Important Questions on Periodic Classification

Q. Why do we classify elements ?

ANSWER -Due to discovery of different elements, it was difficult to  organize the information and properties of the elements,  so we need to find some pattern in the properties of different elements so that they can be grouped together hence we have to classify elements.

Q. What were the two criteria used by Mendeleev to classify the elements in his periodic table ?

ANSWER– two criteria used by Mendeleev to classify the elements in his periodic table are

Atomic mass: elements are arranged in the order of increasing atomic masses.

The formulae of the hydrides and the oxides formed by the various elements.

Q. Why did Mendeleev leave some gaps in his periodic table ?

ANSWER – Mendeleev’s periodic table is an arrangement of the  elements that group similar elements together.  He left blank spaces for the undiscovered elements

Q. In Mendeleev’s periodic table, why was there no mention of noble gases like helium, neon and argon ?

ANSWER-  There was no mention of noble gases like helium,  neon and argon as they were not known at that time and were discovered much later. noble gases are present in very low concentration in atmosphere so difficult to isolate.

Q. Would you place the two isotopes of chlorine, Cl-35 and Cl-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same ? Justify your answer .

ANSWER – In the periodic table of Mendeleev, both the isotopes of chlorine,
i.e., Cl-35 and Cl-37 would be placed in different slots.  because according to mandeleev periodic table property of elements the  periodic function of their atomic masses not atomic numbers

Q. State modern periodic law.

ANSWER – modern periodic law states that  “The physical and chemical properties of the  elements are periodic functions of their atomic numbers”

Q. How does the electronic configuration of the atom of an element relate to its position in the modern periodic table ?

ANSWER -Electronic configuration of an element provides the  information about the valence electrons and number of  shell present in the element. therefore ,  electronic configuration help in knowing the group number and period number of an element, and determine the position of an  element in the periodic table.

Q. How could the modern periodic law remove various anomalies of Mendeleev’s periodic table ? Explain with examples.

ANSWER – In the Modern Periodic table elements are arranged  in the increasing order of their atomic number, removing the anomaly  regarding certain pairs of elements in Mendeleev’s periodic table which is based on atomic mass.
Atomic number of cobalt is 27 and nickel is 28. Hence cobalt  will come before nickel even though its atomic mass is greater.
All isotopes of the same elements have different atomic masses, but same atomic number; therefore they are placed in the same  position in the modern periodic table.

Q. Is it possible to have an element having atomic number 1.5 placed between hydrogen and helium ?

ANSWER – it is not possible  to have an element having atomic number 1.5 placed between hydrogen and helium because Atomic number is the sum of total number of
protons present in an atom. So, these numbers are always present in  whole number and they are never present in fraction

Q. Name the scientist who prepared modern periodic table.

ANSWER -Dmitri Mendeleev is the scientist who prepared modern periodic table

Q. Explain why :
(i) All the elements of a group have similar chemical properties.
(ii) All the elements of a period have different chemical properties.
(b) The atomic radii of three elements X, Y and Z of a period of the periodic table are 186 pm ; 104 pm and 143 pm respectively. Giving a reason, arrange these elements in the increasing order of atomic numbers in the period.


Elements in the same group have similar CHEMICAL properties because they have the same number of valence electrons in their outermost shell.

Elements in the same period have different chemical properties because of regular
increase in valence electrons in all the elements in the period .

Order of radii is B < C < A.

Since atomic size generally decreases along a period, as the atomic number (nuclear charge) increases thus, the element with smaller radii, has the highest atomic number.
Hence, B has the highest atomic number followed by C and A

Q. Why does the size of the atoms progressively become smaller when we move from sodium (Na) to chlorine (Cl) in the third period of the periodic table ?

ANSWER –  The size of atoms decreases progressively, on moving from  sodium (Na) to chlorine (Cl) in the third period because the  atomic number of the elements increases. similarly, the  number of electrons and protons in the atoms also increases.

Q. Helium and neon are nonreactive gases. What, if anything, do their atoms have in common ?

ANSWER – both element belongs to the last group with all the gas whose last shell
are common and they are unreactive while other elements have to  reacts to complete there last shell with total 8 electrons .

Helium and Neon have in common that they have  there valance shell complete and they have very low reactivity.

Q. In the modern Periodic Table, why does cobalt with higher atomic mass of 58.93 appear before nickel having lower atomic mass of 58.71 ?

ANSWER– According to modern periodic table,  elements are arranged according to
their increasing atomic number.  The atomic numbers of cobalt (Co) and nickel (Ni) are 27 and 28, respectively. Therefore, although cobalt  (Co) has higher atomic mass, it appears before nickel (Ni) in the periodic table.

 Q. Why could no fixed position be given to hydrogen in Mendeleev’s periodic table ?

ANSWER – hydrogen has  1 valence electron in its outermost shell. so according to modern periodic table,  hydrogen can be arranged in halogen family . Hence, it was placed in IA group VI B  group and hence could not be given a fixed position in Mendeleev’s Periodic Table.

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