88 MCQs on Chapter 5: Periodic Classification of Elements | Class 10 Science
In this article, you will get high-quality MCQs with answers and suitable explanations on the Periodic Classification of Elements Chapter 5 Class 10 Science. It includes
- “SEBA 10 Years PYQ MCQs”
- “Assertion-Reason MCQs”
- “Statement-Based MCQs”
- “Match the Following MCQs”
To score 100 % marks in your HSLC/CBSE Board exam, we have compiled a collection of important Periodic Classification of Elements MCQs. We have provided simple Explanations so you can learn the underlying periodic trends easily.
Periodic Classification of Elements MCQs
1. Four elements W, X, Y, Z having atomic numbers 11, 13, 16 and 17 respectively. Which one among them has the most metallic property? [ HSLC-2026]
(A) X
(B) W
(C) Z
(D) Y
Explanation: The given atomic numbers belong to Sodium (W = 11), Aluminium (X = 13), Sulfur (Y = 16), and Chlorine (Z = 17). Metallic character represents the ability of an atom to lose electrons. As we move from left to right across a period. Since Sodium (W) is on the extreme left (Group 1), it is the most metallic element among them.
2. An element ‘Z’ has atomic number 9. In which period and group can it be placed in the Modern Periodic table? [ HSLC-2026]
(A) Period 2, Group 7
(B) Period 7, Group 17
(C) Period 7, Group 7
(D) Period 2, Group 17
Explanation: The atomic number is 9, so its electronic configuration is 2, 7.
Since there are 2 shells (K and L), it belongs to Period 2.
It has 7 valence electrons. For elements with more than 2 valence electrons, we add 10 to find the group number. So, 7 + 10 = 17 . So, it belongs to Group 17 (Fluorine).
3. By considering their position in the periodic table, which one of the following elements would you expect to have maximum metallic characteristic? [ HSLC-2025]
(A) Ga
(B) Ge
(C) As
(D) Se
Explanation: Gallium (Ga), Germanium (Ge), Arsenic (As), and Selenium (Se) all belong to the same period (Period 4) and are arranged from left to right in this exact order. As metallic character decreases from left to right across a period, therefore, Gallium (Ga) will be the maximum metallic characteristic.
4. Boron is a non-metal and is placed under group 13 and period 2 of the periodic table. How can boron form bonds with other elements? [ HSLC-2025]
(A) By sharing 5 electrons
(B) By sharing 3 electrons
(C) By sharing 2 electrons
(D) By sharing 1 electron
Explanation: Boron has an atomic number of 5, which gives it an electronic configuration of 2, 3. It has 3 valence electrons in its outermost shell. Being a non-metal with a small atomic size, it prefers to share these 3 electrons to form covalent bonds.
5. What is the trend of valency along the periods in the modern periodic table? [ HSLC-2025]
(A) It increases from left to right
(B) It decreases from right to left
(C) It increases and then decreases
(D) It decreases and then increases
Explanation: Valency is determined by the number of valence electrons. As we move from left to right in a period, the number of valence electrons increases from 1 to 8. Therefore, the valency first increases from 1 to 4 (for Groups 1 to 14) and then decreases from 4 to 0 (for Groups 15 to 18).
6. In the Periodic Table, which of the following properties increase from top to bottom in a group? [ HSLC-2024](i) Valency
(ii) Metallic character
(iii) Effective nuclear charge
(iv) Atomic radius
(A) (i) and (ii)
(B) (ii) and (iv)
(C) (iii) and (iv)
(D) (i) and (iv)
Explanation: Let’s look at the trends down a group from top to bottom:
Valency remains the same.
Metallic character increases because atomic size increases, making it easier to lose electrons.
Effective nuclear charge decreases slightly because of increased shielding.
Atomic radius increases due to the addition of new electron shells.
Therefore, both metallic character (ii) and atomic radius (iv) increase.
7. Element X forms a chloride with the formula XCl₂, which is a solid with high melting point. X would most likely be in same group of the Periodic Table as [ HSLC-2019, 24]
(A) Na
(B) Mg
(C) Al
(D) Si
Explanation: The formula XCl₂ indicates that the valency of element X is 2 (since Chlorine has a valency of 1). On the other hand, Magnesium (Mg) has a valency of 2. So, X would be Mg.
