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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