Question-2. What is Dobereiner's triple law?

Ans- According to Dabereiner's triple law:– If three elements with similar properties are arranged in ascending order of atomic masses, then the atomic mass of the middle element is approximately equal to the average of the atomic masses of the remaining two elements.

Question-3. What are the limitations of Dobereiner's classification?

Ans- Dobereiner's classification has the following limitations—

• All the elements could not be classified on the basis of triad

• This triple rule of his was limited to a few elements.

• Of the elements known up to that time, only triads could be known.

Question-4. What is Newlands' octet theory?

Ans- According to Newlands's octet theory:– Newlands arranged the elements in increasing order of their atomic mass and found that every eighth element has the same property as the first element, just as the eighth note in music is the same as the first note.

Question-5. What are the limits of Newlands' octet theory?

Ans- Newlands' octet theory has the following limitations–

• His theory could be applied only up to calcium.

• This principle did not apply to elements with higher atomic mass.

• Some dissimilar elements were kept in one place.

• This principle was only properly applied to lighter elements.

Question-6. What is Mendeleev's periodic law?

Ans- Mendeleev's periodic law–

According to Mendeleev's periodic law, the properties of elements are periodic functions of their atomic masses.

In other words, according to Mendeleev's periodic law, if the elements are arranged in the order of their increasing atomic masses, then after a certain number of them elements with almost the same properties will be found.

Question-7. Write about Mendeleev's periodic table?

Ans- Mendeleev's periodic table has 6 horizontal rows and 8 vertical columns. Horizontal rows are called periods and vertical columns are called groups or squares. Mendeleev's periodic table was first published in a German magazine in 1872.

Question-8. What is the parameter of Mendeleev's periodic table?

Ans- Mendeleev's periodic table parameter–

• In Mendeleev's periodic table, the atomic masses of the elements were arranged in ascending order.

• Elements with similar properties are grouped together.

• The formulas of hydrides and oxides of elements were classified as a fundamental property.

Question-9. What are the achievements of Mendeleev's periodic table?

Ans- Following are the achievements of Mendeleev's periodic table—

• Mendeleev put elements with similar properties in a group.

• Mendeleev had left blanks for some elements in the periodic table, which were not known at that time.

• When inert gases were discovered later, they were placed in a new group without disturbing Mendeleev's periodic table.

Question-10. What are the main features of Mendeleev's periodic table?

Ans- Following are the main features of Mendeleev's periodic table—

• Facilitates the general study of the elements.

• Facilitates the invention of new elements.

• Knowledge of the nature of compounds of elements.

• Assistance in research work.

Question-11. What were the defects of Mendeleev's periodic table?

Ans- Mendeleev's periodic table had the following defects—

• Hydrogen could not find the correct place in Mendeleev's periodic table.

• There were no separate positions for isotopes in Mendeleev's periodic table.

• Mendeleev's periodic table placed one element in a group, but group VIII placed all three elements together.

• In Mendeleev's periodic table, elements with some similar properties were placed at different times, while they were to be kept at the same time.

Question-12. What is the modern periodic law?

Ans- According to the modern periodic law, the properties of elements are periodic functions of their atomic numbers.

Question-13. Write about the modern periodic table.

Ans- There are 7 periods and 18 groups in the modern periodic table. In these 7 periods, the first period is called very short period. The element on the left side of the modern periodic table is metal. The element on the right is a non-metal and the element shaded is a metalloid. This periodic table is divided into 4 blocks.

Question-14. What are the achievements (properties) of the modern periodic table?

Ans- Following are the achievements of the modern periodic table—

• • Hydrogen has been given a definite place in the modern periodic table.

  • Metals and non-metals are separated by metalloids.

  • Spaces are left in the periodic table for unknown elements.

  • Transition elements and inert gases are kept separate in the modern periodic table.

  • The positions of each of the elements are in the order of their electronic configuration.

  • Each period begins with an alkali metal (except the first period) and ends with an inert element.

Question-15. What is the difference between Mendeleev's periodic table and the modern periodic table?

Ans-

Question-16. How is the period of the modern periodic table determined?

Ans- find the period → The number of orbitals in which an element's electrons are distributed is the periodic number of the elements in the periodic table.

for example:-

• Atomic Number of Carbon (C) → 6

electronic configuration → 2, 4

number of classes → 2

period → 2

• Atomic Number of Calcium (Ca) → 20

electronic configuration →2, 8, 8, 2

number of classes → 4

period → 4

Question-17. How is a group found in the modern periodic table?

Ans- find the group:- The number of valence electrons of the elements will be the number of the group.

for example:-

• Atomic Number of Magnesium (Mg) → 12

electronic configuration → 2, 8, 2

valence electrons → 2

number of group → 2

• Atomic Number of Potassium (K) → 19

electronic configuration → 2, 8, 8, 1

valence electrons → 1

number of group → 1

Question-18. What is connectivity?

