Activity 9 What Determines and Limits an Atom's Mass?

449Coordinated Science for the 21st Century

Activity 9 What Determines and Limits an Atom’s Mass?

What Do You Think?In Activity 4 you learned that the structure of an atom includes a nucleus surrounded by electrons. Most of the mass of an atom is concentrated in the small nucleus that has a positive electriccharge equal in magnitude to the negative charge of all theelectrons surrounding the nucleus.

• What do you think makes up the nucleus of the atom?

Record your ideas about this question in your Active Chemistrylog. Be prepared to discuss your responses with your smallgroup and the class.

InvestigatePart A: What’s in the Nucleus?1. Atomic mass is the average mass of atoms of each element.

Atomic number indicates the number of electrons in the atomand the number of protons located in the nucleus needed toproduce an electrically neutral atom. Refer to the periodictable to answer the following questions:


GOALSIn this activity you will:

• Investigate the compositionof the atom's nucleus.

• Explain why the atomicmasses of some elements arenot whole numbers.

• Use symbols to representdifferent isotopes of anelement.

• Determine the compositionof the nucleus of an atomfrom its isotope symbol.

• Calculate the average atomicmass of an element from thepercent abundance of itsisotopes.

CS_Ch7_PeriodicTbl 2/28/05 10:05 AM Page 449

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a) How many protons are there in ahydrogen atom?

b) To the nearest whole number,what is the atomic mass of ahydrogen atom?

c) How many protons are there in ahelium atom?

d) Since the mass of an electron isnegligible, compared to thenucleus, what would you expectthe atomic mass of a helium atomto be? Explain your answer.

e) To the nearest whole number, whatis the atomic mass of a heliumatom?

2. In Step 1, you found that the heliumatom has a mass that is four timesthe mass of a hydrogen atom, whilethe electric charge on the heliumnucleus is only twice that of thehydrogen atom. This suggests thepresence of another particle in thenucleus, with about the same mass asthe proton but no electric charge.This particle is called a neutron.

Sample:Boron has atomic number 5. Thisinforms you that there are 5 electronsand that the nucleus contains 5 protons. The average atomic massof boron is 10.811 atomic massunits. Most boron has 11 atomicmass units and some has 10 atommass units. Since the mass is the sumof the protons and the neutrons(electrons have very, very little mass)then you can conclude that mostboron nuclides have 5 protons and 6 neutrons in the nucleus.

Refer to your table of atomicnumbers and atomic masses toanswer the following questions:

a) How many protons would youexpect to find in the nucleus of ahelium atom? (Recall that thenumber of protons needs tobalance the number of electrons.)

b) How many neutrons would youexpect to find? (The atomic massis a combination of the mass ofthe protons and the mass of theneutrons.)

c) How many protons and neutronswould you expect to find in thenucleus of an atom of each of thefollowing elements?

• lithium • beryllium• boron • carbon• nitrogen • oxygen• fluorine • neon

3. Refer again to the periodic table.

a) What are the atomic masses ofmagnesium and chlorine? Whatare the atomic masses of sodiumand fluorine? Which set is closerto whole numbers?

b) We expect protons and neutronsto exist in whole numbers. Youcannot have part of a proton inthe nucleus. What would youexpect the atomic masses of mostmagnesium, chlorine, sodium, and fluorine atoms to be? Explain your answer.

4. The fact that some atomic masses arenot close to whole number multiplesof the atomic mass of hydrogen isnow explained by the fact that thenumber of neutrons is not the samein all atoms of a given element. Onlythe number of protons, the atomicnumber, is the same in all atoms of a




given element. Atoms of the sameelement with different number ofneutrons in the nucleus are known asisotopes (meaning “same number ofprotons”). Isotopes are identified bytheir mass number, the sum of thenumber of neutrons plus protons.

Sample:

Lithium has an atomic number of 3 and an average atomic mass of6.941. All lithium atoms have 3protons in the nucleus. A neutralatom of lithium always has 3electrons to balance the charge of thethree protons. The average atomicmass of a lithium atom is 6.941atomic mass units, indicating thatsome lithium atoms have 3 neutrons,to make a total atomic mass of 6 andother lithium atoms have 4 neutrons,to make a total atomic mass of 7.These 2 isotopes are designatedlithium-6 and lithium-7. Since thereare so many more lithium-7 atoms,the average of all of the atoms is veryclose to 7.

