An element can beidentified by its atomic number only. Let us find the atomic number.
Let the number of protons = x
Number of neutrons = x + [x×31.7100 = (x × 0.317x)
Now, Mass no. of element = no. of protons =no. neutrons
81 = x + x + 0-317 x = 2.317 x or x = 812.317 = 35
∴ No. of protons = 35,No. of neutrons = 81 – 35 =46
Atomic number of element (Z) = No. of protons = 35
The element with atomic number (Z) 35 is bromine 8135Br.

Question - 42 : - An ion with mass number 37 possesses one unitof negative charge. If the ion contains 11 -1% more neutrons than theelectrons, find the symbol of the ion.

Answer - 42 : -

Let the no. ofelectron in the ion = x
∴ the no. of protons =x – 1 (as the ion has one unit negative charge)
and the no. of neutrons = x + x×11.1100 = 1.111 x
Mass no. or mass of the ion = No. of protons + No. of neutrons
(x – 1 + 1.111 x)
Given mass of the ion = 37
∴ x- 1 + 1.111 x = 37or 2.111 x = 37 + 1 = 38
x = 382.111 = 18
No. of electrons = 18 ; No. of protons = 18 – 1 = 17
Atomic no. of the ion = 17 ; Atom corresponding to ion = Cl
Symbol of the ion = 3717Cl^–

Question - 43 : - An ion with mass number 56 contains 3 units ofpositive charge and 30.4% more neutrons than electrons. Assign symbol to theion.

Answer - 43 : -

Let the no. ofelectrons in the ion = x
∴ the no. of theprotons = x + 3 (as the ion has three units positive charge)
and the no. of neutrons = x + x×31.7100 = xc + 0.304 x
Now, mass no. of ion = No. of protons + No. of neutrons
= (x + 3) + (x + 0.304x)
∴ 56 = (x + 3) + (x +0.304x) or 2.304x = 56 – 3 = 53
x = 532.304 = 23
Atomic no. of the ion (or element) = 23 + 3 = 26
The element with atomic number 26 is iron (Fe) and the corresponding ion is Fe³⁺.

Question - 44 : - Arrange the following type of radiations inincreasing order of wavelength :
(a) radiation from microwave oven
(b) amber light from traffic signal
(c) radiation from FM radio
(d) cosmic rays from outer space and
(e) X-rays.

Answer - 44 : -

Cosmic rays

Question - 45 : - Nitrogen laser produces radiation ofwavelength of 337.1 nm. If the number of photons emitted is 5.6 x 10²⁴,calculate the power of this laser.

Answer - 45 : -

Question - 46 : - Neon gas is generally used in sign boards. Ifit emits strongly at 616 nm, calculate :
(a) frequency of emission (b) thedistance travelled by this radiation in 30s (c) energy of quantum (d) number ofquanta present if it produces 2 J of energy.

Answer - 46 : -

Question - 47 : - In astronomical observations, signals observedfrom the distant stars are generally weak. If the photon detector receives atotal of 3.15 x 10^-18 J from the radiations of 600 nm,calculate the number of photons received by the detector.

Answer - 47 : -

Question - 48 : - Life times of the molecules in the excitedstates are often measured by using pulsed radiation source of duration nearlyin the nano second range. If the radiation source has the duration of 2 ns andthe number of photons emitted during the pulse source is 2.5 x 10¹⁵,calculate the energy of the source.

Answer - 48 : - Time duration (t) = 2 ns = 2 x 10^-9 s

Energy of one photon,E = hv = (6.626 x 10^-34 Js) x (10⁹/2 s^-1)= 3.25 x 10^-25J
No. of photons = 2.5 x 10⁵
∴ Energy of source =3.3125 x 10^-25 J x 2.5 x 10¹⁵ = 8.28 x 10^-10 J

Question - 49 : - The longest wavelength doublet absorptiontransition is observed at 589 nm and 589.6 nm. Calculate the frequency of eachtransition and energy difference between two excited states.

Answer - 49 : -

Question - 50 : - The work function for cesium atom is 1.9 eV.Calculate (a) the threshold wavelength and (b) the threshold frequency of theradiation. If the cesium element is irradiated with a wavelength 500 nm,calculate the kinetic energy and the velocity of the ejected photoelectron

Answer - 50 : -

Free - Previous Years Question Papers

Chapter 2 Structure of The Atom Solutions

Subscribe Now!