UNIT 3WHAT IS THE BASIC STRUCTURE OF THE ATOM? (pp 52 - 53)

Atomic Theory

1. The idea that matter is made from atoms can be traced to the early Greek philosophers.

2. The first theories were explanations based on their own life experiences so they concluded that matterwas composed of air, earth, fire, and water.

3. One philosopher, DEn'10Ce.lil)J , believed otherwise and proposed that matter was composedof tiny particles called atomos (atoms). His ideas were:

- Matter is composed of empty space through which atoms move.

- Atoms are solid, homogeneous, indestructible and indivisible.

- Different kinds of atoms have different sizes and shapes.

- The differing properties of matter are due to the size, shape and movement of atoms.

- Apparent changes in matter result from changes in the groupings of atoms and not from changes in theatoms themselves.

4. Unfortunately, Aristotle did not agree. Since he was more influential the ideas of Democritus werediscarded and the air, earth, fire and water theory lasted much longer.

Evidence Supporting Atomic Theory

I. Observations of compounds and chemical reactions led to three laws that describe how compounds areformed.

:DE'FI~r~2. U~~J Cl= COfJrVtS 17K).") states that a compound contains the same elements in exactly the sameproportions by mass regardless of the size of the sample or source of the compound.

C lc 1-IIl.C\c

CO,>J.St:IV/A flO,,J 0 I::

3. L0,t.,v OJ;:: rvv·t~S states that when two or more elements react to produce a compound, thetotal mass of the compound is the same as the sum of the masses of the individual elements.

3 (S) -\ 0,,1 ~9) --7 5D;; 19)

Il(()Ln ,:J l!;4. U'l,J Of.=: fR,(J/bR..i7L~Jj applies to different compounds made from the same elements. The massratio for one of the elements that combines with a fixed mass of the other element can be expressed in smallwhole numbers.

H:Ocd '. I

If._ 0,-H',O

OVER

.John Dalton

1. Showed that, collectively, the laws demonstrate the existence of atoms. He argued that these laws couldnot hold true if atoms did not exist.

2. This led to the development of Dalton's Atomic Theory:

- all matter is made up of indivisible and indestructible atoms.

- atoms of a given element are identical in their physical and chemical properties.

- atoms of different elements have different physical and chemical properties.

- atoms of different elements combine in simple, whole number ratios to form chemical compounds.

- atoms cannot be subdivided, created or destroyed when they are combined, separated or rearranged inchemical reactions.

THE STRUCTURE OF THE ATOM (pp. 55 - 57)

Electrons

I. Scientists studying electrical current discovered that current would flow from the negative cathode to thepositive cathode in a tube that was a near vacuum.

2. The remaining gas would glow and the beam was capable of moving a small paddle wheel.

3. This suggested the cathode ray consists of small individual particles.

4. J.J. Thomson studied the cathode ray. He discovered magnetic and electrical fields could deflect therays. By carefully measuring the effect of the fields, Thomson calculated the CHfl'l.0E - TV~ 1/f1,q.~iSIl.q16¥the charged particles.

5. Thomson compared his results with other known information and found that his particles were smallerthan an atom. He had identified the first subatomic particle: the ELECn2().J

6. In 1909, Robert Millikan conducted an experiment, known as the () I L.. D!WP /E,./)Qe,irlll::7VT . Thisexperiment determined the actual charge on the electron. This was set at .- I

7. He then used this information and Thomson's work to calculate the actual mass of an electron.~ lY

nVI SS O~ e ~ o, JJ 'J.. 10 aa-

8. Since matter is neutrally charged, atoms must be neutral. But if all atoms contain electrons, how couldthis be?

9. Thomson theorized that the atom was a ball of positive charge with the negatively charged electronsembedded in the sides. This model became known as the PL1j (]r) PUDDIJoJG 1.(00&:'1-.

Nucleus

I. In 1911, Ernest Rutherford and a group of scientists set out to study how alpha particles would interactwith solid matter. Alpha particles are radioactive and consist of the nucleus of the helium atom. Therefore,the particle has a positive charge.

2. They decided to fire a beam of alpha particles at a thin sheet of gold foil. A screen coated with zincsulfide surrounded the foil. The thought was that the particles would cause a flash of light on the screenwhen they struck it.

3. Rutherford, knowing about Thomson's model, thought the particles would be deflected slightly, ifat all.

4. He discovered that some of the particles had very large deflections. Some even came straight backtoward the source of the alpha particles.

5. Because of this he concluded the plum pudding model was incorrect and set out to develop a newmodel.

6. Rutherford's model states the atom is mostly empty space through which the electrons travel. Heconcluded there is a tiny, dense region of positive charge, which he called the rJvCLt:.::",S . Theelectrons travel around the nucleus.