Refer to your list of atomic masses toanswer the following questions:

a) What isotopes (as indicated bytheir mass numbers)do you expect toaccount for theknown atomic massesof the followingelements?

• carbon (carbon-12 atoms with 6 neutrons andcarbon-13 atomswith 7 neutrons;more carbon-12 atoms)

• hydrogen • beryllium• boron • sodium• magnesium

b) In the notation below, the massnumber is written at the upper leftof the chemical symbol of theelement. The atomic number iswritten at the lower left of thechemical symbol of the element.How many neutrons and protonsare present in the followingisotopes?

i) 32He and 42He

ii) 63Li and 73 Li

iii) 126 C and 13

6 Civ) 14

7 N and 157 N

Part B: Forces within the Atom1. There are two very different forces

acting on the electrons, protons, andneutrons in the atom. In order tobetter understand the atom, youmust first understand these forces.

Cut two strips of transparent tapeabout 12 cm long. Bend one end ofeach strip under to form a tab. Placeone strip sticky-side down on a tableand label the tab “B,” for “bottom.”Place the other strip sticky-side down


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on top of the first strip and label thetab “T,” for “top.”

Peel off the top strip, using the tab,with one hand and then pick up thebottom strip with the other hand.Hold both strips apart, allowingthem to hang down.

Slowly bring the hanging stripstoward each other, but do not letthem touch.

a) Record your observations.

b) If the strips accelerated toward oraway from each other, Newton’sSecond Law tells you that theremust be a force. Is the forcebetween the two strips of tapeattractive or repulsive?

2. Make a second set of strips as in Step 1.

a) Predict what you think willhappen if the two top strips arepicked up, one from each set andbrought toward each other.Record your prediction in yourActive Chemistry log.

Pick up the two top strips by thetabs, allowing both strips to hangdown. Slowly bring them towardeach other.

b) Record your observations.

c) Was the force attractive orrepulsive? Explain.

d) Predict what you think willhappen if the two bottom strips oftape are picked up and broughttoward each other. Record yourprediction.

Pick up the two bottom strips bythe tabs, allowing both strips to

hang down. Slowly bring themtoward each other.

e) Record your observations.

f) Was the force attractive orrepulsive? Explain.

3. The two different strips of tape havedifferent charges. The top strips havea positive electric charge. They havelost some of their electrons. Since the number of protons has remainedthe same, the strips are positive. Thebottom strips have a negative charge.The bottom strips have gained someelectrons. Since the number ofprotons has remained the same, thestrips are negative. The force betweenthe strips is called the electric force.

a) Is the force between two positivestrips repulsive or attractive? Useevidence to justify your answer.

b) Is the force between two negativestrips repulsive or attractive? Useevidence to justify your answer.

c) When a positive and a negativestrip come near each other, is theforce attractive or repulsive?Justify your answer.

4. The nucleus has a positive charge dueto all of the protons there. Theelectrons surrounding the nucleushave negative charges.

a) What kind of electric force(attractive or repulsive) existsbetween the nucleus of an atomand any one of the atom’selectrons?

b) What kind of electric force(attraction or repulsion) existsbetween pairs of protons in thenucleus?




5. The nucleus is a very crowded place.The protons in the nucleus are veryclose to one another. If these protonsare repelling each other by anelectrostatic force (and they are!),there must be another force, anattractive force, that keeps them there.The attractive force is the nuclearforce, also called the strong force.This force is much stronger than theelectric force. It acts between pairs ofprotons, pairs of neutrons, andprotons and neutrons. The electron isnot affected by the nuclear force.

a) Copy and complete the tablebelow in your Active Chemistrylog. The first row has beencompleted for you.

6. If the nucleus were too large, theprotons on one side of the nucleusare too far away to attract theprotons on the other side of thenucleus. The protons can still repelone another since the coulombelectrostatic force is long-range. Therepulsive electrostatic force wins andthe nucleus won’t form.

A large nucleus will break apartwhen the electrostatic repulsionbetween the protons is too great.

The repulsion pushes the fragmentsof the nucleus apart, releasing a greatamount of energy. This process ofsplitting an atom into smaller atomsis called fission. It occurs in uraniumwhen an additional neutron is addedand causes instability.

