IsotopesIsotopes

Atomic Structure - recapAtomic Structure - recap■ There are two regions.There are two regions.■ The The nucleusnucleus..

– With With protonsprotons and and neutrons (nucleons)neutrons (nucleons)

– Positive charge.Positive charge.– Almost all the Almost all the massmass..

■ Electron cloud-Electron cloud- most of most of the the volumevolume of an atom. of an atom.– The region where the The region where the

electron can be electron can be found.found.

– Negative charge.Negative charge.

Properties of the atomProperties of the atom■ Atoms are small.Atoms are small.

■ Nucleus tiny compared to atom.Nucleus tiny compared to atom.

■ If the atom was the size of a stadium, the If the atom was the size of a stadium, the nucleus would be the size of a marble.nucleus would be the size of a marble.

Counting the PiecesCounting the Pieces■ Atomic Number Atomic Number = number of protons= number of protons

– No. of protons determines kind of No. of protons determines kind of atom.atom.

– No. of protons is the same as the No. of protons is the same as the number of electrons in the neutral number of electrons in the neutral atom.atom.

■ Mass Number = Mass Number = the number of protons the number of protons + neutrons.+ neutrons.

IsotopesIsotopes■ Recall Dalton’s view: Atoms of the same Recall Dalton’s view: Atoms of the same

element are element are identicalidentical, those of different atoms , those of different atoms are different.are different.

Dalton was wrong.Dalton was wrong.■ Atoms of the same element can have Atoms of the same element can have

different numbers of different numbers of neutronsneutrons..■ Therefore different Therefore different mass numbers.mass numbers.■ Isotopes -Isotopes - Atoms with the same number of Atoms with the same number of

protons but different number of neutrons.protons but different number of neutrons.

Symbolic Representation of isotopesSymbolic Representation of isotopes

■ Consist of the Consist of the symbolsymbol of the element, of the element, the the mass numbermass number and the and the atomic atomic numbernumber..

X Massnumber

Atomicnumber

Naming IsotopesNaming Isotopes■ Put the mass number after the name of Put the mass number after the name of

the element.the element.■ carbon- 12carbon- 12■ carbon -14carbon -14■ uranium-235uranium-235

SymbolsSymbols■ Find the Find the

– number of protonsnumber of protons– number of neutronsnumber of neutrons– number of electronsnumber of electrons– Atomic numberAtomic number– Mass NumberMass Number– NameName

Na2411

SymbolsSymbols■ Find the Find the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Atomic numberAtomic number

–Mass NumberMass Number– NameName

Br80 35

SymbolsSymbols■ if an element has an atomic if an element has an atomic

number of 34 and a mass number number of 34 and a mass number of 78 what is the of 78 what is the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Complete symbolComplete symbol– NameName

SymbolsSymbols■ if an element has 91 protons and if an element has 91 protons and

140 neutrons what is the 140 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of electronsnumber of electrons

–Complete symbolComplete symbol– NameName

SymbolsSymbols■ if an element has 78 electrons and if an element has 78 electrons and

117 neutrons what is the 117 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of protonsnumber of protons

–Complete symbolComplete symbol– NameName

Atomic MassAtomic Mass■ How heavy is an atom of oxygen?How heavy is an atom of oxygen?■ There are different kinds of oxygen atoms.There are different kinds of oxygen atoms.■ More concerned with More concerned with average average atomic mass.atomic mass.■ Based on Based on abundanceabundance of each element in of each element in

nature.nature.■ Don’t use grams because the numbers Don’t use grams because the numbers

would be too small.would be too small.

Calculating Relative Atomic MassCalculating Relative Atomic Mass

■ Unit is the Unit is the Atomic Mass Unit Atomic Mass Unit (amu)(amu)■ One twelfth the mass of a carbon -12 One twelfth the mass of a carbon -12

atom. atom. ■ 6 p6 p++ and 6 n and 6 n00

■ i.e. Equal to mass of one nucleon■ Each isotope has its own atomic massEach isotope has its own atomic mass■ we get the average using we get the average using percent percent

abundance.abundance.

Calculating averagesCalculating averages■ You have five rocks. Four have a mass of You have five rocks. Four have a mass of

50g, and one has a mass of 60g. What is 50g, and one has a mass of 60g. What is the average mass of the rocks?the average mass of the rocks?