8. Choose the correct answer. [ HSLC-2023]
(i) Which of the following is not an example of metalloid?
(A) Si
(B) Ge
(C) B
(D) Al
9. Using Mendeleev’s periodic table, the formula predicted for the oxide of element K is – [ HSLC-2022]
(A) KO₂
(B) K₂O
(C) K₂O₃
(D) K₂O₂
Explanation: Potassium (K) has a valency of 1. Oxygen has a valency of 2. By crossing over their valencies, we get the chemical formula K₂O.
10. Using Mendeleev’s periodic table, the predicted formula for the oxide of element Al is – [ HSLC-2021]
(A) Al₂O₃
(B) Al₂O₂
(C) AlO₂
(D) Al₃O₂
Explanation: Aluminium (Al) has a valency of 3. Oxygen has a valency of 2. When we cross over their valencies, the predicted formula for the oxide of element Al is Al₂O₃.
11. An atom has electronic configuration 2,8,7. To which of the following elements would it be chemically similar? [ HSLC-2017, 20]
(A) N (7)
(B) F (9)
(C) P (15)
(D) Ar (18)
Explanation: Chemical similarity depends on the number of valence electrons. The given atom has a configuration of 2, 8, 7, meaning it has 7 valence electrons (So, it is Chlorine). Let’s look at the configuration of the options:
N (7) = 2, 5
F (9) = 2, 7
P (15) = 2, 8, 5
Ar (18) = 2, 8, 8
Since Fluorine (F) also has 7 valence electrons, it belongs to the same group (Group 17) and shares similar chemical properties.
12. Which of the following is not a correct statement about the trends when going from left to right across a period of the periodic table? [ HSLC-2018]
(A) The elements become less metallic in nature.
(B) The number of valence electrons increases.
(C) The atoms lose their valence electrons more easily.
(D) The oxides become more acidic.
13. In the periodic table of the elements, in a period from left to right atomic radius gradually – [ HSLC-2016]
(A) increases
(B) decreases
(C) remains same
(D) none of the above
14. According to your text book, the correct order of periodic classification is:
M = Modern periodic table
N = Mendeleev’s periodic table
O = Newlands law of Octaves
P = Dobereiner’s Triads
(A) M→N→O→P
(B) P→N→O→M
(C) P→N→M→O
(D) P→O→N→M
Explanation: This tracks the chronological order of how periodic tables were developed:
P – Dobereiner’s Triads (1817)
O – Newlands’ Law of Octaves (1866)
N – Mendeleev’s Periodic Table (1869)
M – Modern Periodic Table (1913)
15. Assertion (A): In a triad, the three elements have the same gap of atomic mass.
Reason (R): Elements in a triad have similar properties.
(A) Both A and R are correct and R is the correct explanation of A
(B) Both A and R are correct and R is not the correct explanation of A
(C) A is true but R is false
(D) A is false but R is true
Assertion (A) is false: In a Dobereiner’s triad, the middle element’s atomic mass is approximately the average of the atomic masses of the other two elements. The three elements do not have the same gap in atomic mass.
Reason (R) is true: Elements in a triad do have similar chemical properties.
16. A few elements are given below. Which of them will have same chemical properties? (Atomic no. of each elements is given in bracket):
(i) O(8),
(ii) N(7),
(iii) F(9),
(iv) Cl(17)
(A) (i), (ii)
(B) (ii), (iv)
(C) (iii), (iv)
(D) (i), (ii), (iii)
Explanation: Elements in the same vertical group have the same number of valence electrons and identical chemical properties.
F (9) has a configuration of 2, 7 (7 valence electrons).
Cl (17) has a configuration of 2, 8, 7 (7 valence electrons).
Because both have 7 valence electrons. So, they exhibit similar chemical properties.
17. Upto which element, the law of octaves was found to be applicable?
(A) Oxygen
(B) Calcium
(C) Cobalt
(D) Potassium
18. Elements Cl, Br and I are Dobereiner’s triads. If atomic mass of Cl is 35.5 u and of I is 126.9 u then the atomic mass of Br is:
(A) 81.2
(B) 162.4
(C) 91.4
(D) 45.7
Explanation: According to Dobereiner’s law of triads, the atomic mass of the middle element (Bromine) is the arithmetic mean of the first (Chlorine) and third (Iodine) elements.
