5. Periodic Classification of Elements

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Questions :

1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.

Answer: Yes, not all but some Döbereiner’s triads also exist in the columns of Newlands’ Octaves. Like, a döbereiner’s triad was Li, Na, K and can also be found in 2^nd column of Newlands' Octaves.

2. What were the limitations of Döbereiner’s classification?

Answer: There were many limitations of Döbereiner’s classification, which are:
i. It was able to found only 3 triads at that time.
ii. The method was not able to classify all known elements, even at that time and even at present.
iii. The classification couldn't classify elements with high or low atomic mass elements, like hydrogen and oganesson.
iv. Even in the triads, the method had discovered, the atomic mass of the middle element is only roughly average of the other two and not accurate, so it lacks accuracy as in present time, classification with highest accuracy is needed.
v. Even if all the elements had been classified in triads, the number of triads would have been so large and inconvenient that even then they would be of no use.
vi. They cannot give us a complete and large scale way of classification, in which many elements can be classified in one group and also it doesn't give us periodic property's trends.

3. What were the limitations of Newlands’ Law of Octaves?

Answer: There were many limitations of Newlands’ Law of Octaves, which are:
i. It was found that the Law of Octaves was applicable only upto Ca, as after Ca every eighth element did not possess properties similar to that of the first.
ii. Newland didn't left any gaps in his Octaves and the method didn't predicted any new elements, instead it assumed that only 56 elements existed in nature and no more elements would be discovered in the future.
iii. The elements discovered later on did not fit into the Law of Octaves.
iv. He tried to adjust two elements in one slot and also put some unlike elements under the same note. As, in case of Co and Ni placed in halogen group.
v. Fe with similar properties to Co and Ni was placed far away.
vi. It didn't work well with heavier elements.
vii. It was not able to fit noble gases in itself.
viii. Like triads it also, cannot give us a entire network of classification, in which many elements can be classified in one group and also it doesn't give us periodic property's trends.

Questions :

1. Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements:
K, C, AI, Si, Ba.

Answer: The table put K where it stated oxide formulae as R₂O where R is the element's name hence, formulae for oxide of K is K₂O. Similarly, formulae for the oxides of C, Al, Si, Ba are CO₂, Al₂O₃, SiO₂, BaO, respectively.

2. Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (any two)

Answer: Besides Ga, Scandium and Germanium are the two elements, which were similar to Eka–boron and Eka–silicon predicted by him and have since been discovered, which were left by Mendeléev in his Periodic Table.

3. What were the criteria used by Mendeléev in creating his Periodic Table?

Answer: The following criteria were used by Mendeléev in creating his Periodic Table.
i. The hydrides and oxides formed by an element are the basic chemical properties of an element for its classification.
ii. A relation should be must in elements with similar physical and chemical properties and their atomic masses.
iii. The properties of elements are the periodic function of their atomic masses.
iv. Elements should be arranged in order of increasing atomic masses.
v. There should be a periodic recurrence of elements with similar physical and chemical properties, to make understanding easy and to find relation between their various properties and the properties and their causes.

4. Why do you think the noble gases are placed in a separate group?

Answer: Noble gases are placed in a separate group, due to their inert nature and complete octet. All other elements have valency 0 < V < 8 and are reactive but as inert gas's valency=8 is found in no other elements, hence they must be kept different due to their valency. Then as their existence became known when all the other elements were kept in order, (due to their inertness and presence in extremely low concentrations in atmosphere) so, in order not to disturb the existing discoveries and theories and ordered periodic table, they are kept in placed in a separate group, which also improves the quality of periodic table.