■ Total mass = (4 x 50) + (1 x 60) = 260 gTotal mass = (4 x 50) + (1 x 60) = 260 g■ Average mass = Average mass = (4 x 50)(4 x 50) + + (1 x 60)(1 x 60) = = 260260 g g

5 5 5 5 5 5

Calculating averagesCalculating averages■ Average mass = (Average mass = (44 x 50) + ( x 50) + (11 x 60) = x 60) = 260 260

gg 5 5 5 5 5 5■ Average mass = (0.8 x 50) + (0.2 x 60)Average mass = (0.8 x 50) + (0.2 x 60)■ So, 80% of the rocks were 50 gramsSo, 80% of the rocks were 50 grams■ And 20% of the rocks were 60 gramsAnd 20% of the rocks were 60 grams■ Average = % as decimal x mass + Average = % as decimal x mass +

% as decimal x mass + % as decimal x mass + % as decimal x mass + % as decimal x mass +

… …

Relative Atomic MassRelative Atomic Mass■ Calculate the atomic mass of copper if Calculate the atomic mass of copper if

copper has two isotopes. 69.1% have a copper has two isotopes. 69.1% have a mass of 62.93 amu and the rest have a mass of 62.93 amu and the rest have a mass of 64.93 amu.mass of 64.93 amu.

Relative Atomic MassRelative Atomic Mass■ Magnesium has three isotopes. 78.99% Magnesium has three isotopes. 78.99%

magnesium-24 with a mass of 23.9850 magnesium-24 with a mass of 23.9850 amu, 10.00% magnesium-25 with a mass of amu, 10.00% magnesium-25 with a mass of 24.9858 amu, and the rest magnesium-25 24.9858 amu, and the rest magnesium-25 with a mass of 25.9826 amu. What is the with a mass of 25.9826 amu. What is the relative atomic mass of magnesium?relative atomic mass of magnesium?

■ If not told otherwise, the mass of the If not told otherwise, the mass of the isotope is the mass number in amu isotope is the mass number in amu

Relative Atomic MassesRelative Atomic Masses■ Are not a whole numbers because they Are not a whole numbers because they

are averages (‘Weighted Mean’ of the are averages (‘Weighted Mean’ of the istopes present). istopes present).

■ Represented as decimal numbers on Represented as decimal numbers on the periodic table.the periodic table.

Radioactivity■ Not on this Specification

■ STOP HERE!

Unstable Nuclei and RadioactiveUnstable Nuclei and Radioactive Decay (4.4)Decay (4.4)

Up to this point the nucleus of the atom Up to this point the nucleus of the atom has remained unchangedhas remained unchanged– The protons and neutronsThe protons and neutrons

Nuclear reactionsNuclear reactions are reactions that are reactions that involve specifically changes in the involve specifically changes in the nucleus of an atom.nucleus of an atom.

Nuclear reactions are responsible for Nuclear reactions are responsible for producing what we think of as producing what we think of as radiation.radiation.

There are three differentThere are three different forms of forms of nuclear radiationnuclear radiation– AlphaAlpha– BetaBeta– GammaGamma

Particles emitted from nucleus

Spontaneous emission of radiation Spontaneous emission of radiation from an atom isfrom an atom is known asknown as radioactivityradioactivity..

Almost all atoms we’ve encountered Almost all atoms we’ve encountered have stable nucleihave stable nuclei– Not radioactiveNot radioactive

Radioactive atoms have nuclei that are Radioactive atoms have nuclei that are disintegratingdisintegrating

So why are some nuclei stable and So why are some nuclei stable and others unstable?others unstable?

NUCLEAR STABILITYNUCLEAR STABILITY

One of the reasons is the number of One of the reasons is the number of protons and neutrons that they contain.protons and neutrons that they contain.

– Not all combinations of protons Not all combinations of protons and neutrons are stable.and neutrons are stable.

The nucleus is made up of protons and The nucleus is made up of protons and neutrons densely packed together.neutrons densely packed together.

However, protons are positiveHowever, protons are positive and the positive charges shouldand the positive charges shouldbe repelling each otherbe repelling each other ++ ++++

++

++++

So why don’t the protons just fly apart? So why don’t the protons just fly apart? Why are most nuclei stable?Why are most nuclei stable?

There are 2 forces at work:

The repulsive force & a strong nuclear

force.

The neutrons in the nucleus act as the The neutrons in the nucleus act as the glue that holds the nucleus together.glue that holds the nucleus together.

A strong nuclear

force set up by the neutrons

All stable nuclei follow a distinct All stable nuclei follow a distinct pattern. pattern. – For elements with atomic numbers For elements with atomic numbers

between 1 and 20 stable nuclei have between 1 and 20 stable nuclei have almost equal numbers of protons almost equal numbers of protons and neutrons. Beyond 20 protons, and neutrons. Beyond 20 protons, nuclei need increasingly more nuclei need increasingly more neutrons than protons to be stable.neutrons than protons to be stable.

Nuclei are unstable not only if they Nuclei are unstable not only if they contain too few neutrons, but also if contain too few neutrons, but also if they contain too many.they contain too many.