19. Match Column I with Column II:
| 1) Triads | A. Mendeleev |
| 2) Octaves | B. Mosley |
| 3) Atomic number | C. Newlands |
| 4) Atomic mass | D. Dobereiner |
(A) 1→D, 2→C, 3→A, 4→B
(B) 1→C, 2→A, 3→B, 4→D
(C) 1→C, 2→A, 3→D, 4→B
(D) 1→D, 2→C, 3→B, 4→A
20. Which of the following element has twice electron in its valence shell as in its first shell?
(A) Nitrogen
(B) Oxygen
(C) Carbon
(D) Boron
Explanation: Let’s calculate the electronic configuration of the given elements based on their atomic numbers:
Carbon has an atomic number of 6. Its electronic configuration is 2, 4.
The first shell (K shell) has 2 electrons.
The valence shell (L shell) has 4 electrons, which is exactly twice the number of electrons in its first shell $$2 \times 2 = 4$$.
21. Element having electronic configuration 2.8.3 is:
(A) Mg
(B) Al
(C) Si
(D) Ar
Explanation: The total number of electrons is 2 + 8 + 3 = 13. So its atomic number is 13. The element with atomic number 13 is Aluminium (Al).
22. For element X having 13 protons:
(i) It forms amphoteric oxide.
(ii) Its valency is 3.
(iii) The formula of its chloride is XCl₃.
Correct statements:
(A) only (i)
(B) only (ii)
(C) (i) and (ii)
(D) (i), (ii) and (iii)
Explanation: An element with 13 protons has an atomic number of 13, which is Aluminium (Al). Let’s evaluate the statements:
(i) is correct: Aluminium forms Aluminium oxide Al₂O₃, which reacts with both acids and bases, making it an amphoteric oxide.
(ii) is correct: Its electronic configuration is 2, 8, 3, so it has 3 valence electrons and a valency of 3.
(iii) is correct: Since its valency is 3 and Chlorine’s valency is 1, it forms the chloride XCl₃ (AlCl₃).
23. The electronic configuration of the three elements x, y and z are x = 2.4, y = 2.7 and z = 2.1. The formula of the ionic compound formed by the combination of elements is:
(A) xy
(B) yz
(C) xz₃
(D) y₂z
Explanation: To form an ionic compound, we need a metal that loses electrons and a non-metal that gains electrons:
z = 2.1 ( loses 1 electron to get a +1 charge).
y = 2.7 (gains 1 electron to get a ‘-1′ charge).
When z and y combine, they form a stable ionic compound with the formula zy (or yz as listed in the options).
24. Which of the following is a Dobereiner’s triad?
(A) Cl, Br, I
(B) Li, Na, Ca
(C) K, Sr, Ba
(D) Ca, Sr, Be
Explanation: The three classic triads of Dobereiner’s are:
Lithium (Li), Sodium (Na), Potassium (K)
Calcium (Ca), Strontium (Sr), Barium (Ba)
Chlorine (Cl), Bromine (Br), Iodine (I)
25. Assertion (A): Down a group, tendency to lose electrons increases.
Reason (R): Atomic radius increases down a group.
(A) Both A and R are true and R is the correct explanation of A
(B) Both A and R are true and R is not the correct explanation of A
(C) A is true, but R is false
(D) Both A and R are false
Down a group in the periodic table, the atomic size increases because new electron shells are added. So,
- The outermost electrons are further away from the nucleus.
- The attraction between nucleus and valence electrons becomes weaker.
- So, tendency to lose electrons increases.
26. In Modern Periodic Table, while we go down in a group, the atomic size of the elements:
(A) Increases
(B) Decreases
(C) Remains the same
(D) None of the above
Explanation: As we go down a group, a new electron shell is added at each step. This increases the physical distance between the outermost electrons and the nucleus, causing the overall atomic size to increase.