One example of the fission processcan be represented as follows:

23592 U � 1

0n → 9436 Kr � 139

56 Ba � 3 10n

� energy

a) Is the mass number conserved onboth sides of the reaction? What isthe total mass number on eachside?

b) Is the atomic number conservedon both sides of the reaction?What is the total atomic numberon each side?

c) Why does the neutron have a massnumber of 1?

d) Why is the atomic number of aneutron equal to 0?

Small nuclei can also combine toform a larger nucleus and releaseenergy. This process is calledfusion.

electron-proton attractive noneelectron-neutronproton-protonproton-neutronneutron-neutron

Particles Coulomb Strong, electrostatic force nuclear force


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THE NUCLEUS OF AN ATOM

Discovery of the Neutron

The average atomic masses of some elements were known inMendeleev’s time, even though scientists didn’t know much about theactual structure of an atom. In Part A of this activity you exploredthe idea of how the atomic mass relates to the atomic number.Mendeleev began organizing his periodic table by listing all the knownelements in order of atomic mass. However, he found that organizingthe elements in this way did not always make sense in terms of thebehavior of the elements. He concluded that his measurements ofatomic mass were incorrect and in those situations used theproperties of the elements to place them in the table.

As it turned out, Mendeleev’s measurements were not necessarilyflawed.Although early models of the nucleus included the proton, theproton alone could not account for the fact that the mass of a heliumatom is four times the mass of a hydrogen atom while the electriccharge on the helium nucleus is only twice that of the hydrogen atom.Lord Rutherford (after discovering that atoms had a nucleus)addressed this problem when he suggested that another particle waspresent in the nucleus, with about the same mass as the proton butno electric charge. He named this particle the neutron.

The neutron was actually discovered in 1932 (by Chadwick, a Britishphysicist), adding a great deal to the understanding of the nucleus ofthe atom.This discovery did not solve all of the mysteries concerningthe atomic masses of some elements. Scientists today refer to protonsand neutrons as nucleons since they reside in the nucleus and arealmost identical in mass.The mass number tells us the number of nucleons.

Isotopes

In Part A of this activity you also investigated why the atomic mass ofan element is not a whole number. Not all atoms of a given elementhave the same number of neutrons in the nucleus. Only the number ofprotons, the atomic number, is the same in all atoms of a givenelement.Atoms of the same element with different number of

Chem Wordsproton: a positivelycharged subatomicparticle contained in the nucleus of an atom.The mass of a proton is1.673 � 10–24g and has acharge of �1.

neutron: neutralsubatomic particle with a mass of 1.675 � 10–24glocated in the nuclei ofthe atom.




neutrons in the nucleus are known as isotopes (meaning “samenumber of protons”). Isotopes are identified by their mass number, thesum of the number of neutrons plus protons.

You can refer to an element by its name (chlorine), by its atomicsymbol (Cl), or by its atomic number (17).All three identifications areequivalent and used interchangeably in chemistry.The same elementcan have a different number of neutrons in the nucleus. Chlorine,which must have 17 protons in the nucleus, can have 18 or 20neutrons. Chlorine with 20 neutrons and chlorine with 18 neutronsare the isotopes of chlorine (35Cl and 37Cl ).

Electrostatic and Nuclear Forces

In Part B of this activity, when you brought the two positive stripsnear each other, they experienced a repulsive force.This was true fortwo negative strips as well.When a positive and a negative strip werebrought close together, the force was attractive.As you have heard,“opposites attract!”

Inside the nucleus, the protons arerepelling one another. Every pair of protonshas a repulsive force between them.Theforce is very large because the distanceswithin the nucleus are very small.Thenucleus is between 10,000 and 100,000times smaller than the atom.The electricalforce can be described mathematically.

F � �kq

d12

q2�

where F is the force,

k is Coulomb’s constant (a number � 9 � 109 N m2/C2),

q1 and q2 are the charges, and d is the distance between the charges.

As the distance between the charges increases the force weakens.Since the distance in the denominator is squared, if the distance triplesthe electrical force is 9 times (32) weaker or one-ninth as strong.

Chem Wordsisotope: atoms ofthe same elementbut different atomicmasses due todifferent number of neutrons.


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The question then becomes, what holds the protons together in the nucleus? The protons do have an electrical force pushing themapart but they have the larger nuclear force holding them together.The nuclear force is strong at short range.Anywhere beyond adistance of approximately 10�14 m (that’s less than one 10-millionth of one 10-millionth of a meter), the nuclear force is zero. Neutrons in the nucleus are also attracted to each other and to protons withthe nuclear force. Electrons are not affected by the nuclear force.Electrons belong to a different class of particles than protons andneutrons and do not interact with the strong nuclear force.

