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Figuring it out

• By the mid-1800’s there were 62 known elements, but no system to understand them!

How did we get this?

Periodic Table

How did they figure it out?They observed physical and chemical properties and

looked for repeating patterns.

PERIODIC: happening or appearing at regular intervalsregularly repeating patternregularly repeating patternregularly repeating patternregularly repeating patternregularly repeating patternregularly repeating patternregularly repeating patternregularly repeating patternregularly repeating pattern

What does PERIODIC have to do with SCIENCE?

• Scientists use patterns to help us figure things out.

• After all…s’tI namuh erutan ot erugif sgniht tuo…

• The patterns help explain chemical behavior and details about atomic structure.

• Patterns help us predict.

Dobereiner• German chemist in the early 1800’s• Developed concept of TRIADS

– Groups of 3 elements with similar properties in which the middle element has average of the other 2

Cl 36 amu 1.6 g/L

Br 80 amu 3.1 g/L

I 127 amu 5 g/L

Ca 40 amu 1.6 g/mL

Sr 86 amu 2.6 g/mL

Ba 137 amu 3.5 g/mL

Newlands

• English chemist 1865• Law of Octaves

– The properties of elements repeated every 8 elements…like an octave in music

• Everyone laughed!• But he was eventually credited with the idea

of a periodic pattern to the properties of the elements

MENDELEEV

• Russian chemist 1869• Wrote elements their properties on cards and

arranged them different ways according to the different properties

• Noticed periodic repetition of properties when cards were arranged by atomic mass

He had a problem!• Some elements did not fit the pattern• If he sorted by properties, some were out of order

according to masses!• Check Periodic table for examples:

– Ar/K Co/Ni Te/I

• Why? He lived in the 1800’s and didn’t know about the parts of an atom. All he knew was atomic mass. Protons and neutrons and the idea of isotopes would have helped him.

What did he do?

• He boldly broke the pattern of atomic mass to keep the properties together. He even proposed that the masses were wrong!

• He predicted the discovery of elements to fill gaps, and he correctly predicted their properties.

He predicted missing elements. Here’s one.

Property Ekasilicon Germanium

Atomic mass 72 amu 72.59 amu

Density 5.5 g/cc 5.35 g/cc

Melting point high 947 degrees C

Color gray gray

Predicted 1869 Discovered 1882

Father of the Periodic Table

Moseley• English chemist early 1900’s• Solved the problem of elements out of order

according to mass• Developed the concept of ATOMIC NUMBER, the

number of protons in the nucleus.• The correct way to order the chart is by ATOMIC

NUMBER not atomic mass.• Mendeleev’s exceptions fell into place when

ordered by number, not mass. Isotopes (unknown at the time) threw off the order by mass.

ALL of these scientists contributed to the PERIODIC LAW

When elements are arranged in order of INCREASING ATOMIC NUMBER,

their physical and chemical properties show a PERIODIC PATTERN.

(The properties repeat regularly.)

Fromthis To

ths

PERIODIC TRENDSWe can predict many things.

• The periodic table is organized so that the elements with similar electron configurations are in the same column or family.

• Many properties of elements change in a predictable way as you move through the periodic table.

• These variations are called PERIODIC TRENDS.