Atoms and structure

Chapter 2 – Structure of Atom
Page No 65:

Question 2.1:
(i) Calculate the number of electrons which will together weigh one gram.

(ii) Calculate the mass and charge of one mole of electrons.

Answer:

(i) Mass of one electron = 9.10939 × 10 –31 kg

Number of electrons that weigh 9.10939 × 10 –31 kg = 1

Number of electrons that will weigh 1 g = (1 × 10 –3 kg)




= 0.1098 × 10 –3 + 31

= 0.1098 × 10 28

= 1.098 × 10 27

(ii) Mass of one electron = 9.10939 × 10 –31 kg

Mass of one mole of electron = (6.022 × 10 23 ) × (9.10939 ×10 –31 kg)

= 5.48 × 10 –7 kg

Charge on one electron = 1.6022 × 10 –19 coulomb

Charge on one mole of electron = (1.6022 × 10 –19 C) (6.022 × 10 23 )

= 9.65 × 10 4 C



Question 2.2:
(i) Calculate the total number of electrons present in one mole of methane.

(ii) Find (a) the total number and (b) the total mass of neutrons in 7 mg of 14 C.


(Assume that mass of a neutron = 1.675 × 10 –27 kg).

(iii) Find (a) the total number and (b) the total mass of protons in 34 mg of NH 3 at STP.

Will the answer change if the temperature and pressure are changed?

Answer:
(i) Number of electrons present in 1 molecule of methane (CH 4 )

,{1(6) + 4(1)} = 10

Number of electrons present in 1 mole i.e., 6.023 × 10 23 molecules of methane

= 6.022 × 10 23 × 10 = 6.022 × 10 24

(ii) (a) Number of atoms of 14 C in 1 mole= 6.023 × 10 23

Since 1 atom of 14 C contains (14 – 6) i.e., 8 neutrons, the number of neutrons in 14 g
of 14 C is (6.023 × 10 23 ) ×8. Or, 14 g of 14 C contains (6.022 × 10 23 × 8) neutrons.

Number of neutrons in 7 mg




= 2.4092 × 10 21

(b) Mass of one neutron = 1.67493 × 10 –27 kg

Mass of total neutrons in 7 g of 14 C


= (2.4092 × 10 21 ) (1.67493 × 10 –27 kg)

= 4.0352 × 10 –6 kg

(iii) (a) 1 mole of NH 3 = {1(14) + 3(1)} g of NH 3

= 17 g of NH 3

= 6.022× 10 23 molecules of NH 3

Total number of protons present in 1 molecule of NH 3

= {1(7) + 3(1)}

= 10

Number of protons in 6.023 × 10 23 molecules of NH 3

= (6.023 × 10 23 ) (10)

= 6.023 × 10 24

⇒ 17 g of NH 3 contains (6.023 × 10 24 ) protons.

Number of protons in 34 mg of NH 3




= 1.2046 × 10 22

(b) Mass of one proton = 1.67493 × 10 –27 kg

,Total mass of protons in 34 mg of NH 3

= (1.67493 × 10 –27 kg) (1.2046 × 10 22 )

= 2.0176 × 10 –5 kg

The number of protons, electrons, and neutrons in an atom is independent of temperature
and pressure conditions. Hence, the obtained values will remain unchanged if the
temperature and pressure is changed.



Question 2.3:
How many neutrons and protons are there in the following nuclei?

, , , ,




Answer:
13
6 C:

Atomic mass = 13

Atomic number = Number of protons = 6

Number of neutrons = (Atomic mass) – (Atomic number)

= 13 – 6 = 7

:

Atomic mass = 16

Atomic number = 8

Number of protons = 8

Number of neutrons = (Atomic mass) – (Atomic number)

= 16 – 8 = 8

:

Atomic mass = 24

Atomic number = Number of protons = 12

Number of neutrons = (Atomic mass) – (Atomic number)

= 24 – 12 = 12

:

Atomic mass = 56

, Atomic number = Number of protons = 26

Number of neutrons = (Atomic mass) – (Atomic number)

= 56 – 26 = 30

:

Atomic mass = 88

Atomic number = Number of protons = 38

Number of neutrons = (Atomic mass) – (Atomic number)

= 88 – 38 = 50



Question 2.4:
Write the complete symbol for the atom with the given atomic number (Z) and Atomic mass
(A)

(i) Z = 17, A = 35

(ii) Z = 92, A = 233

(iii) Z = 4, A = 9

Answer:

(i) (ii) (iii)



Question 2.5:
Yellow light emitted from a sodium lamp has a wavelength (λ) of 580 nm. Calculate the
frequency (ν) and wave number ( ) of the yellow light.

Answer:
From the expression,




We get,


…….. (i)

Where,

ν = frequency of yellow light

c = velocity of light in vacuum = 3 × 10 8 m/s

λ = wavelength of yellow light = 580 nm = 580 × 10 –9 m