Atom theory

8/7/2019 Atom theory

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Early Atomic Theoryand Structure

Chapter 5

Hein * Best * Pattison * Arena

Eugene Passer Chemistry DepartmentBronx Community College

John Wiley and Sons, Inc

Version 1.0 2

Empedocles ( pronunciation: [em ped okleez] )stated that matter was made of 4elements: earth, air, fire, water.

Democritus ( pronunciation: [de mok ritus])(about 470-370 B.C.) thought that allforms of matter were divisible into tinyindivisible particles. He called thematoms from the Greek atomosindivisible.

The earliest models of the atomwere developed by the ancientGreek philosophers.

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Aristotle (384-322 B.C.) rejected thetheory of Democritus and advancedthe Empedoclean theory

Aristotles influence dominated thethinking of scientists andphilosophers until the beginning of the 17 th century

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Daltons Modelof the Atom

2000 years after Aristotle, John Dalton,an English schoolmaster, proposed hismodel of the atomwhich was based onexperimentation.

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2. Atoms of the same element are alike in mass and size.

3. Atoms of different elements have different masses andsizes.4. Chemical compounds are formed by the union of two or

more atoms of different elements.

Daltons Atomic Theory (1810)

1. Elements are composed of minute indivisible particles calledatoms.

5. Atoms combine to form compounds in simple numericalratios,such as one to one, two to two, two to three, and so on.

6. Atoms of two elements may combine in different ratios to formmore than one compound.

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Changes to the theory

Atoms are composed of subatomic particles electron, neutron, proton

Not all atoms of the same element have thesame mass - carbon 12, carbon 13, carbon 14

Atoms can be decomposed - fission


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Charge may be of two types: positive and negative. Unlike charges attract (positive attracts negative),

and like charges repel (negative repels negative andpositive repels positive). Charge may be transferred from one object to

another, by contact or induction. The less the distance between two charges, the

greater the force of attraction between unlikecharges (or repulsion between identical charges).

Properties of Electric Charge

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kq qF =

r

q1 and q 2 are charges, r is thedistance between charges andk is a constant.

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Discovery of Ions

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Michael Faraday discovered thatcertain substances, when dissolved inwater, conducted an electric current.

He found that atoms of some elementsmoved to the cathode (negativeelectrode) and some moved to theanode (positive electrode).

He concluded they were electricallycharged and called them ions (Greek

wanderer ).16

The anode of an electrolytic cell is positive (cathode is negative), sincethe anode attracts anions from the solution. However, the anode of agalvanic cell is negatively charged, since the spontaneous oxidation at

the anode is the source of the cell's electrons or negative charge. Thecathode of a galvanic cell is its positive terminal.

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Svante Arrhenius reasoned that an ionis an atom (or a group of atoms)carrying a positive or negative electriccharge.

Arrhenius accounted for the electricalconduction of molten sodium chloride(NaCl) by proposing that melted NaCldissociated into the charged ions Na +and Cl -.

NaCl Na + + Cl -

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In the melt the positive Na + ions movedto the cathode (negative electrode).Thus positive ions are called cations .

In the melt the negative Cl - ions movedto the anode (positive electrode). Thusnegative ions are called anions .

NaCl Na + + Cl -


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Subatomic Parts

of the AtomElectron, proton, neutron

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An atom is very Small

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In 1875 Sir William Crookes inventedthe Crookes tube.

Crookes tubes experiments led the wayto an understanding of the subatomicstructure of the atom.

Crookes tube emissions are calledcathode rays .

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In 1897 Sir Joseph Thompson demonstratedthat cathode rays:

travel in straight lines.

are negative in charge.

are deflected by electric andmagnetic fields.

are capable of moving a smallpaddle wheel.

The electron, 9.110 X 10 -28 g, negative charge

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Eugen Goldstein, a German physicist,first observed protons in 1886:

Thompson determined the protonscharacteristics.

Thompson showed that atoms containedboth positive and negative charges.

This disproved the Dalton model of theatom which held that atoms wereindivisible.

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James Chadwick discovered the neutronin 1932.

Its actual mass is slightly greater thanthe mass of a proton.


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Ions

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Positive ions were explained byassuming that a neutral atom loseselectrons.

Negative ions were explained byassuming that extra electrons can beadded to atoms.

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5.4

When one or more electrons are lostfrom an atom, a cation is formed.

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5.4

When one or more electrons are addedto a neutral atom, an anion is formed. 30

The Nuclear Atom


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Radioactivity was discovered by Becquerelin 1896.

Radioactive elements spontaneously emitalpha particles, beta particles and gammarays from their nuclei.

By 1907 Rutherford found that alphaparticles emitted by certain radioactiveelements were helium nuclei.

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The Rutherford Experiment

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Rutherford in 1911 performed experimentsthat shot a stream of alpha particles at agold foil.

Most of the alpha particles passed throughthe foil with little or no deflection.

He found that a few were deflected at largeangles and some alpha particles evenbounced back.

34Rutherfords alpha particle scattering experiment.

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An electron with a mass of 1/1837 amucould not have deflected an alphaparticle with a mass of 4 amu.

Rutherford knew that like chargesrepel.

Rutherford concluded that each goldatom contained a positively chargedmass that occupied a tiny volume. Hecalled this mass the nucleus .

