chemistry

STRUCTURE OF ATOM

TWO MARKS QUESTIONS

1.  Write the complete symbol for the atom with the given atomic number (Z) and

mass number(A).  (a) Z = 17, A = 35            (b) Z = 92 , A = 233

Ans. (a) ³⁵₁₇Cl                        (b) ²³³₉₂U

2.     Using s,p,d and f notation, describe the orbital with the following quantum numbers-

(a) n=1,l=0	(b) n=3, l=1	(c) n=4, l=2 (d) n=4, l=3
Ans. (a) 1s	(b) 3p (c)4d	(d) 4f

3.  How many electrons in an atom have the following quantum numbers?

a.  n=4, m_s= -1/2                     b. n =3 , l=o

Ans. (a) 16 electrons                   (b) 2 electrons.

4.     An element with mass number 81 contains 31.7 % more neutrons as compared to protons. Assign the atomic symbol.

Ans. Mass number = 81, i.e., p + n = 81

If protons = x, then neutrons = x + 31.7 X x = 1.317 x 100

x+1.317x = 81 or 2.317x = 81 x=35

Thus proton = 35, i.e., atomic no. = 35 Hence symbol is ⁸¹₃₅Br

5.     (i) The energy associated with the first orbit in the hydrogen atom is -2.18 x 10^-18J/atom. What is the energy associated with the fifth orbit

(ii) Calculate the radius of Bohr’s fifth orbit for hydrogen atom.

Ans. (i) E_n = -2.18 x 10^-18/ n²        E₅= -2.18 x 10^-18/ 5²   = -8.72 x 10^-20J

(ii) For H atom, r_n= 0.529 x n²     r₅= 0.529 x 5²  = 13.225 A⁰= 1.3225 nm

6.  Explain , giving reasons, which of the following sets of quantum numbers are

not possible.
(a) n=0, l=0; ml = 0, ms= + ½	(c)n=1, l=0; ml = 0, ms= - ½
(b) n=1, l=1; ml =- 0, ms= + ½	(d)	n=2, l=1; ml = 0, ms= + ½
Ans. (a) Not possible because n≠ 0 (c)		Not possible because when n=1, l≠1

(b) Possible                                     (d) Possible

7.     (a)What is the lowest value of n that allows g orbitals to exist?

(b)An electron is in one of the 3d orbitals, Give the possible values of n,l and m_lfor this electron.

Ans.(a) minimum value of n= 5 (b)n=3, l=2, m_l= -2, -1, 0, +1, +2

8.  Calculate the total number of angular nodes and radial nodes present in 30 orbitals.

Ans. For 3p orbitals, n=3, l= 1 Number of angular nodes = l= 1

Number of radial nodes = n-l-1 = 3-1-1= 1

9.     Mention the draw backs of Rutherford’s atomic model.

Ans. 1. It could not explain the stability of an atom.

2.  It could not explain the line spectrum of H- atom.

10.   State de-Broglie concept of dual nature of matter. How do dual nature of electron verified?

Ans. Just as light has dual nature, every material particle in motion has dual nature (particle nature and wave nature). The wave nature has been verified by Davisson

and Germer’sexperiment whereas particle nature by scintillation experiment.