27. If the second shell of an element has three times as many electrons as in the first shell, then the element will be:
(A) N
(B) P
(C) O
(D) S
Explanation: The first shell (K shell) can hold a maximum of 2 electrons. If the second shell (L shell) has three times as many electrons, it must have ( 2 x 3 = 6 ) electrons.
Total electrons = 2 + 6 = 8.
So, the element with atomic number 8 is Oxygen (O).
28. How many elements were known at the time of Mendeleev?
(A) 63
(B) 65
(C) 62
(D) 64
29. Which of the following elements has two electrons in its outermost shell?
(A) Mg
(B) Na
(C) K
(D) Ne
Explanation: Let’s look at the electronic configurations:
Mg (12): 2, 8, 2 (2 valence electrons)
Na (11): 2, 8, 1 (1 valence electron)
K (19): 2, 8, 8, 1 (1 valence electron)
Ne (10): 2, 8 (8 valence electrons)
30. The limitation of Mendeleev’s periodic table is:
(A) He kept some blank space in his table
(B) Some time elements have more atomic mass placed before the element having less atomic mass
(C) The noble gases were rehabilated in new group
(D) The correct position of H₂ is not given
Explanation: One of the major limitations of Mendeleev’s periodic table was that he could not assign a fixed, correct position to Hydrogen.
32. If n = no. of orbits, then the maximum electrons in the orbit:
(A) 2n²
(B) 2n
(C) 4n²
(D) n
33. Which of the following statements is correct statement about the trends when going from left to right across the periods of a modern periodic table:
(A) The elements become less metallic in nature
(B) The number of valence electrons increases
(C) The size of elements decreases
(D) All of the above
34. An element M has 50% of the electrons filled in the 3rd shell as in the 2nd shell. The atomic number of M is:
(A) 10
(B) 12
(C) 14
(D) 18
Explanation: Let’s find the configuration shell by shell:
The 1st shell (K) must be completely full: 2 electrons.
The 2nd shell (L) must also be completely full: 8 electrons.
The 3rd shell (M) has 50% of the number of electrons present in the 2nd shell. 50% of 8 is 4 electrons.
Total number of electrons = 2 + 8 + 4 = 14. The element with atomic number 14 is Silicon (Si).
35. Using Mendeleev’s Periodic Table the formulae predicted for the oxide of element ‘K’ is:
(A) K₂O
(B) KO₂
(C) K₂O₂
(D) K₂O₃
36. Which of the following statements is not a correct statement about the trends when going from left to right across the periodic table:
(A) The elements becomes less metallic in nature
(B) The number of valence electrons increase
(C) The atoms lose their electrons more easily
(D) The oxide becomes more acidic
Explanation: As we move across a period from left to right, the nuclear charge increases and the atomic size decreases. The nucleus pulls outer electrons much tighter, which actually makes it harder for atoms to lose their valence electrons.
37. Which one of the following is metalloid:
(A) Mg
(B) C
(C) P
(D) Ge
38. Newland was from which country:
(A) England
(B) China
(C) Japan
(D) India
39. Which of the following is the correct order of atomic radii of oxygen, fluorine and nitrogen?
(A) F < O < N
(B) N < F < O
(C) O < N < F
(D) F < N < O
Explanation: Nitrogen (7), Oxygen (8), and Fluorine (9) belong to the same period (Period 2). Atomic radius decreases moving from left to right due to increased nuclear pull. Therefore, Nitrogen is the largest, followed by Oxygen, and Fluorine is the smallest (F < O < N).
40. Group 17 elements are called:
(A) Alkali metals
(B) Noble gases
(C) Halogens
(D) None of the above
41. An atom of an element has electronic configuration 2, 8, 2. To which group does it belong?
(A) 4th group
(B) 3rd group
(C) 2nd group
(D) 6th group
Explanation: The group number of an element is determined by its valence electrons (electrons in the outermost shell). The electronic configuration is 2, 8, 2. Since it has 2 valence electrons, it belongs to Group 2.
42. The basis of modern periodic table is:
(A) Atomic mass
(B) Atomic number
(C) Octaves
(D) Triads
43. Assertion: In a group, on moving from top to bottom, atomic size increases.
Reason: A new shell starts as we move down a group.
