus is converted into a proton• Atomic number increases by 1 (new element)• During this conversion an electron and an antineutrino are

ejected from the nucleus• Also known as electron emission• An example of β- decay is:

Electron Antineutrino

Th234

90 Pa234

91e0-1 v

__

Beta Decay

β + Decay (positron emission)• Proton in the nucleus is converted into a neutron• Atomic number decreases by 1 (new element)• During this conversion a positron and a neutrino are

ejected from the nucleus• Also known as positron emission• An example of β+ decay is:

Positron Neutrino

Na 22

11Ne

22

10e0

1 v

Beta Decay

• Given the Greek symbol for gamma, γ (third letter of the Greek alphabet) • Type of ionizing electromagnetic radiation• Most penetrating form • Highest frequency and shortest wavelength in the electromagnetic spectrum• Produced when the nucleus of an atom is in an excited state and then

releases energy, becoming more stable • When a nucleus emits an or β particle, the daughter nucleus is sometimes

left in an excited state. It can then jump down to a lower level by emitting a gamma ray

Gamma Rays

• Nuclear reactions are just like regular chemical reactions• Mass is conserved• Nuclear reactions show the starting material, the type of

radiation given off and the end products• Example:Radioactive atom new radioactive element particle

emitted

Nuclear Reaction Equations

Electron

Alpha particleUranium atom New isotope

U238

92 Th234

90 He4

2

Thorium atom New isotope

Th234

90 Pa234

91 e0-1