1 Le cture 4 • By 1860, sc ientists had alr eady disc overed 60 elements and determined their atomic masses. The Search for a Periodic Table • They not ice d that some el emen ts had simi lar properties. • They wa nted to or gani ze the elements into a system that would show similarities and differences. • It was lo gica l to use ato mic mass as the basis for these early attempts, because no one knew about protons at that time! Döbereiner’s Triads • The e lements in a t riad had similar chemical properties, and their physical properties varied in an orderly way according to their atomic masses. • In 182 9, the G erma n chemist J.W. Döbe reine r clas sifie d some elements into groups of three, which he called “triads”. • The co ncept of triads suggested that t he properties of an element are related to its atomic mass. • For example, tri ads showed a relationship using densities. Within a t riad, density increased with increasing atomic mass. Döbereiner’s Triads
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Lecture 4 • By 1860, scientists had already discovered60 elements and determined their atomicmasses.
The Search for a Periodic Table
• They noticed that some elements had similarproperties.
• They wanted to organize the elements into asystem that would show similarities anddifferences.
• It was logical to use atomic mass as thebasis for these early attempts, because no
one knew about protons at that time!
Döbereiner’s Triads
• The elements in a triad hadsimilar chemical properties,and their physicalproperties varied in anorderly way according totheir atomic masses.
• In 1829, the German chemist J.W.Döbereiner classified someelements into groups ofthree, which he called“triads”.
• The concept of triads suggested that theproperties of an element are related to itsatomic mass.
• For example, triads showed a relationship usingdensities. Within a triad, density increasedwith increasing atomic mass.
Döbereiner’s Triads
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The Periodic TableThe Periodic TableDemetri Mendeleev
•Mendeleev used atomic mass to put theelements known at that time in order.
•He then grouped these elements accordingto their properties.
The Periodic TableThe Periodic TableDemetri Mendeleev
•He found that these chemical propertiesrepeated themselves “periodically”.
•This is the principal known as “ periodicity ”:the tendency for something to re-occur ona regular basis.
•Using this pattern, he was able to predictthe existence of undiscovered elements!
How he did itHow he did it ……A@ B
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First, he put all theknown elements in orderby atomic weight…
Then…
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…he arrangedthem by theirproperties…(here, theproperties arethe specialsymbols)…
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How he did itHow he did it ……A@
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Then, he lookedfor patterns…
His table was sogood, hepredicted newelements thathadn’t even beendiscovered!
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He called hisvertical columns“families” or“groups”,because theyhad similarproperties…
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How he did itHow he did it ……A@
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He called hishorizontal rows“periods”,because thepropertiesrepeatedthemselves“periodically” aseach new row
started.
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How he did itHow he did it ……
Because all the known elements at thattime had been extracted from compounds,Mendeleev had no knowledge of the NobleGases.
Since these elements don’t naturally reactwith anything, they were never part of anycompound and not discovered for yearsafterward!
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Dmitri Mendeleev(1834 – 1907).
Dmitri Mendeleev
Mendeleev’s discovery of theperiodic table came as a result ofattempting to systemize properties ofthe elements for presentation in achemistry textbook.
His highly influential book wentthrough eight editions in his life-timeand five more after his death.
Organization of the elements by relative atomicmass and periodic properties.
A periodic table is atabular arrangementof the elementsthat groups similarelements together.
In the table,Mendeleev arrangedthe elements intoeight groups andtwelve rows. Theformulas R 2O, RO,…,are those of theelements oxides; theformulas RH 4, RH 3,…,are those of theelement hydrides.
MendeleevMendeleev ’’ss tabletableMedeleevMedeleev succeeded where others had failed for two reasons:succeeded where others had failed for two reasons:
He left blank spaces in his table for undiscovered elements.He left blank spaces in his table for undiscovered elements.He corrected some atomic mass values.He corrected some atomic mass values.
The blanks in his table came at atomic massesThe blanks in his table came at atomic masses 4444 (Scandium),(Scandium), 6868(gallium),(gallium), 7272 (germanium) and(germanium) and 100100 (technetium).(technetium).
• Two of the atomic mass values he corrected were those of indium anduranium .
A new group for the periodic tableA new group for the periodic table
One group of elements thatOne group of elements that MedeleevMedeleev diddidnot anticipate was the noble gas. He leftnot anticipate was the noble gas. He leftno blanks for them.no blanks for them.William Ramsay proposed placing them inWilliam Ramsay proposed placing them ina separate group of the table.a separate group of the table.Ramsay placed the new group, which heRamsay placed the new group, which hecalledcalled Group 0Group 0 , between the halogen, between the halogenelements (elements ( Group VIIGroup VII ) and the alkali metals) and the alkali metals((Group IGroup I ).).
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• There are several places in the modern tablewhere an element of higher atomic masscomes before one of lower atomic mass.
• This is because the basis for ordering theelements in the table today is the atomicnumber , not atomic mass.
The Modern Periodic Table
• The atomic number of an element is equal tothe number of protons in the nucleus.
• Atomic numberincreases by oneas you move fromelement toelement across arow.
• Each row (except the first) begins with ametal and ends with a noble gas.
The Modern Periodic Table
• In between, the properties of the elementschange in an orderly progression from leftto right.
• The pattern in properties repeats aftercolumn 18.
• This regular cycle is what we call“periodicity” in the properties of theelements.
The Modern Periodic Table
The statement that……”the physical and chemical properties of the
elements repeat in a regular pattern whenthey are arranged in order of increasingatomic number”…
The reason that “families” of elements exist isthat the elements in a family…
…HAVE THE SAME NUMBER OF VALENCEELECTRONS!!!
(watch the pattern…)
13 14 15 16 17
At
Using the Periodic TableUsing the Periodic Table
Also note that the last number of the familygives the number of valence electrons!!!
A Word AboutA Word About ““MetalloidsMetalloids ””The elements along the “stairstep” betweenthe meals and nonmetals on the PeriodicTable are called “metalloids”.
metalloidsNot only are they between metals andnonmetals on the table, their PROPERTIES are also between metals and nonmetals!
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metalloids
The Modern Periodic Table
Carbon, for example, is very common asgraphite – the same stuff in your pencil lead.
While graphite is brittle, (ever break yourpencil lead?) like a nonmetal…
Graphite also conducts electricity like ametal!
Graphite (carbon) is a metalloid because itacts like a metal AND a nonmetal.
A Final NoteA Final Note ……SEVEN of the elements, when NOT incompounds, are normally found as….
…”DIATOMIC” MOLECULES!
They are: H 2, O2, N2, F2, Cl2, Br2, & I2
It will become very important to know thislater!
(sulfur is usually found as “S 8”, whilephosphorus is usually “P 4”)
