Mendeleev’s Periodic Table Based on Atomic Masses of Elements

Atomic Weight is the basis for Mendeleev’s periodic law. The characteristics of elements are a periodic function of their atomic masses, according to Mendeleev’s periodic law. Mendeleev’s periodic law, which was based on atomic mass, was changed as a result of the discovery of atomic numbers. Knowing the atomic weights of an element’s adjacent elements can occasionally change its atomic weight. As a result, tellurium’s atomic weight should be between 123 and 126, but not greater than 128. (The nuclear weight of tellurium is 127.6, and Mendeleev’s assumption that nuclear weight should increase with the period position was incorrect.)

The characteristics of elements, according to Mendeleev, are a periodic function of their atomic weights. Mendeleev named these elements Eka-Aluminium and Eka-Silicon because he left a space beneath aluminum and a gap under silicon.

Mendeleev’s Periodic Table Based on Atomic Masses of Elements

In 1869, Dmitri Mendeleev came up with a periodic table of chemical elements based on the properties that appeared in the elements. He placed the elements from lightest to heaviest. Before Mendeleev’s Periodic Table, Dobereiner’s triads and Newland’s Law of Octaves were mostly considered for the classification of elements in the form of tables. 

The Mendeleev’s Periodic Table came into the picture after the rejection of Newland’s Law of Octaves. In Mendeleev’s Periodic Table, elements are distributed according to their atomic masses, fundamental property, and chemical properties. 

Mendeleev’s Periodic Law states that “The properties of elements are the periodic function of their atomic masses.”

At the time when Mendeleev formulated this periodic table, only 63 chemical elements had been discovered. After observing all the properties of the elements, Mendeleev concluded that the properties of the elements periodically related to their atomic masses. Other than atomic masses, Mendeleev also used chemical properties to categorize. Formulae of hydrides and oxides of the elements were one of the primary criteria of categorization. He arranged the elements in the periodic table such that all the elements with similar properties would fall in the same vertical columns of the periodic table. He named the vertical columns as “groups” and the horizontal rows as “periods“.

Group		I	II	III	IV	V	VI	VII	VIII
Oxide		R2O	RO	R2O3	RO2	R2O5	RO3	R2O7	RO4
Hydride		RH	RH2	RH3	RH4	RH3	RH2	RH
Periods ⇓		A      B	A       B	A       B	A       B	A       B	A       B	A       B	Transition Series
1		H 1.008
2		Li 6.939	Be 9.012	B 10.81	C 12.011	N 14.007	O 15.999	F 18.998
3		Na 22.99	Mg 24.31	Al 29.98	Si 28.09	P 30.974	S 32.06	Cl 35.453
4	First series:	K 39.102	Ca 40.08	Sc 44.96	Ti 47.9	V 50.94	Cr 50.2	Mn 54.94	Fe 55.85	Co 58.93	Ni 58.71
	Second series:	Cu 63.54	Zn 65.37	Ga 69.72	Ge 72.59	As 74.92	Se 78.96	Br 79.909
5	First series:	Rb 85.47	Sr 87.62	Y 88.91	Zr 91.22	Nb 92.91	Mo 95.94	Tc 99	Ru 101.07	Rh 102.91	Pd 106.4
	Second series:	Ag 107.87	Cd 112.4	In 114.82	Sn 118.69	Sb 121.75	Te 127.6	I 126.9
6	First series:	Ce 132.9	Ba 137.34	La 138.91	Hf 178.49	Ta 180.95	W 183.85		Os 190.2	Ir 192.2	Pt 195.09
	Second Series:	Au 196.97	Hg 200.59	Ti 204.37	Pb 207.19	Bi 208.98

Mendeleev recognized the significance of periodicity. He used a broader range of physical and chemical properties in order to classify the chemical elements. Particularly, he relied on the similarities in the given empirical formulas and properties of the compounds such as oxides and hydrides formed by the elements.

Mendeleev noticed that some of the elements didn’t fit according to the periodic law if the order of atomic masses was followed strictly. He thus ignored the order of atomic masses, assuming that the atomic measurements might be incorrect, and placed the elements with similar properties together. For example, the atomic mass of Tellurium is more than that of Iodine. Yet, he placed Iodine with Group VII and Tellurium in Group VI. This was because Iodine has more similar properties to Chlorine, Fluorine, and Bromine. At the same time, keeping his primary aim of arranging the elements of similar properties in the same group, he proposed that some of the elements were still undiscovered and, therefore, left several gaps in the table. 

Merits

• It was one of the first periodic tables that successfully accommodated all known elements at that time.
• It gave the periodic law based on which the modern periodic law was formed.
• The table had gaps for undiscovered elements, which proved to be beneficial in the discovery of new elements. The periodic table was not disturbed in any way.
• Mendeleev’s Periodic Table in 1869 had only seven groups, but Sir William Ramsay suspected new elements belonging to a hitherto unknown eighth group.
• Ramsey and Rayleigh discovered the noble gases in 1895. These noble gases were added to Mendeleev’s Periodic Table as a separate group.

