Anomalies of Mendeleev’s Periodic Table

The study of a large number of elements is reduced to a few groups of elements if all the elements are divided into a few groups, in such a way that elements in the same group have similar properties. Dobereiner’s Triads, Newland’s law of octaves, Mendeleev’s periodic table all helped to classify the known elements. But there were always some limitations to these classifications until the Modern periodic table. Mendeleev’s periodic table and its anomalies or limitations are discussed below.

What is Mendeleev’s Periodic Table?

In 1869, Mendeleev proposed the periodic law, which states that if elements are arranged in increasing atomic mass order, their properties will be repeated at regular intervals or periods. Mendeleev arranged all 63 elements known at the time in the periodic table in horizontal rows in order of increasing atomic masses, but in such a way that elements with similar properties were placed together in the same vertical column. 

In Mendeleev’s periodic table, there were seven horizontal rows called periods and eight vertical columns called groups. At that time noble gases were not known. So, in Mendeleev’s original periodic table, there was no group of noble gases. To make sure that the elements having similar properties fell in the same vertical column, Mendeleev left some gaps in his table. 

The undiscovered or the unknown elements at the time for which the gaps were left in the periodic table were named by Mendeleev as eka-boron, eka-aluminium, and eka-silicon. When these elements were discovered later on, then eka-boron was named scandium, eka-aluminium was named gallium, and eka-silicon was named germanium. 

Mendeleev also placed a few elements in the wrong order of their atomic masses, keeping the element with the higher atomic mass first and the element with the lower atomic mass later, to ensure that elements with similar properties fell into the same vertical column. When noble gases were discovered, Mendeleev’s periodic table could accommodate them. It could also predict the properties of several elements based on their position in the periodic table.

Anomalies of Mendeleev’s classification of elements

Mendeleev’s periodic table was extremely useful in the study of elements, but it had some shortcomings that could not be explained using Mendeleev’s periodic law. The three major anomalies or limitations in Mendeleev’s element classification are given below:

• Position of Isotopes could not be explained

The atoms of the same element having similar chemical properties but different atomic masses are called isotopes. Since isotopes have different atomic masses, they should be placed in different groups of the periodic table if the elements are arranged according to atomic masses. In Mendeleev’s periodic table, the isotopes were not given their column. In Mendeleev’s periodic table, the isotopes are arranged in the same place. Cl-35 and Cl-37, for example, are two isotopes of chlorine with atomic weights of 35 and 37, respectively. Mendeleev’s periodic law could not explain why these two chlorine isotopes having different atomic masses were placed in the same group of the periodic table.

• Wrong Order of Atomic masses of some elements could not be explained

The elements are arranged in order of increasing atomic masses, according to Mendeleev’s periodic law. As a result, the element with the lower atomic mass should come first, followed by the element with the higher atomic mass. When some elements were grouped, it was discovered that the element with the greater atomic mass came first, followed by the element with the lower atomic mass. When placed in the correct group based on chemical properties, the element cobalt, which has a higher atomic mass of 58.9, comes first, followed by the element nickel, which has a slightly lower atomic mass of 58.7. Mendeleev’s periodic law was unable to explain this unusual situation of atomic masses in the wrong order.

• Correct Position could not be Assigned to Hydrogen in the Periodic table

Hydrogen is in group I alongside the alkali metals in Mendeleev’s periodic table. This is because, like alkali metals (such as sodium), hydrogen reacts with halogens, oxygen, and sulphur to form compounds with similar formulas. The table below shows the similar compounds formed by sodium and hydrogen.

Compounds of hydrogen (H)	Compounds of alkali metal sodium (Na)
HCl	NaCl
H2O	Na2O
H2S	Na2S

This indicates that hydrogen exhibits some properties like alkali metals. Some of the properties of hydrogen are similar to those of halogens (fluorine, chlorine, and bromine). Hydrogen, like halogens (F₂, Cl₂, and Br₂), can be found in the form of diatomic molecules (H₂). Furthermore, hydrogen, like halogens, forms ionic compounds termed hydrides when combined with certain metals and reacts with non-metals to form covalent compounds. All of these characteristics indicate that hydrogen belongs to group VII of the halogen element. As a result, it is possible to conclude that hydrogen belongs to both the alkali metal group and the halogen group based on its properties. Thus, Mendeleev’s periodic law was unable to place hydrogen in the correct position in the periodic table.

The inability of Mendeleev’s periodic law to explain the position of isotopes, the incorrect order of the atomic masses of some elements, and the position of hydrogen led to the conclusion that atomic mass could not be used to classify elements. There was a theory that there must be a more fundamental property of elements that could explain periodicity in element properties better. The atomic number of elements was discovered to be this property. All of Mendeleev’s classification anomalies vanished when the elements were arranged according to increasing atomic numbers.

Solved Questions

Question 1: What was Mendeleev’s reasoning for leaving a gap in his periodic table?

Answer:

Mendeleev left some gaps in his table to ensure that items with similar properties were placed together in the same vertical column or group.

 Question 2: In Mendeleev’s original periodic table which group of elements was missing?

Answer:

Since noble gases were not known at that time, so there was no group of noble gases in Mendeleev’s original periodic table.

Question 3: Give some merits of Mendeleev’s periodic table.

Answer:

Some merits of Mendeleev’s periodic table are.

1. When noble gases were discovered, the periodic table could accommodate them.
2. Mendeleev’s periodic table was able to anticipate the properties of several elements based on their position in the periodic table.
3. Mendeleev’s periodic table anticipated the existence of several elements that were unknown at the time such as gallium, scandium, and germanium.

Question 4: In the modern periodic table, how was the position of cobalt and nickel resolved?

Answer:

In Mendeleev’s periodic table, the wrong order of atomic masses of some elements like cobalt with atomic mass 58.9 and nickel with atomic mass 58.7 could not be explained. To maintain similarity in properties, copper with higher atomic mass had to be placed before nickel. But in the modern periodic table, the elements are arranged in the order of increasing atomic number. So, the problem was resolved because cobalt has a lower atomic number (27) than nickel (28). Therefore nickel with atomic number 28 is placed after cobalt with atomic number 27.

Question 5: In Mendeleev’s original periodic table, how many groups and periods are there?

Answer:

Mendeleev’s original periodic table has eight groups and seven periods.

Question 6: Why was hydrogen not placed in the correct position in Mendeleev’s periodic table?

Answer:

Hydrogen exhibited similar properties to both alkali metals and halogens. So, it was concluded that hydrogen belongs to both the alkali metal group and the halogen group based on its properties. Therefore, hydrogen was not placed in the correct position in Mendeleev’s periodic table.