Periodic Classification of Elements

7 practice questions on Periodic Classification of Elements for NSEJS, with answers

Shishya's practice questions, written with AI. Each answer was checked by an automated second pass, not by a person.

1. 1.In Doebereiner's triads, the atomic mass of the middle element is approximately equal to the:

   • (A)Sum of atomic masses of the first and third elements
   • (B)Average of atomic masses of the first and third elements
   • (C)Product of atomic masses of the first and third elements
   • (D)Difference of atomic masses of the first and third elements
   Show the answer and solution

   Answer: (B) Average of atomic masses of the first and third elements

   Solution: Doebereiner observed that when elements are arranged in groups of three (triads), the atomic mass of the middle element is approximately equal to the arithmetic mean (average) of the atomic masses of the first and third elements. For example, in the triad Li (7), Na (23), K (39), the average of 7 and 39 is 23, which equals sodium's atomic mass.

   Report an error

2. 2.Three elements P, Q, and R have atomic numbers 11, 17, and 19 respectively. Which of the following statements is correct regarding these elements?

   • (A)P and R belong to the same period
   • (B)Q and R belong to the same group
   • (C)P and R belong to the same group
   • (D)All three belong to the same period
   Show the answer and solution

   Answer: (C) P and R belong to the same group

   Solution: Elements with atomic numbers 11 (sodium) and 19 (potassium) belong to Group 1 (alkali metals), as they have the same valence electron configuration.

   Report an error

3. 3.Element X has atomic number 16. In the modern periodic table, element X belongs to:

   • (A)Period 2, Group 16
   • (B)Period 3, Group 16
   • (C)Period 3, Group 6
   • (D)Period 2, Group 6
   Show the answer and solution

   Answer: (B) Period 3, Group 16

   Solution: For element X with atomic number 16, the electronic configuration is 2, 8, 6. The number of shells (3) determines the period, so it belongs to Period 3. The number of valence electrons (6) determines the group number for main group elements, placing it in Group 16. This element is sulfur.

   Report an error

4. 4.Mendeleev arranged elements in his periodic table in order of increasing atomic mass. He left gaps in the table because:

   • (A)He wanted to include noble gases later
   • (B)He predicted the existence of undiscovered elements
   • (C)He made calculation errors in atomic masses
   • (D)He could not determine the properties of those elements
   Show the answer and solution

   Answer: (B) He predicted the existence of undiscovered elements

   Solution: Mendeleev left gaps in his periodic table and predicted that undiscovered elements would fill these gaps. He even predicted the properties of these missing elements. For example, he predicted eka-aluminium and eka-silicon, which were later discovered as gallium and germanium respectively, validating his predictions.

   Report an error

5. 5.As we move from left to right across a period in the modern periodic table, which of the following properties generally increases?

   • (A)Atomic size and metallic character
   • (B)Electronegativity and ionization energy
   • (C)Atomic radius and electronegativity
   • (D)Metallic character and ionization energy
   Show the answer and solution

   Answer: (B) Electronegativity and ionization energy

   Solution: Across a period from left to right, the nuclear charge increases while electrons are added to the same shell. This causes increased attraction between nucleus and electrons, leading to decreased atomic size. As a result, electronegativity (tendency to attract electrons) and ionization energy (energy needed to remove an electron) both increase. Metallic character decreases across a period.

   Report an error

6. 6.Elements A, B, and C have atomic numbers 9, 17, and 35 respectively. Which statement correctly describes these elements?

   • (A)Element A has the largest atomic radius
   • (B)Element C has the highest electronegativity
   • (C)All three elements are halogens with element A being most reactive
   • (D)All three elements belong to the same period
   Show the answer and solution

   Answer: (C) All three elements are halogens with element A being most reactive

   Solution: Elements with atomic numbers 9 (fluorine), 17 (chlorine), and 35 (bromine) all belong to Group 17, the halogens. They are in different periods (2, 3, and 4 respectively). Fluorine (A) has the smallest atomic size and highest electronegativity, making it the most reactive halogen. Atomic radius increases down the group, so A has the smallest radius, not the largest.

   Report an error

7. 7.Consider four elements with atomic numbers 3, 11, 12, and 19. Which pair of elements will have the most similar chemical properties and also form a Doebereiner triad with an element having atomic number between them?

   • (A)3 and 19 with 11 as the middle element
   • (B)11 and 19 with 12 as the middle element
   • (C)3 and 12 with 11 as the middle element
   • (D)11 and 12 with no middle element possible
   Show the answer and solution

   Answer: (A) 3 and 19 with 11 as the middle element

   Solution: Elements with atomic numbers 3 (Li), 11 (Na), and 19 (K) all belong to Group 1 (alkali metals) and have similar chemical properties. They form a Doebereiner triad: Li (atomic mass 7), Na (atomic mass 23), K (atomic mass 39). The average of 7 and 39 is 23, which equals sodium's atomic mass. Element 12 (Mg) belongs to Group 2, so it cannot form a triad with alkali metals.

   Report an error