Question

The energy required for an electron in lithium 2+ ion to be emitted from n = 2 is

Answer 1

Atoms are the basic building blocks of matter which has mass and occupies space. They cannot be chemically subdivided by ordinary means. The word ‘atom’ is derived from the Greek word atom which means indivisible. Matter can be broken into small particles called atoms which are too small to be seen through the naked eye.

As discussed earlier, atoms are composed of three types of particles namely protons, neutrons and electrons. A major proportion of mass of an atom is due to protons and neutrons while electrons have very small mass of about (9.108 X 10-28 grams).Atoms are the basic unit of matter and it is vital to have a thorough knowledge of its components like electrons, protons and neutrons.

We have enlisted the information of various particles along with their charges in the table given below:

  Particle

Charge

Mass (g)

Mass (amu)

Proton

+1

1.6727 x 10-24 g

1.007316

Neutron

0

1.6750 x 10-24 g

1.008701

Electron

-1

9.110 x 10-28 g

0.000549

We discuss some of the illustrations on the structure of an atom:

Problem 1:-

A doubly ionized Lithium atom is hydrogen like with atomic number 3.

(i) Find the wavelength of radiation required to excite the electron in Li++ from the first to the third Bohr Orbit. (Ionization energy of the hydrogen atom equals 13.6 eV).

(ii) How many spectral lines are observed in the emission spectrum of the above excited system?

Solution:-

We are required to find the energy required to excite doubly ionized lithium. We know that, 

En = –13.6 Z2 / n2

Hence, the excitation energy = ΔE = E3 – E1 = –13.6 × (3)2 [1/32 – 1/12]

= +13.6 × (9) [1–1/9] = 13.6 × (9) (8/9) = 108.8 eV.

Wavelength λ = hc / ΔE = (6.63 × 10–34) (3×108) / (13.6 × 8) (1.6 × 10–19)

= (6.63 × (3) / (13.6) (8)(1.16)) 10–7 

= 114.26 × 10–10 m 

= 1143 A

(ii) From the excited state (E3), coming back to ground state, there can be 3C2 = 3 possible radiations.