Periodic Classification of Elements

NCERT Activity Solution

Activity: 5.1

Question:1 Looking at its resemblance to alkali metals and the halogen family, try to assign hydrogen a correct position in Mendeleev's Periodic Table.

Answer: Mendeleev considered formulae of hydrides and oxides formed by an element as one of the basic properties to classify the elements in his Periodic Table.

Hydrogen makes oxides similar to alkali metals, such as H₂O (Hydrogen oxide i.e. water) and hydrides similar to halogen such as LiH (Lithium hydride), so it could be placed with halogens in the same group i.e. in VII^th group. But, hydrogen was placed with alkali metals in I^st group in Mendeleev's Periodic Table.

It appears that hydrogen is placed as starting element in 1^st period and I^st group in the Mendeleev's Periodic Table. Thus, I think it was the correct position as hydrogen has been placed the same position even in Modern Periodic table.

Question:2. To which group and period should hydrogen be assigned?

Answer: Hydrogen should be assigned in 1^st period and I^st group. As I think Mendeleev also thought and considered hydrogen as starting element and gave the first place in his Periodic Table.

Activity: 5.2

Question:1.

Consider the isotopes of chlorine, Cl-35 and Cl-37.

Would you place them in different slots because their atomic masses are different?

Or would you place them in the same position because their chemical properties are the same?

Answer: Although the atomic masses of isotopes of chlorine, Cl-35 and Cl-37 are different even that I would like to place them in the same position because their chemical properties are the same. And Mendeleev also placed many of elements in the same slot even with different atomic masses because their chemical properties are similar, such as Fe (Iron), Cobalt (Co) and Nickel (Ni) were placed in VIII^th collectively.

Activity: 5.3

Question:1. How were the positions of cobalt and nickel resolved in the Modern Periodic Table?

Answer: Modern Periodic Table is based on the Atomic Number of elements and elements are arranged in the increasing order of their atomic number.

Cobalt has atomic number equal to 27 and nickel has atomic number equal to 28. Thus these elements are placed in their correct order of increasing atomic number.

In Mendeleev's Periodic Table also these elements had same position and order.

Question:2. How the position of isotopes of various elements decided in the Modern Periodic Table?

Answer: Modern Periodic Table is based on the Atomic Number of elements and elements are arranged in the increasing order of their atomic number.

Since, isotopes has same atomic number but different atomic masses, and in Modern Periodic Table elements are arranged in the increasing order of their atomic numbers, thus there is no need to assign separate place for them.

Question:3. Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?

Answer: Atomic number 1.5 is not possible as atomic number is equal to number of protons and equal to number of electrons. It is not possible of existence of half proton or electron.

So, there is no question of placement of such element as existence of such element is not possible.

Question:4. Where do you think should hydrogen be placed in Modern Periodic table?

Answer: Since, elements having same valence electrons are placed in same group and hydrogen have valence electron equal to one, so hydrogen is placed with the elements having same number of valence electrons, in first group. Thus, position of hydrogen in Modern Periodic Table is correct.

Activity: 5.4.

Question:1. Look at the group 1 of the Modern Periodic Table, and name elements present in it.

Answer: Name of elements placed in group 1^st in Modern Periodic Table are

Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs) and Francium (Fr).

Question:2. Write down the electronic configuration of the first three elements of group 1.

Answer: First three element in group 1 are Hydrogen (H), Lithium(Li) and Sodium(Na)

Electronic configuration of Hydrogen (Atomic number 1) = 1

Electronic configuration of Lithium (Atomic number: 3) = 2, 1

Electronic configuration of Sodium (Atomic number: 11) = 2, 8, 1

Question:3. What similarity do you find in their electronic configuration?

Answer: Hydrogen (H), Lithium(Li) and Sodium(Na) all have one electron in their outermost orbit (shell).

Question:4. How many valence electrons are present in these three elements?

Answer:

Valence electron present in Hydrogen (H) = 1

Valence electron present in Lithium (Li) = 1

Valence electron present in Sodium (Na) = 1

Activity: 5.5.

Question:1. If you look at the long form of the Periodic Table, you will find that the elements Li, Be, B, C, N, O F, and Ne are present in the second period. Write down their electronic configuration.

Answer:

Electronic configuration of Lithium (Atomic number: 3) = 2, 1

Electronic configuration of Beryllium (Atomic number: 4) = 2, 2

Electronic configuration of Boron (Atomic number: 5) = 2, 3

Electronic configuration of Carbon (Atomic number: 6) = 2, 4

Electronic configuration of Nitrogen (Atomic number: 7) = 2, 5

Electronic configuration of Oxygen (Atomic number: 8) = 2, 6

Electronic configuration of Fluorine (Atomic number: 9) = 2, 7

Electronic configuration of Neon (Atomic number: 10) = 2, 8

Question:2. Do these elements contain the same number of valence electrons?

Answer: Now these elements do not contain same number of valence electrons.

Valence electron of Lithium (Li) = 1

Valence electron of Beryllium (Be) = 2

Valence electron of Boron (B) = 3

Valence electrons of Carbon (C) = 4

Valence electron of Nitrogen (N) = 5

Valence electron of Oxygen (O) = 6

Valence electron of Fluorine (F) = 7

Valence electrons of Neon (Ne) = 0

[Valence electrons becomes zero if outermost orbital is completely filled, thus all inert gases have valence electrons equal to zero as all of them have outermost orbit completely filled. And this is the cause they are unreactive gases.]

Question:3. Do they contain the same number of shells?

Answer: Yes they all contain same number of shells i.e. two. Because of this they all are placed in period 2 (two).