ATOMS, MOLECULES AND NUCLEI

1. What is the minimum angular momentum of the electron in an

hydrogen atom?

Ans: h/2π.

2. Which physical quantity of an atomic electron has the dimensions

same as that of h?

Ans: Angular momentum.

3. How do the potential and kinetic energies of the electron in a

hydrogen atom change when the atom is raised from the ground

sate to an excited state?

Ans: For the electron in a hydrogen atom,

potential energy = - 4

2 2 20

me

4 h n and

kinetic energy = 4

2 2 20

me

8 h n

Thus, when the atom is raised to one of its excited sates (higher n),

the electron’s potential energy increases (becomes less negative)

and kinetic energy decreases.

4. How many spectral series are possible in the hydrogen spectrum?

Ans: Infinite. The last(sixth) of the named series in the hydrogen

spectrum is the Humphreys series which results from transitions to

nf = 6 from ni = 7, 8, 9, .... etc. in the far – infrared region. Further

series are unnamed but follow the same pattern. Their lines are

increasingly faint showing that they correspond to increasingly rare

atomic events.

[Note: There are infinite numbers of lines in each series. The

spacing between adjacent lines decreases with decreasing

wavelength, converging to the so- called series limit or short –

wavelength limit.

5. Which hydrogen spectral series was found first?

Ans: The Balmer series in the visible region.

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