7. Rutherford also concluded that most of the atom's mass resides in the nucleus.

Protons and Neutrons

I. Rutherford refined his model and concluded the nucleus contained positively charged particles calledPt.;; '11f,",j,s

2. The proton's charge is equal to, but opposite that of an electron.

3. Scientists still had a dilemma because the mass of the known elements, except hydrogen, had beenfound to be heavier than that predicted by the protons and electrons.

4. In I932, James Chadwick showed that the nucleus contained another particle. This particle had thesame mass as the proton, but had no charge. It was called a oI/El//lloA/

PROPERTIES OF SUBATOMIC PARTICLES

PARTICLE SYMBOL LOCATION RELATIVE RELATIVE ACTUALCHARGE MASS MASS (2:)

electron e - t'(i..~1. ,_\ .,.. , _L. -u,\J(,'CU;V-'s /3'1/0 q.If'f.If)

proton (J -i- {'JIX ..l8J..I +1 I'. ,-,l.'1

(, fc 7 '00neutron o ,\) v Cl!:l.JS cI I

~2.'1n I, It; 7 't./o

HOW ATOMS DIFFER

Atomic Number

l. Not long after Rutherford's gold foil experiment, one of his fellow scientists beamed X rays at samplesof the elements and discovered that the wavelength of the resultant X rays decreased with increasing atomicweight.

2. This scientist, Henry Moseley, concluded that the number of electrons around the nucleus must havebeen increasing. If this were so, then the number of protons in the nucleus must also increase in order tomaintain a neutral charge.

3. Atoms of different elements contain a unique positive charge and therefore a unique number of protonsin the nucleus.

OVER

4. The number of protons in the nucleus is referred to as the A TDml C tJ u mBE)t

5. If one knows the atomic number, one also knows the number of electrons present since the atom isneutral.

Isotopes and Mass Number (pp. 60 - 63)

1. While the number of protons in an atom is fixed, the number of neutrons in the nucleus can vary. Evenfor the same element.

2. Atoms of the same element that have the same umber of protons, but different neutrons are called.ISOTO PES

3. Despite this, isotopes still have very similar chemical properties.

4. To make it easier to identify each of the various isotopes of an element, chemists add a number aftereach's name. The number is called the MASS tJ\,1()1tlb7l

fi,~I\SS AlVfV/,(j(:;71:= # ,4e1.Jf7),.,J.S -+ # ;Jf=lJ77Z0dS

***** Determine the mass number for the isotope of potassium containing 19 protons and 20 neutrons.Write its name.

***** Determine the mass number of the isotope of mercury containing 80 protons and 124 neutrons.Write its name.

tv\ A s.s J:t z: 80 -+ I ;;llf ;; ao If

Th' b I' k "....sO·TOm~ 3Ynl8bL;IS sym 0 IS nown as an .s: "-_'{_I... _

':0(\"""""h".,\SS tt

5. An isotope can also be represented by a symbol.It looks like:***** Write the isotopic symbol for each of the following:

uranium-238

iron-57 57 r:J7 re

ca\Cium-46

***** One of the most harmful components of nuclear waste is a radioactive isotope ofstrontium-90. Itcan be deposited in your bones where it replaces the calcium. Write the isotopic symbol for strontium-90.How many protons are in the nucleus? How many neutrons?

p,eCTOJJ-S ,,; 3 i qo .s. q ~(' S ;18 r.08)~1--J5 z: (J - vcr-'=' ;>

Mass of an Atom

I. Because the actual mass of a proton, neutron and electron are so small, the actual mass of an atom is avery small number.

2. These number are hard to work with so chemists developed a method of measuring the mass of an atomrelative to the mass of a specifically chosen atomic standard.

3. The standard is the O.J.~otJ -/3.exactly 12 atomic mass units.

4. One f\TVIl~1 ( n'Y-\S.s UN lr (all1uJs equal to

atom. Scientists assigned the carbon-12 atom a mass of

.LI d the mass of the carbon-12 atom.

5. The atomic masses of the elements are listed on the periodic table, but they are not whole numbers.

6. This is because the ATlln1lt W/-4SJ. .of an element is the weighted average mass of that element.I<.8-ADVt:

7. Weighted averages depend on two factors: the VI'1 As.s and the A~\J.uMI\.I CE of each

isotope: (n-V\>1) ( ~tL. (~S(} •.J) + (iW\$5) eR.a. ~'~IiN).1 •••

A'T1)i1'liC n114SS -= It) 0

***** Calculate the average atomic mass for silicon. Silicon has three naturally occurring isotopes:

silicon-28silicon-29silicon-30

27.98 amu28/98 amu29.97 amu

92.21%4.700%3.090%

-.-.... ,~--.. ---~-( OJ y, 17) (a. (I 9t;!)

lOG

***** Calculate the atomic mass of oxygen. Oxygen has three naturally occurring isotopes:

oxygen-l 6oxygen-I 7oxygen-I 8

15.99 amu17.00 amu18.00 amu

99.76%0.03800%0.2000%

o = /00

IS,qCjaI'Vlti

OVER

CALCULA nONS AND THE RULES FOR SIGNIFICANT FIGURES

I. Each measured quantity has some degree of error in it. When measurements are added, subtracted,multiplied and divided small error may either cancel each other or add up to a larger error.