(A) Both assertion and reason are true and reason correctly explains assertion
(B) Both true but reason is not the correct explanation
(C) Assertion true, reason false
(D) Assertion false, reason true
Explanation: As we move from top to bottom down in a group, a completely new electron shell is added. This increases the distance between the outermost electrons and the nucleus, which causes the atomic size to increase.
44. Which property does NOT increase on moving down a group in periodic table?
(A) Atomic radius
(B) Metallic character
(C) Valence electron
(D) Number of shells
Explanation: Let’s look at what happens down a group:
Atomic radius increases due to extra shells.
Metallic character increases because electrons are lost more easily.
Number of shells increases by one at each period level.
Valence electrons remain identical for all elements within the same group
45. Besides gallium, which other elements were predicted by Mendeleev and discovered later?
(A) Sc and Ti
(B) Sc and Ge
(C) Ge and Cr
(D) Ge and V
Explanation: Mendeleev left gaps in his periodic table for elements that were undiscovered at the time. He named them Eka-Boron, Eka-Aluminium, and Eka-Silicon. When discovered later, they matched his predictions perfectly:
Eka-Boron became Scandium (Sc)
Eka-Aluminium became Gallium (Ga)
Eka-Silicon became Germanium (Ge)
47. Which of the following is NOT correct about trends across a period (left to right) in the periodic table?
(A) Number of valence electrons increases
(B) Tendency to lose electrons increases
(C) Atomic radius decreases
(D) Electronegativity increases
48. Outermost shell has 25% of electrons in L–shell. Valency is –
(A) 1
(B) 2
(C) 3
(D) 4
Explanation: Let’s look at the shell structure:
The K-shell is the 1st shell, holding its maximum of 2 electrons.
The L-shell can hold a maximum of 8 electrons. If it is filled to 25% capacity:
$$\text{Electrons in L-shell} = 25\% \text{ of } 8 $$
$$= \frac{25}{100} \times 8 $$
$$= 2 \text{ electrons}$$Therefore, valency is 2.
49. Group–1 elements are known as –
(A) Halogens
(B) Alkali metals
(C) Lanthanoids
(D) Actinoids
50. In a period, as we move from left to right, the effective nuclear charge acting on the valence electrons gradually:
(A) Increases
(B) Decreases
(C) Remains the same
(D) None of the above
51. ‘Te’ (Tellurium) is:
(A) A metal
(B) A non-metal
(C) A metalloid
(D) None of the above
52. An atom has electronic configuration 2.8.7. The element is:
(A) C
(B) Ca
(C) Cl
(D) Ar
Explanation: The given electronic configuration is 2, 8, 7. Adding the electrons (2 + 8 + 7 = 17) gives us an atomic number of 17. The element with atomic number 17 is Chlorine (Cl).
53. In which Dobereiner triad are elements in increasing order of atomic mass?
(A) Na, Li, K
(B) Ba, Sr, Ca
(C) Cl, Br, I
(D) Li, Ca, Cl
55. Limitations of Mendeleev’s classification:
i) Position of hydrogen not justified.
ii) No place for isotopes.
iii) Noble gases, discovered later, could be accommodated without disturbing order.
iv) Increasing order of atomic mass not strictly maintained.
Options:
(A) i, ii, iii
(B) ii, iii, iv
(C) i, ii, iv
(D) i, iii, iv
Explanation: Let’s look closely at the listed points:
(i) is a limitation: Hydrogen’s correct position was never fully resolved.
(ii) is a limitation: Since his table was based on atomic mass, isotopes of the same element (having different masses) could not be assigned unique slots.
(iv) is a limitation: He occasionally had to place an element with a slightly higher atomic mass before a lighter element to keep similar properties together (e.g., Cobalt before Nickel).
Statement (iii) is an achievement, not a limitation. Because noble gases are inert, they seamlessly fit into a brand-new group later on without disrupting his existing system.
57. The electronic configurations of three elements X, Y and Z are:
X – 2, 8; Y – 2, 8, 7; Z – 2, 8, 2.
Which of the following is correct?