Radioactive elements emit different Radioactive elements emit different kinds of radiationkinds of radiation

– Alpha Alpha – BetaBeta– GammaGamma

Each differs in charge, mass, and Each differs in charge, mass, and penetrating powerpenetrating power

Alpha radiation consists of a stream of Alpha radiation consists of a stream of high-energy alpha particleshigh-energy alpha particles

Consists of 2 protons and 2 neutrons Consists of 2 protons and 2 neutrons and is identical to a helium-4 nucleusand is identical to a helium-4 nucleus

Can be represented by the symbolCan be represented by the symbol

Alpha ParticlesAlpha Particles

Alpha particles do not have much Alpha particles do not have much penetrating power.penetrating power.

They are able to travel only a few They are able to travel only a few centimeters through air and are easily centimeters through air and are easily stopped by paper or clothingstopped by paper or clothing

Not normally harmful to humansNot normally harmful to humans

Alpha ParticlesAlpha Particles

Beta radiation consists of a stream of Beta radiation consists of a stream of high-speed electronshigh-speed electrons

A neutron changes into a proton and A neutron changes into a proton and an electronan electron– The proton remains in the nucleus The proton remains in the nucleus

and the electron is propelled out of and the electron is propelled out of the nucleusthe nucleus

Beta radiation is represented by the Beta radiation is represented by the symbolsymbol

Beta ParticlesBeta Particles

The mass number is zeroThe mass number is zero

100 times more penetrating then alpha 100 times more penetrating then alpha radiationradiation

Can damage the skinCan damage the skin

Beta ParticlesBeta Particles

A Gamma ray is highly energetic light, A Gamma ray is highly energetic light, similar to x-rayssimilar to x-rays

Does not consist of particlesDoes not consist of particles Gamma radiation accompanies alpha Gamma radiation accompanies alpha

and beta radiationand beta radiation Much more penetrating than either of Much more penetrating than either of αα

or or ββ It is able to penetrate deeply into solid It is able to penetrate deeply into solid

material, including body tissuematerial, including body tissue Symbolized by: Symbolized by:

Gamma RaysGamma Rays

Properties of Some RadiationsProperties of Some Radiations

PropertyPropertyAlphaAlpha

RadiationRadiationBeta Beta

RadiationRadiationGamma RadiationGamma Radiation

CompositionComposition Alpha ParticleAlpha Particle Beta ParticleBeta Particle High-energy photonHigh-energy photon

SymbolSymbol αα, He, He ββ, e, e γγ

ChargeCharge 2+2+ 1-1- 00

Mass (amu)Mass (amu) 44 1/18371/1837 00

4422

00-1-1

Lead, concreteLead, concreteMetal foilMetal foilPaper, clothingPaper, clothingShieldingShielding

Very highVery high(penetrates body easily)(penetrates body easily)

ModerateModerate(4mm body tissue)(4mm body tissue)

Low Low (0.05mm body (0.05mm body tissue)tissue)PowerPower

1MeV1MeV0.05 to 1 MeV0.05 to 1 MeV5 MeV5 MeVAppox.Appox.energyenergy

Cobalt-60Cobalt-60Carbon-14Carbon-14Radium-226Radium-226Common Common sourcesource

Gamma RadiationGamma RadiationBeta Beta

RadiationRadiationAlphaAlpha

RadiationRadiationPropertyProperty

Properties of Some RadiationsProperties of Some Radiations

When an atom emits one of these kinds When an atom emits one of these kinds of radiation, it is said to be decaying.of radiation, it is said to be decaying.

An atom may undergo an alpha or beta An atom may undergo an alpha or beta decay.decay.

The radiation is called decay because The radiation is called decay because the nucleus decomposes to form a new the nucleus decomposes to form a new nucleus, called transmutationnucleus, called transmutation

The best way to understand a decay is The best way to understand a decay is with a nuclear equationwith a nuclear equation

Radioactive DecayRadioactive Decay

An alpha particle is a particle An alpha particle is a particle composed of 2 protons and 2 neutrons.composed of 2 protons and 2 neutrons.

With each expulsion of an alpha particle With each expulsion of an alpha particle from the atom’s nucleus the atom loses from the atom’s nucleus the atom loses 4 units of mass & 2 protons (+2 charged 4 units of mass & 2 protons (+2 charged particle)particle)

Any change in #’s of protons changes Any change in #’s of protons changes the type of atom, this is transmutation.the type of atom, this is transmutation.

Alpha Decay EquationsAlpha Decay Equations

If you remember a If you remember a β β is an eis an e-- that is that is expelled from an atomexpelled from an atom

This electron is the result of one of the This electron is the result of one of the atom’s neutrons decomposing into a atom’s neutrons decomposing into a proton and an electron.proton and an electron.

This results in the atom having one This results in the atom having one more proton which causes it to mutate more proton which causes it to mutate into a different atom.into a different atom.

Beta Decay EquationsBeta Decay Equations