The nucleus is held together by a new force—the strong nuclearforce.The nuclear force:

• is very, very strong at small distances;• acts only between nucleons (proton-proton, proton-neutron,

neutron-neutron);• is always attractive;• is very short range (if nucleons are more than 10�14 m apart,

the nuclear force is zero).The atom is held together by the electrostatic coulomb force.Theelectrostatic force:

• is strong at small distances, weak at large distances;• acts only between charged particles (proton-proton, electron-

electron, proton-electron);• is attractive or repulsive;• is long range (the force gets weaker at large distances).

All the nucleons are attracted by the nuclear force.The electrostaticforce repelling protons in the nucleus is overwhelmed by the attractivenuclear force between these protons.

Unstable Atoms

You might expect to find nuclei of atoms with all sorts ofcombinations of neutrons and protons.Yet the quantity of isotopes foreach element is rather small, and the number of elements is alsolimited. Moreover, elements do not occur in nature with atomicnumber greater than 92, and the highest atomic number for an atomcreated in the laboratory is 117.




There are two stable masses of chlorine, chlorine-35 and chlorine-37.The key word in this statement is “stable.” There are other isotopes ofchlorine, both heavier and lighter than chlorine-35 and chlorine-37, butthey are not stable.The unstable isotopes can convert to a more stablecombination of neutrons and protons, and they do so according to asystematic pattern in time.These other isotopes of chlorine are said tobe radioactive. Understanding why certain elements are radioactiverequires a deeper understanding of the structure of the nucleus.Scientists are still trying to fully understand stability of the elements.

If the nucleus of an atom is too large, the protons on one side of thenucleus are too far away to attract the protons on the other side ofthe nucleus.The protons can still repel one another since the coulombelectrostatic force is long-range.The interaction between the repulsiveelectrostatic force and the attractive nuclear force is one determiningfactor on the maximum size of a nucleus.

The stability of an atom varies with the elements. Light elements becomemore stable as the atomic mass (the number of nucleons) increases.Themost stable element is iron (atomic number 26) with an atomic mass of56. Elements with larger atomic masses become less stable.

In general, elements with nuclear mass much, much less than 56 cancombine to gain mass, become more stable, and give off energy. Thisprocess is called fusion. Elements with nuclear mass much, muchgreater than 56 can break apart to lose mass, become more stable,and give off energy.This process is called fission.

Fusion is the process of small nuclei combining to increase their mass.The best example of fusion processes is what occurs in the Sun andother stars.The fusion process is ideal for supplying safe energybecause it releases very large amounts of energy without leaving muchdangerous radioactive residue. However, it is very difficult toaccomplish this on an industrial level at the present time. In the futurewe hope scientists will figure out how to harness the energy ofnuclear fusion, because it would be an excellent source of energy for society.

The process of splitting an atom into smaller atoms is called fission.This is the process that is used to produce nuclear energy. It is usedto power nuclear submarines and to produce electrical energy innuclear power plants all over the world.

Chem Wordsradioactive: an atomthat has an unstablenuclei and will emitalpha, positron, orbeta particles inorder to achievemore stable nuclei.

fusion: nuclei oflighter atomscombining to formnuclei with greatermass and release ofa large amount ofenergy.

fission: the processof breaking apartnuclei into smallernuclei and with therelease of a largeamount of energy.


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The use of nuclear energy for the production of electricity is quiteapparent as you look at the numerous states that depend on nuclearenergy. For example, over 40% of Illinois’ electricity is produced bynuclear energy. Nuclear fission does create some major problems:(1) Security, (2) Radiation, (3) Removal of spent rods, and (4) Disposalof waste. With these problems, there is a need for continuedresearch. Numerous universities and government facilities are tryingto improve the efficiency of nuclear fission and at the same time,trying to develop nuclear fusion for commercial use.

This ongoing research is expensive and depends on the government,industry, and other organizations to continue supporting thisresearch. If we can learn how to harness nuclear fusion we canalleviate our nation’s electrical problems while decreasing pollution.The field of nuclear science is going to continue to grow and thefuture will provide great opportunities for a young scientist like youto get involved.

