NCERT Solutions for Class 10 Science Chapter 5: Periodic Classification of Elements

Periodic Classification of Elements

NCERT Solutions for Class 10 Science • Chapter 5

NCERT In-Text Questions

Question 1. Did Doberiener's triads also exist in the columns of Newlands' Octaves? Compare and find out.

Answer: Yes, some of Doberiener's triads did exist in the columns of Newlands' Octaves.

Explanation: For example, the triad of Lithium (Li), Sodium (Na), and Potassium (K) exists in both Doberiener's triads and Newlands' Octaves as they share similar chemical properties.

Question 2. What were the limitations of Doberiener's Triads?

Answer: All known elements could not be classified into groups of triads.

Explanation:

• Not all groups of elements obeyed the Law of Triads based on their atomic masses and properties.
• Only a limited number of triads could be identified at that time, making the system inadequate for overall classification.

Question 3. What were the limitations of Newlands' Law of Octaves?

Answer: The main limitations of Newlands' Law of Octaves are:

1. The law was found to be applicable only up to calcium. It was not applicable to elements of higher atomic masses.
2. He assumed that only 56 elements existed in nature and no more elements would be discovered in the future. Newly discovered elements did not fit into his octaves.
3. In order to fit elements into octaves, Newland adjusted two elements in the same slot (like Cobalt and Nickel) which were not similar in properties.

Question 4. Use Mendeleev's periodic table to predict the formulae for the oxides of the elements: K, C, Al, Si, Ba.

Answer:

• Potassium (K): \( K_2O \)
• Carbon (C): \( CO_2 \)
• Aluminium (Al): \( Al_2O_3 \)
• Silicon (Si): \( SiO_2 \)
• Barium (Ba): \( BaO \)

Explanation: Oxygen is a member of group VIA with a valency of 2. The formula can be predicted from their respective groups in Mendeleev's table:

• K is in group IA (valency 1) -> \( K_2O \)
• C is in group IVA (valency 4) -> \( C_2O_4 \) simplifies to \( CO_2 \)
• Al is in group IIIA (valency 3) -> \( Al_2O_3 \)
• Si is in group IVA (valency 4) -> \( SiO_2 \)
• Ba is in group IIA (valency 2) -> \( Ba_2O_2 \) simplifies to \( BaO \)

Question 5. Besides gallium, which two other elements have since been discovered that fill the gaps left by Mendeleev in his periodic table?

Answer: Scandium (Sc) and Germanium (Ge).

Explanation: Mendeleev predicted the existence of these elements and named them Eka-boron (Scandium) and Eka-silicon (Germanium), leaving gaps for them in his periodic table.

Question 6. What were the criteria used by Mendeleev in creating his periodic table?

Answer: Mendeleev used the atomic masses of the elements as the main criteria for creating his periodic table. He arranged the elements in the increasing order of their atomic masses and grouped elements with similar chemical properties together.

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

Answer: Noble gases are placed in a separate group (Group 18 or zero group) because they possess completely filled valence shells and have zero valency.

Explanation: In Mendeleev's Periodic Table, elements were arranged on the basis of valency. Since noble gases (He, Ne, Ar, Kr, Xe, Rn) are extremely unreactive and have zero valency, they did not fit into existing groups. Thus, they were added later into a separate 'zero group' without disturbing the existing order.

Question 8. How could the Modern Periodic Table remove various anomalies of Mendeleev's Periodic Table?

Answer: The Modern Periodic Table removed anomalies by arranging elements in the increasing order of their atomic numbers rather than atomic masses.

Explanation: Formulated by Henry Moseley in 1913, the Modern Periodic Law states that properties of elements are periodic functions of their atomic numbers. This resolved issues like:

• Isotopes: Since isotopes have the same atomic number, they did not need separate slots.
• Wrong Order of Atomic Masses: Anomalies like placing Cobalt before Nickel were resolved, as Cobalt (Z=27) correctly comes before Nickel (Z=28).

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

Answer: Calcium (Ca) and Strontium (Sr).

Explanation: Magnesium (Mg) belongs to group 2 (Alkaline Earth Metals). The basis for this choice is their electronic configuration; all these elements have exactly 2 electrons in their outermost valence shell, resulting in similar chemical properties.

Question 10. Name:
1. three elements that have a single electron in their outermost shells.
2. three elements that have two electrons in their outermost shells.
3. three elements with filled outermost shells.

Answer:

1. Lithium, Sodium, Potassium (Alkali metals, Group 1)
2. Beryllium, Magnesium, Calcium (Alkaline earth metals, Group 2)
3. Helium, Neon, Argon (Noble gases, Group 18)

Chapter End Questions

Question 1. 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.

Answer: (c) The atoms lose their electrons more easily.

Explanation: As we move from left to right across a period, the effective nuclear charge increases, which pulls the valence electrons closer to the nucleus. This makes it more difficult for atoms to lose electrons, meaning they become less metallic.

Question 2. Element X forms a chloride with the formula XCl2 (represented as XCl in some texts), which is a solid with a high melting point. X is most likely to be in the same group of the periodic table as:
(a) Na
(b) Mg
(c) Al
(d) Si

Answer: (b) Mg

Explanation: The formula indicates that the element X has a valency of 2 (forming an ionic chloride). Magnesium (Mg) belongs to Group 2 and has a valency of 2, forming \( MgCl_2 \), which is a solid with a high melting point.

Question 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 the valence shell?
(d) a total of two shells with three electrons in the valence shell?
(e) twice as many electrons in the second shell as in the first shell?

Answer:

• (a) Neon (Ne): Configuration is 2, 8.
• (b) Magnesium (Mg): Atomic number is 12 (2+8+2).
• (c) Silicon (Si): Configuration is 2, 8, 4.
• (d) Boron (B): Configuration is 2, 3.
• (e) Carbon (C): Configuration is 2, 4 (4 is twice of 2).

Question 4.
(a) Which property do all elements in the same column of the periodic table as boron have in common?
(b) Which 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 as Boron (Group 13) have exactly 3 valence electrons.
(b) All elements in the same column as Fluorine (Group 17) have exactly 7 valence electrons.

Explanation: Elements in the same group of the modern periodic table have identical outer shell electronic configurations, which is why they exhibit similar chemical properties.

Question 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 is 17 (Chlorine).
(b) It would be chemically similar to Fluorine, F(9).

Explanation:

• Atomic number = sum of electrons = 2 + 8 + 7 = 17.
• Fluorine has an atomic number of 9, meaning its electronic configuration is 2, 7. Both elements have 7 valence electrons, placing them in Group 17 (Halogens).

Question 6. Compare and contrast the arrangement of elements in Mendeleev's Periodic Table and the Modern Periodic Table.

Answer:

Mendeleev's Periodic Table	Modern Periodic Table
Based on atomic masses of the elements.	Based on atomic numbers of the elements.
Isotopes pose a problem as they have different atomic masses but similar properties.	Isotopes are easily accommodated since they have the same atomic number.
No justification for placing Hydrogen in Group IA.	Hydrogen's position is justified with alkali metals as both have one valence electron.
Except Group VIII, groups are divided into sub-groups A and B.	No sub-groups; all 18 groups are independent in nature.
Position of certain elements is anomalous (e.g., Cobalt before Nickel).	Anomalies are resolved (Cobalt Z=27 correctly placed before Nickel Z=28).