Periodic Classification of Elements (Question Answer) 

QUESTIONS

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

Answer: Döbereiner’s triads also exist in the columns of Newlands’ Octaves. Eg- Li, Na, K. If Li is the first element then Na is the eighth element and their properties are the same. Similarly, if Na is considered to be the first element, then the element K will be the eighth element.

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

Answer: Only three triads could be identified from among the elements known at that time. Therefore, this method of classification as a triad was not useful.

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

Answer: The limitations of Newlands' Law of octave are-

1. Newlands' Law of octave was found to be applicable only up to calcium. After calcium, the properties of eighth elements was not the same as the first element.

2. Newlands' knew that there were only 56 elements and assumed that no more elements would be discovered in the future. However, with the subsequent discovery of many new elements, their properties did not conform to the Octave law.

3. Newlands' not only placed the two elements in the same place in the arrangement of the elements in his list, but also includes some elements with different properties in the same column. For example, cobalt and nickel have the same place in the list and are placed in the same column as fluorine, chlorine and bromine. However, their properties are quite different from each other. Properties of iron are similar to that of cobalt and nickel, although this element are placed far apart from each other. Thus, Newlands' octave formula seemed to be only applicable to lighter elements.

4. Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements: 

K, C, AI, Si, Ba. 

Answer: Formulae for the oxides of these  elements: 

             K2O, CO2, Al2O3, SiO2, BaO

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

Answer: Scandium and Germanium.

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

Answer: Mendeleev created his periodic table based on the relationship between the atomic mass of the elements and their physical and chemical properties. He also focused on the tendency of elements to form oxides and hydrides.

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

Answer: Noble gases are chemically inert and exhibit similar properties. This is why they are placed in a separate group.

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

Answer: The Modern Periodic Table removes the various anomalies of Mendeleev's periodic Table as follows:

            (1) Mendeleev's Periodic Table could not give the correct place to hydrogen. However, the modern Periodic Table eliminates this problem and gives hydrogen a specific place.

            (2) The isotopes of the elements were discovered after Mendeleev's classification and created a challenge against Mendeleev's Periodic Table. But the modern Periodic Table eliminates this problem and gives the isotopes the same place.

            (3) Atomic number is the basis for determining the place of an element in the modern Periodic Table. So the problem of locating the elements was solved.

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

Answer: Beryllium (Be) and Calcium (Ca) are two elements that show chemical reactions similar to magnesium. This is because their electronic configuration is similar to that of magnesium [Mg (12) - 2,8,2; Be (4) - 2,2; Ca (20) - 2,8,8,2) and each of them has two electrons in the outermost shell. 

10. 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) Lithium [Li (3) - 2,1], Sodium [Na (11) - 2,8,1] and Potassium [K (19) - 2,8,8,1]

                 (b) Beryllium [Be (4) - 2,2] and Magnesium [Mg (12) - 2,8,2]

                 (c) Helium [He (2) - 2], Neon [Ne (10) - 2,8], Argon [Ar (18) - 2,8,8]

11. (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) These elements have atomic similarities. Each of them has one electron in the outermost shell.

(b) The outermost shells of the helium and neon atoms are completely filled with two electrons. Therefore, they are inactive.

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

Answer: Lithium, beryllium and boron are metals.

13. 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: Be has maximum metallic characteristic.

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. 

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) Al        (d) Si 

Answer: (b) Mg

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 (10) - 2,8]

                 (b) Magnesium [Mg(12)- 2,8,2]

                (c) Silicon [Si (14) -2,8,4]

                 (d) Boron [B (5) - 2,3]

                 (e) Carbon [C (2,4)]

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) Each of the elements in the same column with Baron in the periodic table has three electrons in the outermost shell and therefore has the same chemical properties.

              (b) Each of the elements in the same column with fluorine in the periodic table has seven electrons in the outermost shell and therefore has the same chemical properties.

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) Atomic number of the atom having electronic configuration 2,8,7 is 17.

(b) The element will be chemically similar to F(9).

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) A is a non-metal.

(b) C is less reactive than A.

(c) A will form anion.

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 electro-negative? Why? 

Answer: Electronic configuration of-

                Nitrogen (7) is - 2,5

                Phosphorus (15) is - 2,8,5

        Nitrogen will be more electro-negative because the outermost shell of nitrogen is second shell and because it is close to the nucleus, nitrogen can easily accept electrons. Therefore, nitrogen will be more electro-negative than phosphorus.

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

Answer: The electronic configuration of an atom is intimately linked to its place in the modern periodic table. From the electronic configuration of any element, the position of the element can be determined in the periodic table. In the electron configuration of an atom, the number of electrons in its outermost shell indicates the group number of the element. On the other hand, the total number of cells in an element indicates the period at which the element will remain. For example, the electronic configuration of sodium is 2,8,1; Thus, sodium will be the group I in the periodic table and since the element has three orbits, it will be in the third period.

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:  The electronic configurations of the elements will be-

                 Calcium (20) - 2,8,8,2

                 Atomic number (12) - 2,8,2

                 Atomic number (19) - 2,8,8,1

                 Atomic number (21) - 2,8,8,3

                 Atomic number (38) - 2,8,18,8,2

        Thus, Elements with atomic number 12 i.e. magnesium and strontium ( atomic no.-38) will have physical and chemical properties resembling calcium, because both of them are in the same group in the periodic table with equal number of electrons in the outermost shell.

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

Answer: 

Mendeleev's periodic table	Modern periodic table
1. The elements are arranged in increasing order of their atomic masses.	1. The elements are arranged in increasing order of their atomic numbers.
2. There are 9 groups in Mendeleev's periodic table.	2. The modern periodic table has 18 groups.
3. There is no place for noble gases.	3. There is a designated place for noble gas.
4. The hydrogen could not be assigned a specific place.	4. Hydrogen has a definite place.
5. The isotopes could not be assigned a specific place.	5. Isotopes have specific place.