1868 Dmitri Mendeleev

The First Periodic Table

In 1863 there were 56 known elements with a new element being discovered at a rate of approximately one per year. A system of organizing the elements was increasingly needed.

After becoming a teacher, Mendeleev wrote the definitive two-volume textbook at that time: Principles of Chemistry (1868-1870). As he attempted to classify the elements according to their chemical properties, he noticed patterns that led him to postulate his Periodic Table.

Mendeleev ordered the elements according to increasing atomic weight in vertical rows so that the horizontal rows contain similar elements, still ordered by increasing atomic weight, one obtains the following arrangement, from which a few general conclusions may be derived.

Mendeleev’s Periodic Table Ti=50 Zr=90 ?=180

V=51 Nb=94 Ta=182

Cr=52 Mo=96 W=186

Mn=55 Rh=104,4 Pt=197,4

Fe=56 Ru=104,4 Ir=198

Ni=Co=59 Pd=106,6 Os=199

H=1 Cu=63,4 Ag=108 Hg=200

Be=9,4 Mg=24 Zn=65,2 Cd=112

B=11 Al=27,4 ? =68 Ur=116 Au=197?

C=12 Si=28 ? =70 Sn=118

N=14 P=31 As=75 Sb=122 Bi=210?

O=16 S=32 Se=79,4 Te=128?

F=19 Cl=35,5 Br=80 J=127 Li=7 Na=23 K=39 Rb=85,4 Cs=133 Tl=204 Ca=40 Sr=87,6 Ba=137 Pb=207

?=45 Ce=92

?Er=56 La=94

?Yt=60 Di=95

?In=75,6 Th=118?

In 1869, Mendeleev made a formal presentation to the Russian Chemical Society, in which he described elements according to both atomic weight and valence. In this presentation, he made some bold predictions about elements that were not yet known. He had left spaces in his table where he predicted new elements would eventually fit and he included predictions about the properties they would possess.

Only a few months later, Lothar Meyer published a virtually identical table. Some consider Meyer and Mendeleev the co-creators of the periodic table, but virtually everybody agrees that Mendeleev's accurate prediction of the qualities of what he called ekasilicon, ekaaluminium and ekaboron (germanium, gallium and scandium, respectively) qualifies him for the majority of the credit for the table.

Mendeleev was proven to be correct when Ga (gallium), Sc (scandium), and Ge (germanium) were found in 1875, 1879, and 1886 respectively, fitting perfectly into the missing spaces.

Problems with Mendeleev’s Table

Misplaced elements of iodine + tellurium Heavy elements did not fit well into his table There was no room for new families of elements

A New Family of Elements 1894 John William Strutt, 3rd Baron Rayleigh

An English physicist who, with William Ramsay, discovered the element argon (Ar), an achievement for which he earned the Nobel Prize for Physics in 1904.

Argon is present in the Earth's atmosphere at 0.94% (3rd in proportion after nitrogen and oxygen). Argon has approximately the same solubility in water as oxygen gas and is 2.5 times more soluble in water than nitrogen gas. Argon is colorless, odorless, tasteless and nontoxic in both its liquid and gaseous forms. Argon is chemically inert under most conditions and forms no confirmed stable compounds at room temperature.

How did they discover it?

They had found out that nitrogen produced from chemical compounds was one-half percent lighter than nitrogen from the atmosphere. The difference seemed insignificant, but it was important enough to attract their attention for many months. They decided to remove all of the oxygen, carbon dioxide, water and nitrogen from a sample of clean air. What was left afterward was a small amount of gas which was proven to be a new element that they called argon.

(In Greek, argon means “inactive”)

Due to its inert properties and high abundance in nature, argon has thousands of uses and around 700,000 tons are used each year.

With the discovery of argon, a mystery presented itself. Were there other un-reactive elements hiding in nature yet to be discovered?

The search was on!

1895 William Ramsay Three more “noble gases”

These were discovered when Ramsay cooled a sample of air until it became a liquid, then warmed the liquid and captured the gases one by one as they boiled off (fractional distillation). The gases that boiled off included krypton, xenon, and neon. Element Boiling Pt. % abundance (Kelvin) in air Rn 211 1 x 10-19 Xe 165 0.000009 Kr 120 0.000114 Ar 87 0.94 Ne 27 0.00182 He 4 0.000524

In Greek, neon means “new one”; krypton means “hidden”; and xenon means “stranger”.

Mendeleev’s table didn’t accommodate these new elements and a new system was needed. 1913 Henry Moseley

In 1913, by using x-ray spectra obtained by diffraction in crystals, he found a systematic relation between wavelength and atomic number, that came to be known as Moseley's law.

This discovery allowed Moseley to put the elements into their current order by atomic number.

When World War I started, Moseley left Oxford University to enlist in the Royal Engineers. He was killed by a sniper during the Battle of Gallipoli in 1915, at the age of 27.