Q1.   Did DobereinerA’s triads also exist in the columns of Newland’s Octaves? Compare and find out.
Ans. Yes, it exists in the Newland’s octave. Li, Na and K constitute the triad whereas its also found in the second column of Newland’s octave.
Q2.   What were the limitations of Dobereiner’s classification?

Ans:- Two important limitations are:

i)Dobereiner could find only three triads; i.e. a total of 9 elements only.
thus all known elements could not be classified into groups of triads on the basis of their properties.

ii)Not all groups obeyed the Law of Triads. For example, the nitrogen family does not obey the Law of Triads.
Q3. What were the limitations of Newland’s Law of Octaves?
Ans:- 1) It was applicable only up to calcium, as after calcium every 8th element did not possess properties similar to that of the first.
2) Newlands assumed that only 56 elements existed and no more elements will be discovered. But, later on, several new elements were discovered that did not fit into Law of Octaves.
3) In order to fit elements in his table, Newlands adjusted 2 elements in the same slot but also put some unlike elements under the same slot. e.g. Co and Ni were put in one slot containing F, Cl, Br which have different properties. Similarly, iron which resembles Co and Ni was placed far away.
Q4.   Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements:
K, C, Al, 1Si, Ba
Ans:-
$K-K_{2}O\linebreak&space;C-CO_{2}\linebreak&space;Al-Al_{2}O_{3}\linebreak&space;Si-SiO_{2}\linebreak&space;Ba-BaO&space;\linebreak$
Q5.   Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two)
Ans. Besides gallium, scandium(group III B) and germanium(group IV A)  have been discovered.
Q6.   What were the criteria used by Mendeleev in creating his Periodic Table?
Ans.  The criteria used by Mendeleev in creating his Periodic Table are as follows —-
(i)Mendeleev arranged the elements on the basis of their  Increasing order of atomic mass.
(ii) he kept all elements in a group with similar properties.
(iii) he treated the formula of hydrides and oxides formed by an element as one of the basic properties for its classification.
Q7.   Why do you think noble gases are placed in a separate group?
Ans. Noble gases namely helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and Radon (Rn) are placed separately in the periodic table because :

A) Their outer electronic shell configuration is completely filled. So they are inert (unreactive) in nature.
B)They exist in very low concentration in nature.
C) They all exhibit very different properties from all other elements of the periodic table.

Q8.   How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?

Ans:- The Modern Periodic Table could remove various anomalies of Mendeleevs Periodic Table:

a)there is no separate provisions for isotopes in the Mendeleev’s table but When the elements are arranged according to their atomic numbers on the basis of the modern periodic law, all the anomalies of Mendeleev’s classification disappear. For example-all the isotopes of an element have the same atomic number, they can be put at one place in the same group of the periodic table.

b)wrong order of argon, potassium, cobalt and nickel was removed.

