The periodic table

THE PERIODIC TABLE
HISTORICAL BACKGROUND
The idea that all things are elements or combinations of elements can be found in the writings of the ancient Greeks and other early peoples. Although this idea may seem simple, it is very difficult to decide if a substance is indeed an element or a compound. There were twelve chemical elements discovered in ancient times, but it was not known that they were elements. Another 76 elements were discovered between 1557 and 1925. Many mistakes were made in the discovery of the elements and the term ‘chemist’ was often interchangeable with ‘alchemist’. Chemistry as we know it today can be said to have found its footing as a science in 1661 when Robert Boyle wrote a paper that differentiated between the two.
This distinction was necessary because in those times, chemistry was a trial and error enterprise, as exemplified by the discovery of phosphorus in 1675. Henning Brand was a German merchant and amateur alchemist. The colour of human urine somehow convinced him that gold could be distilled from it. With various complicated processes he refined several buckets of it into a glowing waxy paste. This result was obviously not gold, but it was interesting in its own right because it spontaneously burst into flame when exposed to air. This was the foundation for discovery of elements in chemistry.
In 1771, a French tax collector named Antoine-Laurent Lavoisier had not discovered any elements but he was keenly interested in science and analysed the discoveries of others. He was the first to truly recognise the elements. He identified some elements properly and disproved the existence of others, such as phlogiston. He also co-authored the Methode de Nomeclature Chimique, which was used as the standard for naming new elements.
Table 1: Antoine Lavoisier's 1789 classification of substances into four 'element' groups acid-making gas-like metallic earthy elements sulphur Light cobalt, mercury, tin magnesia (magnesium oxide) phosphorus caloric (heat) copper, nickel, iron, silex (silicon dioxide) charcoal (carbon)
Oxygen
gold, lead, silver, zinc lime (calcium oxide)

azote (nitrogen) manganese, tungsten argilla (aluminium oxide)

Hydrogen platina (platinum) barytes (barium sulphate)
In 1799, a scientist named Benjamin Thompson founded the Royal Institution and appointed Humphry Davy, a brilliant young scientist, as the professor of chemistry. Davy proceeded to discover several new elements (including potassium, magnesium, calcium, strontium and aluminium). An addiction to laughing gas ended his life in 1829, with a total of twelve elements credited to him.
By the 1800s, the existence of atoms as the building blocks of all elements was a widely accepted fact. In 1808, John Dalton stated that ‘Atoms are featureless spheres. The only difference between elements is their weight...’ and listed the weights of 20 elements known at the time in a famous table.

Figure John Dalton's weight of the 20 known elements

THE BIRTH OF THE PERIODIC TABLE
In 1829, Johann Dobereiner obsereved that some elements exhibited similar properties and he placed them in groups of three. These groups were called triads. For example, he classed Iodine, Chlorine and Bromine into one triad, Barium, Strontium and Calcium in another and Lithium, Sodium and Potassium were in a third.
By 1863 even more elements had been discovered, and scientists began to see patterns in the characteristics. That same year, English chemist John Newlands divided the then discovered 56 elements into 11 groups, based on characteristics. He suggested that elements be arranged in “octaves” because he noticed (after arranging the elements in order of increasing atomic mass) that certain properties repeated every 8th element, inadvertently discovering periodicity. He also did not insist on classifying elements according to whether they were metals or non-metals. The table was not accurate and there were quite some mix-ups but the periodic table as we know it today was emerging.

Figure John Newlands' Table of Octaves
Newlands' claim of observing a repeating pattern amongst the elements was ridiculed by all upon its announcement. At gatherings, some members would ask him to get his octaves to play music. Disheartened, he stopped trying to get the scientists of the day to embrace the idea and was not heard from again.
In 1869 Russian chemist Dimitri Mendeleyev (also spelled Mendeleef or Mendeleev) started the development of the periodic table. Previously elements were grouped either according to atomic mass or common characteristics, e.g gaseous, metallic or non-metallic. Mendeleyev was the first to attempt to combine the two criteria in a single table. It is believed that he was inspired by the game of solitaire. In this game, playing cards are arranged from top to bottom by number and from left to right by suit. He wrote the names and atomic masses of all the elements on cards. The he arranged the cards one after the other in ascending order of atomic mass. He noticed that some properties were repeated after a certain interval.