Questions :

1. How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?

Answer: It can do so in various ways, which are:
i. By Modern Periodic Law that properties of elements are a periodic function of their atomic number and not atomic mass as stated by Mendeleev. As, these removed the anomaly when an element with a slightly greater atomic mass was placed before an element with a slightly lower atomic mass, like in Co (atomic mass 58.9) appeared before Ni (atomic mass 58.7).
ii. These removed anomaly in case of isotopes as if they are placed according to Periodic Law, they should be kept in different positions and the entire table would have become chaos as now the isotopes which have similar chemical properties shouldn't have matched the formulae given on top. But, due to Modern Periodic Law the anomaly was solved as now all the isotopes were assumed to state in same position with atomic mass, the average of the atomic mass of all isotopes, with respect to their percentage on Earth.
iii. These removed anomaly in case of isobars too as according to Periodic Law, they should be kept in same position even if they had different chemical properties which had made everything chaos. But, due to Modern Periodic Law the anomaly was solved as now they are kept in different positions.

2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?

Answer: The 2 elements are Beryllium(Be) and Calcium(Ca), also Sr and Ba will show chemical reactions similar to Mg. As, we know that chemical reactions are based on the chemical properties of reactants and as chemical properties are dependent on the valency which is similar in elements of same group according to modern periodic table, hence elements in alkaline earth metal group, the group of Mg will show chemical reactions similar to Mg like, Be, Ca, Ba, Sr.

3. Name

(a) three elements that have a single electron in their outermost shells.
(b) two elements that have two electrons in their outermost shells.
(c) three elements with filled outermost shells.

Answer: (a) Sodium(Na), Hydrogen(H), Potassium(K) are the 3 elements that have a single electron in their outermost shells.
(b) Magnesium(Mg) and Calcium(Ca) are the 2 elements that have two electrons in their outermost shells.
(c) Helium(He), Neon(Ne) and Argon(Ar) are the 3 elements with filled outermost shells.

4. (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?

Answer: (a) Yes, there are lots of similarities in them which are:
i. All are of same group, alkali metal group.
ii. All have only 1 valence electron in their outermost shell.
iii. All are most electropositive and reactive elements, hence can easily react with water or even moisture in air.
iv. All are metals thus, form hydroxides on reacting with water, evolving hydrogen gas.

(b) The atoms have many things in common:
i. Both are inert or noble gases.
ii. Both have a complete outermost shell, hence are unreactive.
iii. Both don't react with even steam or any other substance, excluding some very rare cases.
iv. Both are gases at room temperature and have boiling point near to absolute zero.

5. In the Modern Periodic Table, which are the metals among the first ten elements?

Answer: In the Modern Periodic Table, Lithium(Li) and Beryllium(Be) are the metals among the first ten elements.

6. 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

Answer: In the following elements, Gallium is expected to have maximum metallic characteristic, due to his large atomic radius and ultimately greater tendency to lose electrons easily even it is in right side of Be. The order of elements with increasing metallic characteristic is Se < As < Ge < Be < Ga.

Exercises :

1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of 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.

Answer: (c) The atoms lose their electrons more easily. Reason: As, we know that on moving from left to right across the periods effective nuclear charge increases and hence, ability to lose electrons decreases not increases.

2. Element X forms a chloride with the formula XCl2, which is a 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) AI
(d) Si

Answer: (b) Mg. Reason: The formula is such that the valency of element X should be 2, as valency of Cl is 1 and there are 2 Cl atoms to be satisfied. Then, the element X should form a ionic bond with Cl not a covalent bond as an ionic bond has high melting point and is solid, in most cases. And, Mg in the options satisfy both the condition and form MgCl₂ hence it is the correct option.

3. 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?

Answer: (a) Neon(Ne) is the element with 2 shells, both of which are completely filled with electrons. As its electronic configuration is 2, 8.
(b) Magnesium(Mg) is the element which has the electronic configuration 2, 8, 2. As, it's atomic number is 12.
(c) Silicon(Si) is the element with a total of 3 shells, with 4 electrons in its valence shell. As, it's atomic number is 14 and electronic configuration is 2, 8, 4.
(d) Boron(B) is the element with a total of 2 shells, with 3 electrons in its valence shell. As, it's atomic number is 5 and electronic configuration is 2, 3.
(e) Carbon(C) is the element with twice as many electrons in its second shell as in its first shell. As, electrons to be in 2^nd shell, 1^st shell with 2 electrons is to be completed and so the required element will have electronic configuration of 2, 4, which is of C.

4. (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?