Q9.   Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Ans. Calcium and barium.
Reason: both Calcium and barium have the same valency and lie in the same group of the periodic table as Mg.
Q10.   Name:
(a) Three elements that have a single electron in their outermost shells.
(b) vo elements that have two electrons in their outermost shells.
(c) Three elements with filled outermost shells.
Ans:- (a)Lithium (2.1), Sodium (2.8.1), and potassium (2.8.8.1).
(b) beryllium (2.2) and magnesium (2.8.2)
(c)Helium (2) Neon (2.8) and Argon (2.8.8)
Q11.   (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) All these metals have the same number of outermost electrons i.e. 1 and can readily lose electrons to become positive ions.
(b) Helium and neon have completely filled outermost shell (inert gas).
Q12.   In the Modem Periodic Table, which are the metals among the first ten elements.
Ans. Lithium and beryllium are metals.
Q13.   By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristics?
Ga, Ge, s, Se, Be
Ans. Among the given elements Be will show maximum metallic characteristics as it belongs to the extreme left of the Periodic Table.
EXERCISE —
Q1.   Which of the following statements is not a correct statement about the trends when going from, left to right across the periods of the Periodic Table?
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily,
(d) The oxides become more acidic.
Ans. (c) The atoms lose their electrons more easily.
Q2.   Element X forms a chloride with the formula,$XCl_{2}$ which is solid with a high melting point. X would most likely be in the same group of the Periodic Table as
(a) Na           (b) Mg           (c) Al           (d) Si
Ans. (b) Mg.
Q3.   Which element has
(a) two shells, both of which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as in its first shell?
Ans. (a) Ne (2, 8)
(b) Mg (2, 8, 2)
(c) Si (2, 8, 4)
(d) B (2, 3)
(e) C (2, 4)
Q4.   (a) What property do all elements in the same column of the Periodic Table as boron have in common?
(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?
Ans. (a) All the elements which lie in the same column as that of boron belong to group 13. hence they have the same valence electrons and their valency is 3.
(b)The elements which lie in the same column as fluorine belong to group 17 and thus have 7 electrons in the valence shell and hence valency is 1. All these elements (fluorine, chlorine, bromine, iodine and astatine) are called halogens and these are non-metals.
Q5.   An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
(b) To which of the following elements would it be chemically similar?
(Atomic numbers are given in parentheses)
N(7) F(9) P(15) Ar(18)
Ans. (a) The atomic number of the element is 17.
(b) F(9) (2, 7) will be chemically similar to the given element.
Q6.   The position of three elements A, B and C in the Periodic Table is shown below:

(a) State whether A is a metal or non-metal.
(b) State whether Cis more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion, cation or anion, will be formed by element A?

Ans:- (a) ‘A’ is non-metal.

(b) ‘C’ is less reactive than ‘A’
(c) ‘C’ is smaller in size than ‘B’
(d) ‘A’ will form an anion.
Q7.   Nitrogen (atomic number 7) and phosphorous (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of the these will be more electronegative? Why?
Ans. Nitrogen atomic number 7 → 2, 5
Phosphorus atomic number 15 → 2, 8, 5
Nitrogen with two shells will be more electronegative because it can easily gain electron due to its smaller size of an atom than Phosphorus.
Q8.   How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Ans. Electronic configuration of an element tells us the no. of the electron in it which is used in finding the valence electron and the atomic number of the element.
The no. of an electron in the outer most shell specifies the position of an element in the periodic table. For example Oxygen.The atomic number of Oxygen is 16
Electronic configuration of oxygen – 2 8 6.
For any element number of valence electrons is equal to its group number. Here oxygen contains 6 valence electrons hence, it belongs to VI A group.
For any element, the valence shell number is equal to its period number.
In oxygen 3rd shell is the outermost shell (Valence shell) hence, it belongs to period 3.
Q9.   In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?
Ans. calcium atomic number — 20
Electronic configuration — 2, 8, 8, 2
Elements with atomic number 12 → 2, 8, 2
and atomic number 38 → 2, 8, 18, 8, 2
will resemble calcium as they all have same valence electrons and their chemical properties are also same.
Q10.   Compare and contrast the arrangement of elements in Mendeleev’s, Periodic Table and the Modern Periodic Table.
Ans:- 1) In Mendeleev’s periodic table, Elements are arranged in the increasing order of their atomic masses.
In the Modern periodic table, Elements are arranged in the increasing order of their atomic numbers.
2) In Mendeleev’s periodic table, there are 8 Groups.
In the Modern periodic table, there are 18 Groups.
3)In Mendeleev’s periodic table, Each group are subdivided into subgroup ‘a’ and ‘b’
In the Modern periodic table, groups are not subdivided into sub-groups.
4) In Mendeleev’s periodic table, groups for Noble gas was not present as noble gases were not discovered by that time.
In the Modern periodic table, A separate group is meant for noble gases.
5) In Mendeleev’s periodic table, there was no place for isotopes.
In the Modern periodic table, This problem has been rectified as slots are determined according to atomic number.