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If a positive alpha particle approachedclose enough to the positive mass itwas deflected.

Most of the alpha particles passedthrough the gold foil. This ledRutherford to conclude that a goldatom was mostly empty space.


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Because alpha particles have relativelyhigh masses, the extent of thereflections led Rutherford to concludethat the nucleus was very heavy anddense.

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5.5

Deflection and scattering of alpha particles by positive gold nuclei.

Deflection

Scattering

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GeneralArrangement of

SubatomicParticles

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Rutherfords experiment showed that anatom had a dense, positively chargednucleus.

Chadwicks work in 1932 demonstratedthe atom contains neutrons.

Rutherford also noted that light,negatively charged electrons werepresent in an atom and offset the positivenuclear charge.

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Rutherford put forward a model of theatom in which a dense, positivelycharged nucleus is located at theatoms center.

The negative electrons surround thenucleus.

The nucleus contains protons andneutrons

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Atomic Numbersof the Elements

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The atomic number of an element isequal to the number of protons in thenucleus of that element.

The atomic number of an atomdetermines which element the atom is.

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Every atom with an atomicnumber of 1 is a hydrogen atom.

Every hydrogen atom contains 1proton in its nucleus.

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Every atom with an atomicnumber of 6 is a carbon atom.

Every carbon atom contains 6protons in its nucleus.

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1 proton in thenucleus

atomicnumber

Every atom with anatomic number of 1is a hydrogen atom.

1H 486 protons in thenucleus 6Catomic

number

Every atom with anatomic number of 6is a carbon atom.


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92 protonsin thenucleus 92 U

atomicnumber

Every atom with anatomic number of 92 is a uraniumatom.

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Isotopes of theElements

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Atoms of the same element can havedifferent masses.

They always have the same number of protons, but they can have differentnumbers of neutrons in their nuclei.

The difference in the number of neutronsaccounts for the difference in mass.

These are isotopes of the same element.52

Isotopes of the SameElement Have

Equal numbers of protons

Different numbers of

neutrons

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Isotopic Notation

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Isotopic Notation

C6 6 protons

126 protons + 6 neutrons


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Isotopic Notation

C6 6 protons


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Isotopic Notation

88 protons

16 8 protons + 8 neutronsO

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Isotopic Notation

88 protons

17 8 protons + 9 neutrons

O

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Isotopic Notation

88 protons


O

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Hydrogen has three isotopes

1 proton

0 neutrons

1 proton

1 neutron

1 proton

2 neutrons60

Examples of Isotopes

Element Protons Electrons Neutrons SymbolHydrogen 1 1 0 11HHydrogen 1 1 1 12H

Hydrogen 1 1 2 13H

Uranium 92 92 143 92235UUranium 92 92 146 92238U

Chlorine 17 17 18 1735ClChlorine 17 17 20 1737Cl


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Atomic Mass

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The mass of a single atom is too small tomeasure on a balance.

Using a mass spectrometer, the mass of the hydrogen atom was determined.

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A Modern Mass Spectrometer

A massspectrogram

is recorded.

From the intensity and positionsof the lines on the massspectrogram, the differentisotopes and their relativeamounts can be determined.

Positive ionsformed fromsample. Electrical field

at slitsacceleratespositive ions.

Deflection of positive ionsoccurs atmagnetic field.

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5.9

A typical reading from a mass spectrometer. The two

principal isotopes of copper are shown with theabundance (%) given.

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Using a mass spectrometer, the mass of onehydrogen atom was determined to be 1.673x 10 -24 g

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smallsmallsmallsmallsmallsmallsmallsmallsmallsmallsmallsmallsmallsmallsmall

smallsmall

smallsmall

small

This number is very small.


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1.673 x 10 -24 g

The mass of a hydrogen atom isvery small.

Numbers of this size are too small forpractical use.To overcome this problem a system of relative atomic masses using atomicmass units was devised to express themasses of elements using simplenumbers.

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126 C

The standard to which the masses of allother atoms are compared to was chosento be the most abundant isotope of

carbon.

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12

6 C

A mass of exactly 12 atomic mass units(amu) was assigned to

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1 amu is defined as exactly equal tothe mass of a carbon-12 atom 12

1

12

6 C

1 amu = 1.6606 x 10 -24 g

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HAverage atomic mass 1.00797 amu.

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KAverage atomic mass 39.098 amu.


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UAverage atomic mass 248.029 amu.

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Average RelativeAtomic Mass

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Most elements occur as mixtures of isotopes.

Isotopes of the same element havedifferent masses.

The listed atomic mass of an element isthe average relative mass of the isotopesof that element compared to the mass of carbon-12 (exactly 12.0000amu

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To calculate the atomic mass multiply theatomic mass of each isotope by its percentabundance and add the results.

(62.9998 amu) 0.6909 = 43.48 amu

(64.9278 amu) 0.3091 = 20.07 amu63.55 amu

63.5530.9164.9278

69.0962.9298

Averageatomic mass

(amu)

Abundance(%)

Isotopic mass(amu)

Isotope

6329 Cu6529 Cu

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Relationship Between MassNumber and Atomic Number

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The mass number minus the atomicnumber equals the number of neutrons inthe nucleus.

47109 Ag

mass numberatomicnumber

number of neutrons

- =

62109 - 47 =

mass

number atomicnumber


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