Answer: (a) All elements in the same column of the Periodic Table as boron have valency equal to 3 and electropositive characteristic in common and ultimately, chemical properties in common. Though, B is a metalloid and all other members of the group are metals, still they have chemical properties nearly same.
(b) All elements in the same column of the Periodic Table as fluorine have valency equal to 1 and high electronegative characteristic in common and ultimately, chemical properties in common.

5. 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)

Answer: (a) The atomic number of this element is 17 as the total number of electrons is equal to the atomic number, in case of neutral atom.
(b) Of the following it would be similar to F(9) as it has the same number of valence electrons as the given element Cl have or they are in same group.

6. The position of three elements A, B and C in the Periodic Table are shown below –

Group 16	Group 17
-	-
-	A
-	-
B	C

(a) State whether A is a metal or non-metal.
(b) State whether C is 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?

Answer: (a) Element A is a non-metal as elements in group 17 are all non-metals.
(b) Element C is less reactive than A. Because, the elements in group 17 are reactive due to their high electronegativity and electronegativity increases from bottom to top and element A is at top, with respect to element C.
(c) C will be smaller in size than B as, we know that atomic size decreases from left to right in a period and C and B are in same group.
(d) Element A will form a anion of charge equal to -1 as it is in group 17 and 2^nd position in group and is Cl element.

7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?

Answer: The electronic configuration of nitrogen is 2, 5 and electronic configuration of phosphorous is 2, 8, 5. In P and N, nitrogen(N) will be more electronegative as we know that in periodic table electronegativity increases as we move upward in a group, due to increasing effective nuclear charge or decrease in number of electron shell and atomic radius and ultimately, higher attractive force of nucleus on outermost shell's electrons.

8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?

Answer: Electronic configuration of an atom relates directly to its position in the Modern Periodic Table. As, we know that E.C tells us the number of shells of an atom which in turn decides the position of an atom, as the number of shells in an atom is equal to its period number. Then, as we know that the number of electrons in the valence shell of an atom decides its group number, with respect to period to which it belongs. If, the atom has less than 4 valence electrons, then it will be a part of group in left side of periodic table and if more than 4 then in right side. Also, if an atom have only 1 valence electron then it will be in 1^st group, if 2 then in 2^nd group and if 8 then in 18^th group, if 7 then in 17^th and in such lots of ways. So, in such ways the electronic configuration of an atom relate to its position in the Modern Periodic Table.

9. 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?

Answer: In these element with atomic number 12 will have properties resembling Ca the most as, it is like E.C of Ca, 2,8,8,2 it also have E.C 2,8,2 and as they both have same valency, hence there will be similarities in their physical and chemical properties, also element with atomic number 38 will have similarity with it but as the difference in electropositivity increases as we move down in group, so it will not be as similar to it as Mg. Then, as the remaining K and Sc have different valency their physical and chemical properties will not resemble Ca.
*E.C=Electronic Configuration.

10. Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.

Answer:

Mendeléev’s Periodic Table	Modern Periodic Table
1. It have 6 periods and 8 groups.	1. It have 7 periods and 18 groups.
2. The elements are arranged according to Periodic Law that is with respect to atomic mass.	2. The elements are arranged according to Modern Periodic Law that is with respect to atomic number.
3. There are subdivisions present in it.	3. There are no subdivisions present in it.
4. Every element doesn't have a unique position for it.	4. Every element have a unique position for it.
5. The arrangement not even follows Periodic Law completely.	5. The arrangement follows Modern Periodic Law completely.
6. The isobars would have been placed in same position.	6. The isobars are placed in different positions.
7. Oxides and hydroxides of the element is considered to be a fundamental criterion.	7. Oxides and hydroxides of the element is not considered to be a fundamental criterion.
8. Transition elements were kept in last group.	8. Transition elements are kept in center of table.
9. Group of noble gases is not present only.	9. There is a special group of noble gases.
10. Lanthanides and Actinides are not present separately.	10. Lanthanides and Actinides are present separately.
11. A space is not present between Be and B.	11. A space is present between Be and B.