Ans- The number of electrons an element gives up, or gains, in order to complete its octet, is called the valency of the element.

जैसे– Atomic Number of Sodium (Na) → 11

electronic configuration → 2, 8, 1

Here it will give up 1 electron to complete the octet.

∴ Its valency will be → 1.

Question-19. Why does the metallic property of elements decrease while going from left to right in a period, and the non-metallic property increases?

Ans- From left to right in a period, the metallic properties of the elements decrease and the non-metallic properties increase, as the size of the atoms increases, which increases the force of attraction of the nucleus on the electron, making it difficult to remove electrons from atoms but less external. It is easy to add electrons. Therefore, from left to right in a period, the metallic properties of the elements decrease, and the non-metallic properties increase.

Question-20. Electronegativity increases from top to bottom in a group of metals, and electronegativity decreases in a group of non-metals, why?

Ans- The tendency of elements to lose electrons increases from top to bottom in a group of metals. Therefore, electronegativity increases in the group of metals whereas the tendency of the elements to accept electrons decreases in the group of non-metals. Therefore, the electronegativity decreases in the group of non-metals.

Question-21. Are Dobereiner's triples also found in Newlands's octave column?

Ans- Yes, Dobereiner's triads are also found in Newlands's octal column. Eg: - Lithium (Li), sodium (Na), and potassium (K) are Dabereiner's triads, which are also found in Newlands's Octave column.

Question-22. What other elements have been discovered so far apart from gallium, for which Mendeleev left a blank in his periodic table? Give two examples.

Ans- Scandium and Germanium.

Question-23. Why do you think noble gases were placed in a separate group?

Ans- Noble gases were discovered after the creation of Mendeleev's periodic table and these gases do not reactive. Therefore it was placed in a separate group without disturbing Mendeleev's periodic table.

Question-24. Name two elements showing chemical reactivity like magnesium. What is the basis of your selection?

Ans- Names of two elements showing chemical reactivity like magnesium are calcium and magnesium.

The basis of our selection is as follows—

• • Elements of the same group have the same chemical reactivity.

  • They have the same valence electron.

Question-25. Name the following.

(a) Three elements that have 1 electron in their outermost shell.

Ans-

∵ There is 1 electron in the outer shell.

∴ The element belongs to the group → 1.

So the three group 1 elements are Lithium (Li), Sodium (Na), Potassium (K).

(b) दो तत्वों जिनके सबसे बाहरी कोश में दो इलेक्ट्रॉन उपस्थित हो।

Ans-

∵ has 2 electrons in the outer shell

∴ The element belongs to the group → 2.

Therefore, the elements of group 2 are the two elements → magnesium (Mg), calcium (Ca).

(c) Three elements whose outer shell is complete.

Ans-

∵ The outer shell is complete.

∴ The element belongs to the group → 18

So the three elements of group 18 are helium (He), neon (Ne), argon (Ar).

Question-26. Lithium, sodium, potassium, all these metals react with water to release hydrogen gas. Do the atoms of these elements have any similarities?

Ans- Yes, there is a similarity in the atoms of these three elements, because these three elements react similarly with water. So all the three elements belong to the same group. Therefore, these three elements will have the same number of electrons in the outermost shell.

Question-27. Helium is an inert gas, whereas the reactivity of neon is very low. Do their atoms have any similarities?

Ans- Both helium and neon have low reactivity, that is, both are elements of group 18. Therefore the outermost orbitals of these elements are completely filled with electrons.

Question-28. Which metal is in the first ten elements of the modern periodic table?

Ans- The first 10 elements in the modern periodic table—lithium, beryllium—are two metals, boron is a metalloid, and the remaining seven elements—hydrogen, helium, carbon, nitrogen, oxygen, fluorine, and neon—are nonmetals.

Question-29. Which of these elements is characterized by the most metallic characteristics in place of them in the periodic table? Ga, Ge, As, Se, Be

Ans- Among these elements, Be is located on the left side of the periodic table, and the rest of the elements are located on the right side. The metallic properties of elements decrease as we move from left to right in the periodic table. Therefore, the Be element will have the most metallic character.

Question-30. Element X forms a chloride with the formula XCl₂, which is a solid and has a high melting point. Which group in the periodic table would it possibly belong to?

Ans-

∵ Element X chloride forms XCl₂.

∴ The valency of element X is 2.

Question-31. What properties do all the elements of the boron column in the periodic table have in common?

Ans- The valence electron of all the elements in the column of boron in the periodic table is 3. Hence all elements will be electropositive i.e. all elements are metals. Therefore, all the elements will be good conductors of electricity and heat and will have similar chemical properties.

