Question 1

Which of the following elements is located in Period 3, Group 13 of the periodic table?

Accepted Answer

The periodic table is organized by periods (horizontal rows) and groups (vertical columns). Period 3 contains elements with 3 electron shells (atomic numbers 11–18). Group 13 (or IIIA in older notation) contains elements with 3 valence electrons in their outermost shell. The element at the intersection of Period 3 and Group 13 is Aluminium (Al), with atomic number 13 and electron configuration 2, 8, 3. This places it in the p-block of the periodic table. Therefore, the answer is B.

Question 2

An element has atomic number 17 and mass number 35. How many neutrons are present in an atom of this element?

Accepted Answer

The number of neutrons in an atom is calculated using the formula: Number of neutrons = Mass number − Atomic number. Given: Atomic number (Z) = 17, Mass number (A) = 35. Therefore, Number of neutrons = 35 − 17 = 18. This element is Chlorine-35, a stable isotope of chlorine. The atomic number represents the number of protons (which equals electrons in a neutral atom), while the mass number represents the total count of protons and neutrons in the nucleus. Therefore, answer is C.

Question 3

Which block of the periodic table contains elements whose valence electrons are in d-orbitals?

Accepted Answer

The periodic table is divided into four blocks (s, p, d, f) based on which orbital the valence electrons occupy. The d-block contains transition metals where the last electron enters a d-orbital. Elements in the d-block have valence electrons in d-orbitals and are characterized by variable oxidation states and colored compounds. This classification is part of the modern periodic table structure taught in NCERT Class 10.

Question 4

The atomic radius of elements generally decreases across a period (left to right) in the periodic table. Which of the following best explains this trend?

Accepted Answer

As we move from left to right across a period, the atomic number increases, meaning more protons are added to the nucleus. However, electrons are added to the same shell (same period). The increased nuclear charge pulls the electrons closer, decreasing the atomic radius. This is a key periodic trend taught in NCERT Class 10 chemistry and is due to the balance between increasing nuclear charge and constant shielding.

Question 5

An atom of element X has 8 protons and 8 neutrons. An atom of element Y has 8 protons and 10 neutrons. What is the relationship between X and Y?

Accepted Answer

Element X has atomic number 8 (8 protons), which is oxygen. Element Y also has 8 protons (same atomic number), but 10 neutrons instead of 8. Since they have the same atomic number but different mass numbers, they are isotopes of the same element (oxygen). Isotopes are atoms of the same element with different numbers of neutrons, hence different mass numbers. This is a fundamental concept from NCERT Class 9 chemistry.

Question 6

Which of the following elements belongs to Group 17 (Halogens) of the periodic table?

Accepted Answer

Group 17 elements are called halogens and have 7 valence electrons in their outermost shell. Chlorine (Cl, atomic number 17) has electron configuration 2, 8, 7 and belongs to Group 17. Halogens are highly reactive non-metals that readily gain one electron to form −1 ions. This classification is based on the modern periodic table structure taught in NCERT Class 10.

Question 7

An element has atomic number 17 and mass number 35. How many neutrons does this atom contain?

Accepted Answer

The number of neutrons is calculated using: Number of neutrons = Mass number − Atomic number. Here, neutrons = 35 − 17 = 18. This is a fundamental concept from Bohr's atomic model where atomic number represents protons (and electrons in a neutral atom), and mass number is the sum of protons and neutrons.

Question 8

An element has an atomic number of 17 and a mass number of 35. How many neutrons does this atom contain, and in which block of the periodic table is this element located?

Accepted Answer

The number of neutrons is calculated using: Number of neutrons = Mass number − Atomic number = 35 − 17 = 18 neutrons. The element with atomic number 17 is chlorine (Cl). Chlorine has electron configuration 2, 8, 7 (or [Ne]3s²3p⁵), meaning its valence electrons are in the p-orbital. Elements whose last electron enters a p-orbital are classified in the p-block of the periodic table. Therefore, the answer is C.

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