2. To avoid the possibility of adding extra error, rules for significant figures are used in calculations.The rules for addition and subtractions are different from the rules for multiplication and division.

3. When adding or subtracting: The answer can have no more digits to the right of the decimal pointthan there are in the measurement with the smallest number of digits to the right of the decimal point.

***** Add the following number together and round the answer to the proper number of significantfigures:

3.95+2.879+213.6: &;)0,'-/;)1 .::

83-2004 ;;: Vd,{P z; &3

4. When multiplying or dividing: The answer can have no more significant figures than there are inthe measurement with the smallest number of significant figures.

* * * * * Perform the following calculations and express the answer in the proper number of significantfigures: .

(l2.257)x(1.162) ~ IlL ;Jl(,J(P z:

(5.61) x (7.891)

[(0.871)x(0.23)]+(5.871)- O.() 3lf/&IQSSq s:o,03V

***** Perform the following calculations and express the answer in the correct number of significantfigures.

37.4+2.7+0.0015 -: <(t:,fbIS'= 1./0"

256.3-137+2+10.11 ;0 ,3/,'-1'.:: /3/

129+29.2 :-: tl,Yn.fof~{q .: '/. V:)

[(257) x(3.1)] +(547) s: f, !/S~I/RC;qI/J: I. S

[(12.4) x (7.943)] + .0064

[(246.83) + (26.3)] - 1.349 :: 3"; 0 30 2....:: ,p-. C If

SCIENTIFIC NOTATION

1. Sometimes known as exponential notation.

2. This system helps us avoid the uncertainty of whether zeros at the end of a number are significant.

3. The exponent tells us how many decimal places to move the decimal point. The sign (+) or (-) tellsus whether to move the decimal point to the left or the right.

4. Scientific notation is a method for making very large or very small numbers more compact andeasier to write.

5. It expresses a number as a :-,1,-~2=:,O,-::'D::-l_~C..:...·-=-' ofa number between __ 1_4--,--"--'_'.::.__ 1_0 _and the appropriate "(0 j,~t:;1l.. of 10

***** 7q, ;3 X {O

6. When the decimal point is moved to the Ll.:;.i=='T , the power often is PO.:iITlifl:

***** (), 0001'<>7 :::

7. When the decimal point is moved ill 0 HT , the power often is tJ. E6177~/t7

***** Express the following numbers to three significant figures in scientific notation:, ~ ~

3,500,000 3, SO'/. (0 0.00134 /• .Jl( Y 10

0.00055-Ifs . so '/..10 4003

10,300

8. To convert a number expressed in scientific notation into its normal expanded form simply movethe decimal point the appropriate number of spaces in the direction indicated by the sign on the poweroften.

***** Express the following numbers in their normal expanded form.

2.38 X 105

1.5 X 10-3 C). 0010

OVER

.....-.---~--... ... _----_ ... _------ .- ..-------~ ----

9. To add or subtract numbers expressed in scientific notation, the exponents must be the same:5 .3

;J,OO ""0 -+ 3.oo~IO. .:3 :3

::; .;J ~ o . 0 ...,.. I 0 -+ .3. 00 Y.. I 0 ~3

010 a. BO 'f... (0

10. When multiplying numbers expressed in scientific notation, carry out the operation and then addthe exponents together. .3

(3,O'j;105)(4,O"'f.IO-'2..)-:. ,~ 1-.,0 z: /.~"'IOIf

-,),) ( 4)(I.O~~'O J.b? "f...fO ::

Q)

.:J. } L/ X 10

11. When dividing, carry out the operation and then subtract the exponent in the denominator from theexponent in the numerator.

,/ --if1?,O'IC(O-~ &'.O'f...IO -l/_(-l.}= -2 :. ';;,0)",10;."",'0-2- f,foYIO

--

12. When raising a number expressed in scientific notation to a power, carry out the operation, thenmultiply the exponent by the power.

-~ ~ . -~( ,?, 0 -.,../0) z: (o~ 'f.. 10 ::-1

0.l(X/O

3) 3(q. (03 ,'" fO =

7qq, cl S OlJCf7 X 10

= qq,,3 '/.10 i

10