(A) X is a metal, Y is non-metal, Z is a metal
(B) Y is a metal
(C) Z is a non-metal
(D) Y is non-metal, Z is metal
Explanation: Let’s break down each element’s configuration:
X (2, 8): This is Neon, a stable inert noble gas.
Y (2, 8, 7): This is Chlorine. It has 7 valence electrons and readily gains 1 electron, so it is a non-metal.
Z (2, 8, 2): This is Magnesium. It has 2 valence electrons and readily loses them, so it is a metal.
Therefore, statement (d) is correct.
58. Which of the given elements A, B, C, D and E with atomic number 2, 3, 7, 10 and 30 respectively belong to the same period?
(A) A, B, C
(B) B, C, D
(C) A, D, E
(D) B, D, E
Explanation: Elements share the same period if they have the same number of electron shells. Let’s write out the configurations:
A (2) → 2 (1 shell → Period 1)
B (3) → 2, 1 (2 shells → Period 2)
C (7) → 2, 5 (2 shells → Period 2)
D (10) → 2, 8 (2 shells → Period 2)
E (30) → 2, 8, 18, 2 (4 shells → Period 4)
Elements B, C, and D all use exactly two shells and belong to Period 2.
59. The element Al is present in which period and group respectively?
(A) 3, 12
(B) 3, 13
(C) 4, 13
(D) 2, 13
Explanation: Aluminium (Al) has an atomic number of 13, giving it an electronic configuration of 2, 8, 3.
It occupies 3 shells (K, L, M), so in Period 3.
It has 3 valence electrons. For main-group elements with 3 or more valence electrons, we find the group number by adding 10 (3 + 10 = 13). This places it in Group 13.
62. Which of the below is the correct decreasing order of elements in terms of their atomic number?
(A) Cl > Al > Si > Ne > Mg
(B) Ne > Mg > Al > Cl > Si
(C) Cl > Si > Al > Ne > Mg
(D) Cl > Si > Al > Mg > Ne
Explanation: The atomic numbers of the given elements are:
Chlorine (Cl) = 17
Silicon (Si) = 14
Aluminium (Al) = 13
Magnesium (Mg) = 12
Neon (Ne) = 10
63. Which of the below statements are true for the Modern Periodic Table?
(A) There are 18 groups.
(B) The atomic radius of elements increases moving from left to right along a period.
(C) The metallic character of elements decreases when moving down in a group.
Choose the correct options-
(A) Only A
(B) A and B correct
(C) Only C
(D) B and C correct
Explanation: Let’s review the statements given in question 63:
(A) is true: The Modern Periodic Table contains exactly 18 vertical columns known as groups.
(B) is false: The atomic radius decreases (not increases) from left to right across a period.
(C) is false: The metallic character increases (not decreases) down a group.
65. About modern periodic table, which statements are incorrect?
i) It has 18 horizontal rows called periods
ii) Electronegativity increases from top to bottom in a group
iii) It has 7 horizontal rows called periods
iv) Effective nuclear charge on valence electrons increases across a period
(A) (i) and (ii)
(B) (i) and (iv)
(C) (ii) and (iii)
(D) (iii) and (iv)
Explanation: Let’s check each statement to identify the incorrect ones:
(i) is incorrect: The table has 18 vertical columns called groups, and 7 horizontal rows called periods.
(ii) is incorrect: Electronegativity decreases from top to bottom down a group because the atomic size grows.
(iii) is correct: It has 7 horizontal rows called periods.
(iv) is correct: Effective nuclear charge increases across a period.
Therefore, statements (i) and (ii) are incorrect, matching option (a).
66. Arrange Be, Ne, He, Na, Cl, Al in increasing order of atomic number –
(A) Na < Ne < Cl < Al < Be < He
(B) He < Be < Na < Ne < Cl < Al
(C) Cl < Al < Na < Ne < Be < He
(D) He < Be < Ne < Na < Al < Cl
67. Match Column A with Column B:
| Column A | Column B |
| (i) Metalloids | (p) Cl |
| (ii) Noble gas | (q) Na |
| (iii) Halogen | (r) Si |
| (iv) Alkali metal | (s) Ar |
(A) (i)→p, (ii)→q, (iii)→r, (iv)→s
(B) (i)→r, (ii)→s, (iii)→q, (iv)→p
(C) (i)→r, (ii)→s, (iii)→p, (iv)→q
(D) (i)→r, (ii)→p, (iii)→s, (iv)→q
68. The elements P, Q and R belong to group 1, 14 and 17 respectively of the periodic table. Which elements will form ionic compounds?