Figure Mendeleyev's initial arrangement of the elements

Keeping the cards in order of increasing atomic number, he started to re-arrange them such that when he got to a repeating property he placed that element underneath the previous one with the same property. Following this method, the elements were arranged horizontally in periods and vertically in groups.

Figure Mendeleyev's arrangement according to periodicity

The scientific community was immediately struck by the relationships between the elements that immediately became obvious. Elements in the same period exhibited similar properties such as being metals or gases. Elements in the same group were found to be arranged in order of increasing atomic number. This led Mendeleyev to state the Periodic Law as:
‘The properties of the elements are a periodic function of their atomic weights.’

Figure Mendeleyev's first draft of the periodic table
However there was a glaring inconsistency in this seemingly flawless arrangement; when some elements that were supposed to belong to the same family were arranged as such, the masses no longer fit sequentially. When they were arranged sequentially, the elements no longer fell into neat groups. Mendeleyev believed this discrepancy was because of the inaccuracies of measuring the atomic mass of the elements and that it would be eliminated when scientists invented a better way to measure the atomic mass. However, he was able to correctly predict the properties of elements that had not yet been discovered (using interpolation) and leave spaces for them in his table.

Table 2: Prediction of Properties of an Unknown Element

Predicted - Ekasilicon
Discovered - Germanium
Atomic weight
72
72.32
Specific gravity
5.5
5.47
Color
dark grey greyish-white Formula of oxide
EsO2
GeO2
Specific gravity of oxide
4.7
4.70
Formula of chloride
EsCl4
GeCl4
Specific gravity of chloride
1.9
1.887
Boiling point of chloride below 100°C
83°C
http://genesismission.jpl.nasa.gov/educate/scimodule/cosmic/explore_1ST.pdf

Figure The first draft of the modern periodic table http://www.learner.org/courses/physics/visual/visual.html?shortname=periodictable_2 Independently, a German chemist by the name of Julius Lothar Meyer was also working on a way to arrange the elements based on their atomic weights. Like Mendeleyev, he was inspired by the Chemists’ Congress held at Karlsruhe in 1860 and the light it shed on Avagadro’s theorem. He set out to publish a textbook that would order the elements neatly. In 1864, he had successfully classified about half the elements of the time according to their atomic weights. He was astute enough to observe that the valence changed periodically and was dependent on the atomic weight. By 1868, he had expanded the table to contain all the known elements. Like Mendeleyev, he also left spaces in his table for elements that had not been discovered. He asked a colleague to critique it. Unfortunately, in 1869, Mendeleyev published his own version of the table before Meyer’s was ready in 1870. This is the reason Mendeleyev is credited with the creation of the periodic table to this day.
Table 3: Periodic table according to Lothar Meyer, 1870
I.
II.
III.
IV.
V.
VI.
VII.
VIII. B=11,0
Al=27,3

-- ?In=113,4
Tl=202.7

-- -- -- C=11,97
Si=28

-- Sn=117,8

Ti=48 Zr=89,7 -- N=14,01
P=30,9

As=74,9 Sb=122,1

V=51,2 Nb=93,7 Ta=182,2 O=15,96
S=31,98

Se=78 Te=128?

Cr=52,4 Mo=95,6 W=183,5
--
F=19,1
Cl=35,38

Br=79,75 J=126,5

Mn=54,8 Ru=103,5 Os=198,6 ?

Fe=55,9 Rh=104,1 Ir=196,7

Co=Ni=58,6 Pd=106,2 Pt=196,7
Li=7,01
Na=22,99
K=39,04

Rb=85,2 Cs=132,7

Cu=63,3 Ag=107,66 Au=196,2
?Be=9.3
Mg=23,9
Ca=39,9

Sr=87,0 Ba=136,8

Zn=64,9 Cd=111,6 Hg=199,8

In 1913, an English scientist Henry Moseley used a process called x-ray diffraction to correctly determine that while the mass of an atom and the number of protons it had were close, they were not the same thing. If the number of protons, or ‘atomic number’ was used in arranging the elements accordingly, the discrepancies noticed by Mendeleyev, and indeed the rest of the scientific community, were immediately eliminated.

Figure Comparison of atomic number with atomic mass, courtesy of Henry Moseley's work

Therefore, Mendeleyev might have discovered the principle of arranging the elements into a table, but Moseley was responsible for the corrections that gave rise to the periodic table as we know it today.
He therefore restated the periodic law as:
‘When the elements are arranged in increasing order of atomic number, the chemical properties repeat periodically.’