37.THE PERIODIC TABLE- History (2)-Dimitri Mendeleev.

The Father of the periodic table .

 A child was born on 8th February 1834, in  a small town Tobolsk in Siberia. He was the youngest of 14 siblings.His father was a high school teacher and mother managed a glass factory.His father went blind in the same year of his birth and thus his mother had to work to support the family.As he started growing up,his mother gathered that her youngest son was intellectually strong and so she decided to give him a good education.When this child was 15 years old, his mother started a journey with him, from Tobolsk to Moscow(around 2500kms !) on a horse carriage.She wanted to admit her child in a good school in Moscow.However, he was not admitted in any school in Moscow and so they travelled further more till St.Petersberg. Here this child was admitted to a teacher’s training program , which his father had attended.

She got the 16 year old skinny boy to St.Petersberg University and with a very heavy heart,bid him good bye.Tired from this long journey,Maria Mendeleev, his mother, died soon after that.Little did she know that the very street she travelled from to leave her son in St.petersberg, would be soon named after her son – Dimitri Mendeleev!!!

monument-to-dmitriy-mendeleev-in-st-petersburg.jpg
Mendeleev statue in St.Petersberg.

dimendeleevcab

Dimitri Mendeleev is aptly called the ‘The father of the Periodic Table’ as he was the first chemist to find out the order amongst the different elements and to lay these elements in a tabular form.He not only classified the elements known at that time, but also predicted with accuracy the properties of missing elements like Gallium (which he called eka-Aluminium)and Germanium(eka-silicium) and scandium (eka-boron)Eka means one after in Sanskrit .

While writing a textbook of Chemistry he started his calculations on the atomic weights of elements and found a pattern there.He was sitting on a railway station, playing a card game(patience) when BOOM something struck him! And he came up with the periodic table!

He could make a sense of data before him and organise the elements in close to perfect order.His table looked as follows –

periodic_table_by_mendeleev_1871-svg

The horizontal dashes ,in the above table, represent the missing elements.He called his table ‘Periodic System‘.This table was based on Mendeleev’s periodic law,

“The physical and chemical properties are periodic properties of their atomic weights”

This means that when elements were arranged in increasing order of their atomic weights, certain properties of elements showed a regularity.The properties repeated after a periodic interval and properties of elements in a column were similar.
e.g. All alkali metals (Group IA) showed similar properties like they were all soft metals and could be cut with a knife.They had low  melting and boiling points and they all were highly reactive.
All such elements were  placed in one column ,one below the other to categorise them.He figured that some spots had to be left vacant for the undiscovered elements.Later, he not only predicted their properties but also gave a quantitative judgement of their properties.

e.g.- He predicted that there must be an element called ‘ Eka- silicium’ after silicon and before tin  in that respective column.He predicted the following properties of this unknown element –

Property

Predictions(eka-silicon)

Actual values(Germanium)

Atomic weight

72 g/mol

72.59 g/mol

Boiling point 

High

958 deg celsius

Density 

5.5g/cm3

5.36 g/cm3

That was so close!

A story goes where a French scientist discovered an element and named it Gallium and  measured it’s density to be 5.9.The element’s properties seemed to perfectly match with Mendeleev’s prediction of eka-aluminium except for its density.Mendeleev,who had never seen the element  confidently wrote to the Frenchman to recalculate the density.After precise measurement it was found that Mendeleev was indeed correct ! The density of Gallium was 4.7 indeed!! This was the genius of Dimitri Mendeleev!

Even after such a great discovery he never receieved a Noble prize. Although, an synthetic element(atomic number 101)has been named after Mendeleev, in the modern periodic table.

Limitations of Mendeleev Periodic table – 

  1. william_ramsay
    Sir William Ramsay-The man who discovered Noble gases!

    Sir William Ramsay,a British chemist, discovered Argon gas and this gas had no place in the periodic table! Mendeleev argued that this was not an element in the first place as it didnt react with anything! However, a few years later Ramsay was successful in isolating Helium, which was already known to be present in the sun .And even this element didnt find a place in Mendeleev’s periodic table! With the discovery of krypton,Xenon and Radon, Mendeleev added a new column of Noble gases to his periodic Table but the problem of their correct position persisted  as their atomic weights were more than their succesive elements in the table.Thus, in these  cases the element with higher weight had to be placed before the one with lower atomic weight.e.g. Argon (At.wt A= 39.948) was placed before Potassium (A= 39.098) and  Cobalt (A= 58.933) was placed before Nickel(A= 58.693).

  2. Mendeleev could not assign a correct position to hydrogen in the periodic table as hydrogen resembled both alkali metals and halogens.
  3. Isotopes were given the same place in the periodic table, though they differed in atomic mass.

So, with the above drawbacks it was necessary to make some amendments to the table and  find a better method to arrange the elements correctly.Who was the man who finally gave us the modern Periodic table? More about him in our next post.Till then,

Be a perpetual student of life and keep learning …

Good day!

References and Further Reading –

1)https://en.wikipedia.org/wiki/Dmitri_Mendeleev
2)MIT 3.091 lecture 2 video by Professor Sadoway
3)Father of the Periodic Table “Dmitri Mendeleev” documentary.
4)THE MYSTERY OF MATTER: Episode 2, Unruly Elements documentary.

Image source –

  1. By -. Original uploader was Serge Lachinov at ru.wikipedia – Transferred from ru.wikipedia(Original text : кабинет академика Михаила Михайловича Шульца – фото любезно передано мне в собственность вдовой М.М.Шульца Ниной Дмитриевной Шульц.), Public Domain, https://commons.wikimedia.org/w/index.php?curid=7347916
  2. http://www.saint-petersburg.com/monuments/dmitriy-mendeleev/
  3. https://en.wikipedia.org/wiki/Periodic_table#/media/File:Periodic_table_by_Mendeleev,_1871.svg
  4. By Elliott & Fry – http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1904/ramsay.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=23455
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