Demerits

• Hydrogen, the first element to begin with didn’t have a definitive place on the table. It was placed differently from the rest of the elements.
• Isotopes are variants of a particular element that have the same atomic number but differ in atomic mass. These isotopes violated Mendeleev’s Periodic Table as they couldn’t be placed.
• The increase in the atomic mass of the elements is irregular and not in a fixed pattern. Thus, it was not possible to exactly determine how many elements were yet to be discovered.

Predictions

• Mendeleev predicted several elements after observing his periodic table. He estimated that some unfilled spots would be similar to elements existing in the given group.
• He named some elements as (eka)-(similar element), where Eka means ‘one’ in Sanskrit.
• Mendeleev predicted eka-boron (Eb), eka-aluminium (Ea), eka-manganese (Em), and eka-silicon (Es).
• These elements were later discovered to be scandium, gallium, technetium, and germanium respectively.
• Mendeleev also predicted an element between thorium and uranium amongst others.

Mendeleev’s Predictions and actual properties on discovery:

Property	Eka-silicon (predicted)	Germanium (found)	Eka-aluminium (predicted)	Gallium (found)
Atomic Weight	72	72.6	68	70
Density	5.5	5.36	5.9	5.94
Melting Point	High	1231	Low	302.93
Formula of oxide	EO2	GeO2	E2O3	Ga2O3
Formula of chloride	ECl4	GeCl4	ECl3	GaCl3

To conclude, in his time, Dmitri Mendeleev made a very insightful Periodic Table based on Atomic Masses of Elements. It was the foundation of the Modern Periodic Table and successfully predicted several unknown elements of that time.

Sample Questions

Question 1: How is Mendeleev’s Periodic Table different from the Modern Periodic Table?

Answer:

The difference between Mendeleev’s Periodic Table and Modern Periodic Table are:

Mendeleev’s Periodic Table	Modern Periodic Table
This table is based on the atomic masses of the given elements.	This table is based on the atomic number of the given elements.
The chemical elements are arranged in the increasing order of their atomic masses.	The chemical elements are arranged in the increasing order of their atomic numbers.
Hydrogen doesn’t have a fixed place in this table.	Hydrogen has a fixed place in this table.
This table has 7 groups and 6 periods.	This table has 18 groups and 7 periods.
Isotopes violate this table and have no allotted place.	Isotopes are placed in the same element as the atomic number is constant throughout.

Question 2: Why did Mendeleev’s Periodic Table fail?

Answer:

Mendeleev’s Periodic Table had some shortcomings due to which it was discarded.

• Firstly, the increase in the atomic mass of the elements was found to be irregular and not in a fixed pattern.
• Secondly, Hydrogen didn’t have a fixed place on the table.
• Next, isotopes couldn’t be placed as same elements had different atomic masses.
• Lastly, as more elements were being discovered, they couldn’t get a specified place on the table.

Question 3: How did Mendeleev calculate the atomic masses of the chemical elements?

Answer:

Mendeleev did not calculate atomic masses on his own. He used the masses of existing elements to place them in the table and predict the masses of undiscovered elements. Mendeleev predicted eka-boron (Eb), eka-aluminium (Ea), eka-manganese (Em), and eka-silicon (Es), which were later discovered to be scandium, gallium, technetium and germanium respectively.

Question 4: Did Mendeleev violate the periodic law in the table? Explain.

Answer:

Mendeleev arranged the elements according to the atomic masses. Yet, there were instances where he had to place elements not in sequence to ensure that elements with similar properties were in the same group. Therefore, he had to violate the periodic law and ignore the order of atomic masses in some cases. For example, the atomic mass of Tellurium is more than that of Iodine. Yet, he placed Iodine with Group VII and Tellurium in Group VI. This was because Iodine has more similar properties to Chlorine, Fluorine and Bromine.

Question 5: Why couldn’t Mendeleev place Hydrogen in the periodic table?

Answer:

Hydrogen reacts with metals to form ionic compounds called hydrides and also with non-metals to form covalent compounds. Mendeleev’s periodic law could not assign a fixed position to hydrogen in the periodic table because hydrogen resembled both alkali metals (Group 1) and halogens (Group 17) in some of its properties. 

Question 6: How did isotopes violate Mendeleev’s Periodic Table?

Answer:

Isotopes are variants of a particular element that have the same atomic number but differ in atomic mass. As Mendeleev’s Periodic Table is according to the atomic masses, he couldn’t place one element twice in the table. Thus, isotopes violate the table and can’t be placed.