(A) P and Q
(B) P and R
(C) Q and R
(D) P, Q and R
Explanation: An ionic compound is formed by the complete transfer of electrons from a highly reactive metal to a highly reactive non-metal:
P belongs to Group 1 (Alkali metals), so it easily loses 1 electron ($P^+$).
R belongs to Group 17 (Halogens), so it easily gains 1 electron ($R^-$).
Together, P and R form a stable ionic compound ($PR$).
(On the other hand, Element Q belongs to Group 14, like Carbon, which forms covalent bonds).
69. Assertion (A): Isotopes are placed together in the modern periodic table.
Reason (R): Isotopes have same atomic number.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is not the correct explanation of A.
(C) A is true but R is false.
(D) A is false but R is true.
Explanation: Assertion (A) is true: In the Modern Periodic Table, all isotopes of an element occupy the exact same slot.
Reason (R) is true: The modern table sorts elements by atomic number. Because isotopes of an element share the same number of protons, they have the same atomic number and naturally belong in the same position.
70. Using Mendeleev’s Periodic table, the formula predicted for the oxide of element ‘K’ is:
(A) KO₂
(B) K₂O
(C) K₂O₃
(D) K₂O₂
Explanation: Potassium (K) has a valency of 1. Oxygen has a valency of 2. By crossing over their valencies, the formula for the oxide of Potassium is determined to be K₂O.
71. The atomic number of four elements A, B, C and D is 2, 8, 14 and 19 respectively. Which of the following elements have four number of shells?
(A) A
(B) B
(C) C
(D) D
Explanation: The number of shells an atom has determines its period number and depends on its electronic configuration. Let’s find the configuration for each:
A (2) → 2 (1 shell: K)
B (8) → 2, 6 (2 shells: K, L)
C (14) →2, 8, 4 (3 shells: K, L, M)
D (19) → 2, 8, 8, 1 (4 shells: K, L, M, N)
Therefore, element D (Potassium) uses exactly four electron shells.
73. An element ‘X’ has 50% of electrons filled in the third shell as in the second shell. The atomic number of ‘X’ is:
(A) 10
(B) 12
(C) 14
(D) 18
74. In the periodic table which of the following properties increase from top to bottom in a group:
(i) Atomic radius
(ii) Valency
(iii) Metallic character
(iv) Effective Nuclear charge
(A) (i) and (ii)
(B) (i) and (iii)
(C) (i) and (iv)
(D) (ii) and (iv)
75. Which of the given elements P, Q, R, S and T of modern periodic table with atomic number 2, 4, 7, 10 and 20 respectively belong to the same period:
(A) P, Q, R
(B) Q, R, S
(C) Q, R, T
(D) P, Q, T
Explanation: Elements belong to the same period if they use the same number of electron shells. Let’s look at their configurations:
- P (2) → 2 → Period 1
- Q (4) → 2, 2 → Period 2
- R (7) → 2, 5 → Period 2
- S (10) → 2, 8 → Period 2
- T (20) → 2, 8, 8, 2 → Period 4
So, Q, R, and S all have two electron shells and belong to Period 2.
77. Chlorine element with atomic number 17 and atomic mass 35.5 is arranged in the modern periodic table at:
(A) Period-2, Group-17
(B) Period-3, Group-7
(C) Period-3, Group-17
(D) Period-4, Group-17
Explanation:
Chlorine has atomic number 17, so its electronic configuration is 2, 8, 7.
- It has 3 electron shells, so it belongs to Period 3.
- It has 7 valence electrons, so it is placed in Group( 7 + 10 =17 ) (halogens).
Therefore, chlorine is placed in Period 3 and Group 17.
78. The electronic configuration of three elements X, Y and Z are X-2.8; Y-2.8.7 and Z-2.8.2. Which of the following is correct?