Question-32. What properties do all the elements in the column of fluorine have in common in the periodic table?

Ans- The valence electrons of all the elements in the column of fluorine (atomic number 9) in the periodic table are 7. Therefore, all the elements will be electronegative i.e. all the elements are non-metals. Therefore, all these elements will be poor conductors of electricity and heat and will have similar chemical properties.

Question-33. The electronic configuration of an atom is 2, 8, 7.

(a) What is the atomic number of this element.

Ans-

Electronic configuration of atom → 2, 8, 7

Atomic number → 2 + 8 + 7 = 17

(b) With which of the following elements will they have chemical similarity?

N(7), F(9), P(15), Ar(18)

Ans-

Electronic configuration of F(9)→ 2, 7

Electronic configuration of the given element → 2, 8, 7

Therefore, element with atomic number 17 will have chemical similarity with element F(9), as both have the same number of valence electrons.

Question-34. Nitrogen (atomic number 7) and phosphorus (atomic number 15) are elements of group 15 of the periodic table. Write the electronic configuration of these two elements. Which of these elements will be more electronegative and why?

Ans-

Electronic configuration of nitrogen (atomic number 7) → 2, 5

Electronic configuration of phosphorus (atomic number 15) → 2, 8, 5

The valency electron of these two elements is 5. So both the elements are elements of group 15. In group 15, nitrogen (N) comes up and phosphorus (P) comes down. Electricity decreases from top to bottom in a group of non-metals. Hence nitrogen will be more electronegative.

Question-35. How is the electronic configuration of elements related to the position of the elements in the modern periodic table?

Ans- The modern periodic table is based on the atomic numbers of the elements. The basis of classification of elements is their electronic configuration. By electronic configuration we can find its period and group. Through the electronic configuration of the elements, we can divide the elements into metals and non-metals and also know their chemical properties, thus the electronic configuration of the elements is related to the position of the element in the modern periodic table.

Question-36. Compare the arrangement of elements in Mendeleev's periodic table and modern periodic table, that is, find the difference between the two.

Question-37. In the modern periodic table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these elements have similar physical and chemical properties to calcium?

Ans-

element atomic number electronic configuration

• 20 2, 8, 8, 2

• 12 2, 8, 2

• 19 2, 8, 8, 1

• 21 2, 8, 8, 3

• 38 2, 8, 18, 8, 2

Since elements with atomic numbers 20, 12 and 38 have 2 electrons in their outer shell. Hence, they have the same number of valence electrons. Therefore all three are members of the same group (class). All the elements of a group have the same chemical properties, but there are common elements with atomic number 12 and 20 and transition element with atomic number 38. Normal elements and transition elements are in the same group, even though their chemical properties differ. Therefore, elements with atomic numbers 12 and 20 will have similar physical and chemical properties.

Question-38. The positions of the three elements A, B and C in the periodic table are as follows.

Group 16 Group 17

............. .............

............. A

............. .............

B C

tell that,

(a) Is A a metal or a non-metal?

⇒ non-metal

(b) Is C more reactive than A, or less?

⇒ less

(c) Will the size of C be bigger or smaller than B?

⇒ smaller

(d) What type of ion will element A form, cation or anion?

⇒ anion

Question-39. in which element,

(a) Has two shells, and both are filled with electrons?

⇒ Neon (Ne) → [Ne(10)= 2, 8]

(b) The electronic configuration is 2, 8, 2?

⇒ Magnesium [Mg(12)= 2, 8, 2]

(c) There are three shells in total and four electrons in the valence shell?

⇒ Silicon (Si) = [Si(14)= 2, 8, 4]

(d) There are two shells in total and three electrons in the valence shell?

⇒ Boron (B)= [B(15)= 2, 3]

(e) The second shell has twice as many electrons as the first shell?

⇒ Carbon(C)= [C(6)= 2, 4]

Question-40. How did the modern periodic table remove various anomalies in Mendeleev's periodic table?

Ans- Various discrepancies in Mendeleev's periodic table were removed by the modern periodic table as follows—

• The position of hydrogen in Mendeleev's periodic table was uncertain, whereas in the modern periodic table, hydrogen was kept at one place.

• When the elements are arranged in increasing order of their atomic number, the elements with higher atomic mass which came before the elements of lower atomic mass are automatically corrected.

• In Mendeleev's periodic table, three elements were placed together in group VIII, but in the modern periodic table they automatically moved to separate groups based on atomic number.

• Inert gases had no place in Mendeleev's periodic table, but in the modern periodic table, inert gases were placed in the 18th group.

Question-41. Using Mendeleev's periodic table, estimate the formula for the oxides of the following elements.

K, C, Al, Si, Ba

Ans-

element Oxide Formula

K → K₂O

C → CO₂

Al → Al₂O₃

Si → SiO₂