"AKC SCIENCE CLASSES"
CLASS 10 TH (CBSE AND MP BOARD)
CHAPTER 05
PERIODIC CLASSIFICATION OF ELEMENTS
INTEXT QUESTIONS AND ANSWERS
Q.01:- Did Döbereiner's triads also exist in the columns of Newlands' Octaves? Compare and find out.
Ans:- Döbereiner's triad were also found in Newlands's Octave. For example, Li, Na, K.
Q.02:- What were the limitations of Döbereiner's classifications?
Ans:- Döbereiner could form only three triads from the elements known at that time.
Q.03:- What were the limitations of Newlands' Law of Octaves?
Ans:- There are only 56 elements known at the time of Newland. Only elements up to calcium (At. wt. = 40) could be fitted into his scheme octaves. In order to fit elements into his table, he adjusted two elements in the same slot. For example, Co and Ni were put in one slot in the note containing F, Cl, Br which have different properties. Similarly iron which resembles Co and Ni was placed far way.
NCERT INTEXT QUESTIONS (PAGE NO. 85)
Q.01:- Use Mendeleev's Periodic Table to predict the formulae for the oxides of the following elements :-
K, C, Al, Si, Ba
Ans:- K2O, C02, Al2O3, SiO2, BaO.
Q.02:- Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic table?(any two)
Ans:- Scandanium and Germanium
Q.03:- What were the criteria used by Mendeleev in creating his Periodic Table?
Ans:- Mendeleev used the relationship between the atomic masses of the elements and their physical and chemical properties. Among chemical properties he examined the compound formed by elements with oxygen and hydrogen. He found that if the 63 elements known at that time were arranged in the increasing order of their atomic masses, the properties of elements and also formulae of their oxides and hydrides gradually changed from element to element and at a certain interval they suddenly started almost repeating. This relationship was expressed by Mendeleev as Periodic Law, i.e., "the properties of elements are the periodic functions of their atomic masses."
Q.04:- Why do you think the noble gases are placed in a separate group?
Ans:- Noble gases like helium (He), neon (Ne), and argon (Ar) are chemically inert and present in atmosphere in extremely low concentrations. Thus owing to their similar inert behavior and and similar electron configuration, they are justified to be placed in a separate group.
NCERT INTEXT QUESTIONS (PAGE NO. 90)
Q.01:- How could the Modern Periodic Table remove various anomalies of Mendeleev's Periodic Table?
Ans:- Because of placement of elements in increasing atomic masses in Mendeleev's periodic table, certain chemically similar elements were separated and chemically dissimilar elements were to be grouped together. For example, Argon with atomic mass 39.948 needs to be placed after potassium with atomic mass 39.0983. Nickel with atomic mass 58.69 was placed before cobalt with atomic mass 58.93. But as per their chemical properties, these are placed in reversed order.
As the modern law is based on atomic number or number of electrons, elements with similar valence electron fall in the same group. Since the chemical properties of elements depend on their valence electrons, anomalies in the Mendeleev's table are automatically removed.
Further placement of hydrogen with alkali metals is now justified on the basis of its electronic configuration.
Q.02:- Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis of your choice?
Ans:- Beryllium (Be) and Calcium (Ca).
Basis of choice :- Both Be (At. no. 4) and Ca (At. no. 20) have electronic configuration similar to magnesium, i.e., two electrons in outermost shells.
Mg 2, 8, 2
Be 2, 2
Ca 2, 8, 8, 2
Both Be and Ca react with oxygen to give basic oxides :- BeO and CaO.
Q.03:- Name
(a) three elements that have a single electrons in their outermost shells.
(b) two elements that have two electrons in their outermost shells.
(c) three elements with filled outermost shells.
Ans:- (a) Lithium, sodium, potassium.
(b) Beryllium, magnesium.
(c) Helium, Neon, Argon.
Q.04:- (a) Lithium, sodium, potassium, are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What if anything, do their atoms have in common?
Ans:- (a) The atoms of lithium, sodium, and potassium all have only one electron in their outermost shells.
(b) The atoms of helium and neon have their outermost shell completely filled, i.e., are octets.
Q.05:- In the Modern Periodic Table, which are the metals among the first ten elements?
Ans:- Li and Be are metals.
Q.06:- By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?
Ga, Ge, As, Se, Be
Ans:- Be.
