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Chp 5.1 - Organizing the Elements
Pg. 126-129
Search for Order Until 1750, scientist had
identified only 17 elements Mainly metals such as
copper and iron As the number of known
elements grew, so did the need to organize them into groups based on their properties
In 1789, Antoine Lavoisier grouped the known elements into categories he called metals, nonmetals, gases, and earths.
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Mendeleev’s Periodic Table
1860’s, Mendeleev found a way to arrange the element’s while playing solitaire, a favorite card game
In solitaire the player must sort the cards into four columns by suit and value to win
Mendeleev’s strategy for organizing the elements was modeled on the card game
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Mendeleev’s Proposal
Used the element’s name, mass, and properties on cards
When he lined the cards up in order of increasing mass, a pattern emerged
Mendeleev arranged the elements into rows in order of increasing mass so that elements with similar properties were in the same column
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Mendeleev’s Proposal cont.
Columns were organized by propertiesWithin a column, the masses increased
from top to bottomMendeleev’s chart was a periodic tablePeriodic table - is an arrangement of
elements in columns, based on a set of properties that repeat from row to row.
Mendeleev’s Prediction Placed bromine (Br) with chlorine (Cl) because they had
similar properties He also used the masses of arsenic and selenium to place them
into the table He couldn’t complete his chart because many elements
had not been discovered yet He left spaces in the table for those elements He offered the best explanation for how the properties of
an element were related to its location in his table
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Mendeleev’s Prediction cont.
A good way to test for the correctness of a scientific model is whether the model can be used to make accurate predictions
Mendeleev was confident that the gaps in his table would be filled by new elements
He used the properties of elements located near the blank spaces to predict properties for the undiscovered elements
Evidence supporting Mendeleev’s Table
Predicted that the element that fit the space below aluminum would have a low melting point and density of 5.9 g/cm3
He named it eka-aluminum Later gallium (Ga) was discovered in 1875 and it is a
soft metal with a melting point of 29.7 o C and a density of 5.9 g/cm3
Scientists concluded that eka-aluminum and Gallium are the same element
The close match between Mendeleev’s predictions and the actual properties of new elements showed how useful his periodic table could be
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The End
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