## Atomic Structure Questions and Answers Part-1

1. The electronic configuration of an element is 1s2, 2s2, 2p6, 3s2, 3p6, 3d5, 4s1. This represents its:
a) excited state
b) ground state
c) cationic form
d) anionic form

Explanation: The given electronic configuration is ground state for chromium.

2. Select the pairs of ions which have same electronic configuration?
a) Cr3+, Fe3+
b) Fe3+, Mn2+
c) Fe3+, Co3+
d) Se3+, Cr3+

Explanation: Fe3+ and Mn2+ have same electronic configuration

3. Many elements have non-integral atomic masses because:
a) they have isotopes
b) their isotopes have non-integral masses
c) their isotopes have different masses
d) Both a and c

Explanation: Non-integral atomic masses are due to isotopes which have different masses.

4. Light of wavelength l shines on a metal surface with intensity x and the metal emits electrons per second of average energy, Z. What will happen to Y and Z if x is doubled?
a) Y will be double and Z will become half
b) Y will remain same and Z will be doubled
c) Both Y and Z will be doubled
d) Y will be doubled but Z will remain same

Explanation: When intensity is doubled, number of electrons emitted per second is also doubled but average energy of photoelectrons emitted remains the same.

5. Difference between nth and (n + 1)th Bohr's radius of H-atom is equal to its (n – 1)th Bohr's radius. The value of n is:
a) 1
b) 2
c) 3
d) 4

Explanation: rn n2
But rn + 1 – rn = rn – 1
(n + 1)2 – n2 = (n – 1)2
n = 4

6. Atoms consists of protons, neutrons and electrons. If the mass of neutrons and electrons were made half and two times respectively to their actual masses, then the atomic mass of $_{6}{{C}^{12}}$
a) Will remain approximately the same
b) Will become approximately two times
c) Will remain approximately half
d) Will be reduced by 25%

Explanation: No change by doubling mass of electrons however by reducing mass of neutron to half total atomic mass becomes $6+3$ instead of $6+6$ . Thus, reduced by 25%.

7. Principal, azimuthal and magnetic quantum numbers are respectively related to:
a) size, orientation and shape
b) size, shape and orientation
c) shape, size and orientation
d) none of these

Explanation: Principal gives size, azimuthal gives shape and magnetic quantum number gives the orientation.

8. The number of d-electrons retained in Fe2+ (At.no. of Fe = 26) ion is
a) 4
b) 5
c) 6
d) 3

Explanation: Configuration of Fe+2 = 3d6.4s°

9. Consider the ground state of Cr atom (Z=24). The number of electrons with the azimuthal quantum numbers 1 = 1 and 2 are respectively
a) 12 and 4
b) 16 and 5
c) 16 and 4
d) 12 and 5

Explanation: Configuration of Cr (Z = 24) is 1s22s22p63s23p63d54s*
1 = 1, denotes p-electrons (2p6, 3p6)
1 = 2, denotes d-electrons (3d5)

10. The electrons identified by quantum numbers n and l
1) n = 4, l = 1 2) n = 4, l = 0
3) n = 3, l = 2 4) n = 3, l = 1
can be placed in order of increasing energy as
a) c < d < b < a
b) d < b < c < a
c) b < d < a < c
d) a < c < b < d