(A) X is noble gas and Y is metal
(B) Y is non metal and Z is metal
(C) Y is metal Z is non metal
(D) X and Z both are metals
Explanation: Let’s look closely at each element’s valence electrons:
X (2, 8): Has a completely filled outer shell. This is Neon, a noble gas.
Y (2, 8, 7): Has 7 valence electrons. It readily gains 1 electron to complete its octet, making it a non-metal (Chlorine).
Z (2, 8, 2): Has 2 valence electrons. It easily loses these 2 electrons to form a positive ion, making it a metal (Magnesium).
Therefore, statement (B) is completely correct.
79. Electronic configuration of Al³⁺ is:
(A) 2.8.3
(B) 2.8.8
(C) 2.8
(D) 2.8.8.3
Explanation:
Aluminium (Al) has atomic number 13, so its electronic configuration is 2, 8, 3.
For Al³⁺, it loses 3 electrons from the outermost shell:
- Al (neutral): 2, 8, 3
- After losing 3 electrons → 2, 8
So, the electronic configuration of Al³⁺ is 2, 8.
80. Identify the group which is not a Dobereiner triad:
(A) Li, Na, K
(B) Be, Mg, Ca
(C) Ca, Sr, Ba
(D) Cl, Br, I
81. An element X (2,8,2) combines separately with (SO₄)²⁻ and (PO₄)³⁻ radicals. The chemical formulas of the compounds are:
(A) X₂SO₄; X₃(PO₄)₂
(B) XSO₄; X₃(PO₄)₂
(C) X(SO₄)₂; X₂(PO₄)₃
(D) XSO₄; X₃(PO₄)₃
82. The number of electrons in the valence shell of alkali metals is –
(A) 1
(B) 2
(C) 3
(D) 4
Explanation: Alkali metals belong to Group 1 of the Modern Periodic Table (e.g., Lithium, Sodium, Potassium). All elements in this group possess exactly 1 electron in their outermost valence shell.
83. Which of the following elements is not a metalloid?
(A) Arsenic
(B) Antimony
(C) Tellurium
(D) Aluminium
84. In a periodic table of elements, which of the following properties decrease gradually from left to right?
(i) Metallic character
(ii) Effective nuclear charge
(iii) Atomic size
(iv) Electronegativity
(A) (i) and (ii)
(B) (i) and (iii)
(C) (ii) and (iii)
(D) (ii) and (iv)
Explanation: Let’s look at the periodic trends when moving from left to right across a period:
(i) Metallic character decreases as elements turn from metals to non-metals.
(ii) Effective nuclear charge increases because protons are added to the nucleus.
(iii) Atomic size decreases because the increasing nuclear charge pulls the same electron shell closer.
(iv) Electronegativity increases as atoms have a stronger pull for shared electrons.
Therefore, both metallic character (i) and atomic size (iii) decrease.
85. Which of the following sets of elements does not form Dobereiner’s triads?
(A) N, P, As
(B) Li, Na, K
(C) Ca, Sr, Ba
(D) Cl, Br, I
86. Elements were arranged in Mendeleev’s periodic table –
(i) On the basis of atomic masses of the elements
(ii) On the basis of atomic numbers of the elements
(iii) On the basis of similarity of chemical properties of elements
(iv) On the basis of similarity of physical properties of elements
(A) Only (ii)
(B) (i) and (iii)
(C) (ii) and (iv)
(D) (ii) and (iii)
Explanation: Mendeleev organized his periodic table based on two main pillars:
(i) Arranging the elements in the increasing order of their atomic masses.
(iii) Grouping elements together based on the similarity of their chemical properties (specifically the formulas of their hydrides and oxides).
87. ‘X’ is a halogen. Our body needs ‘X’ to prepare thyroxin hormone. Identify ‘X’.
(A) Iodine
(B) Bromine
(C) Chlorine
(D) Fluorine
Explanation: The thyroid gland requires Iodine to synthesize the hormone thyroxin. Iodine is located in Group 17, making it a halogen.
88. Which of the following statement is not correct when we move from left to right in a period of periodic table
(A) The elements become less metallic in nature.
(B) The number of valence electrons increases.
(C) The atoms lose their electron more easily.
(D) The oxide become more acidic.