1. Explain the differencebetween atomicmass and atomicnumber.

2. What two forces areat work in thenucleus of an atom?Explain how eachworks.

3. What is an isotope?

4. Why are someisotopes unstable?

5. Construct a table ordiagram to compareand contrast thenuclear processes offission and fusion.

Checking Up




Reflecting on the Activity and the Challenge

In Part A of this activity you learnedthat the mass of an atom, concentratedin the nucleus, is due to two types ofparticles, the proton and the neutron.Elements are identified by their atomicnumber, the number of protons in thenucleus. The atomic mass, the averagemass of an atom of a given element,listed on the periodic table is areflection of the variety of isotopes of agiven element that exist. How will youincorporate your expandedunderstanding of the contents of anatom’s nucleus, average atomic massesand isotopes into your game about theperiodic table?

In Part B of this activity you alsolearned that only some combinations of

neutrons and protons in a nucleus arestable, depending on the balancebetween the strong force holding thenuclear particles together and theelectric force pushing them apart. Thenuclear force is a short-range force.Beyond a distance of approximately10�14 m, the nuclear force has nostrength. Within that distance, this forcebetween protons and protons, protonsand neutrons, and neutrons andneutrons is quite strong. Recognizingthe interplay between the electric forcein the nucleus and the strong, attractivenuclear force provides an insight intothe size of nuclei and the maximum sizeof a nucleus. These insights can beincorporated into your periodic tablegame in a creative way.

1. If lithium loses an electron to become Li+, what is the average atomic mass of thelithium ion? Explain how you arrived at your answer.

2. Hydrogen has 3 isotopes with mass numbers of 1, 2, and 3. Write the completechemical symbol for each isotope.

3. Give the complete chemical symbol for the element that contains 16 protons, 16 electrons, and 17 neutrons.

4. Complete the table below: (Use the periodic table.)

Chemical symbol 3919 K

Atomic number 9Number of protons 15Number of electrons 53Number of neutrons 10 16Atomic mass 127

5. Neutrons can be used to bombard the nucleus of an atom like uranium. Whywould it be more difficult to inject the nucleus of uranium with a proton?


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6. Complete the following reaction: 23592 U � 1

0 n → 9438 Sr � _____ � 2 1

0 n

7. Radon is a threat to the well-being of people in their homes because it emitsradioactive particles at a significant rate. Complete the following radioactivedecay equation: 22286 Rn → 218

84 Po � _____

8. Explain why a helium atom is able to exist. What keeps the 2 electrons, 2 protons, and 2 neutrons together?

Inquiring Further

Calculating average atomic mass

If you know the percentages ofabundance for the isotopes of achemical element and the known massesof those isotopes, you can calculate theaverage atomic mass of that element.The process is similar to calculating theaverage age of students in your class —add up each person’s age and divide bythe number of students in your class.However, if you had to average the ageof all of the students in your highschool, you might choose another route.It would be easier to find out howmany students are fourteen, how manyare fifteen, and so on. Then you couldmultiply the number of students in eachage group by that age. Then you wouldadd these subsets together and divide bythe total number of students.

A similar process is used to average themasses of different isotopes of anelement. Consider the element chlorine.There are two stable isotopes ofchlorine, chlorine-35 and chlorine-37. Ofall the chlorine atoms on Earth, 75.77%of them are the isotope chlorine-35, eachhaving a mass of 34.96885. The other24.23% of stable chlorine atoms are theisotope chlorine-37, each having a massof 36.96590. This means that 75.77 out

of 100 chlorine atoms have a mass of34.96885 and 24.23 have a mass of36.96590. To find the average mass, thenumber of each isotope is multiplied bythat isotope’s mass. Then the productsare added together. The sum is dividedby 100, since the information pertainedto 100 chlorine atoms. The result is anaverage atomic mass of 35.45 forchlorine, the same value stated in theperiodic table. The math is shown below:

Chlorine-35 34.96885 � 75.77 � 2649.6

Chlorine-37 36.96590 � 24.23 � 895.73545.3 ÷ 100 � 35.453

Magnesium, another isotope youinvestigated, has three stable isotopes as follows:

mass isotopic % number mass abundance

24 23.98504 78.99

25 24.98594 10.00

26 25.98259 11.01

Calculate the average atomic mass formagnesium. Describe how you arrivedat your answer. You may use theprocess described above or challengeyourself to develop your own process.


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Activity 9 What Determines and Limits an Atom's Mass?