ATOMIC SPECTRUM OF HYDROGEN

ATOMIC SPECTRUM OF HYDROGEN

The emission line spectrum

 of hydrogen can be obtained by passing electric discharge through

the gas contained in a discharge tube at low pressure. The light radiation emitted is then examined

with the help of a spectroscope. The bright lines recorded on the photographic plate constitute the

atomic spectrum of hydrogen 

In 1884 J.J. Balmer observed that there were four prominent coloured lines in the visible hydrogen

spectrum :

(1) a red line with a wavelength of 6563 Å.

(2) a blue-green line with a wavelength 4861 Å.

(3) a blue line with a wavelength 4340 Å.

(4) a violet line with a wavelength 4102 Å.

Hydrogen

discharge tube

The examination of the atomic spectrum of hydrogen with a spectroscope.

The above series of four lines in the visible spectrum of hydrogen was named as the Balmer

Series. By carefully studying the wavelengths of the observed lines, Balmer was able empirically to

give an equation which related the wavelengths (λ) of the observed lines. The Balmer Equation is

1/λ=R( 1/2^2-1/n^2)

where R is a constant called the Rydberg Constant which has the value 109, 677 cm– 1 and n = 3, 4,

5, 6 etc. That is, if we substitute the values of 3, 4, 5 and 6 for n, we get, respectively, the wavelength

of the four lines of the hydrogen spectrum.

Blue-green Blue Violet

6563 4861 434

Balmer series in the Hydrogen spectrum. 

In addition to Balmer Series, four other spectral series were discovered in the infrared and

ultraviolet regions of the hydrogen spectrum. These bear the names of the discoverers. Thus in all

we have Five Spectral Series in the atomic spectrum of hydrogen :

Name                                  Region where located                        

(1) Lyman Series Ultraviolet ( 1)     UV

(2) Balmer Series Visible       (2)  visible

(3) Paschen Series Infrared  (3) Infrared

(4) Brackett Series Infrared   (4) infrared

(5) Pfund Series                     (5)infrared

    Balmer equation had no theoretical basis at all. Nobody had any idea how it worked so

accurately in finding the wavelengths of the spectral lines of hydrogen atom. However, in 1913 Bohr

put forward his theory which immediately explained the observed hydrogen atom spectrum. Before

we can understand Bohr theory of the atomic structure, it is necessary to acquaint ourselves with the

quantum theory of energy

CONTINUOUS SPECTRUM:

 White light is radiant energy coming from the sun or from incandescent lamps. It is composed of

light waves in the range 4000-8000 Å. Each wave has a characteristic colour. When a beam of white

light is passed through a prism, different wavelengths are refracted (or bent) through different

angles. When received on a screen, these form a continuous series of colour bands : violet, indigo,

blue, green, yellow, orange and red (VIBGYOR). This series of bands that form a continuous

rainbow of colours, is called a Continuous SpectrumWhite light is radiant energy coming from the sun or from incandescent lamps. It is composed of

light waves in the range 4000-8000 Å. Each wave has a characteristic colour. When a beam of white

light is passed through a prism, different wavelengths are refracted (or bent) through different

angles. When received on a screen, these form a continuous series of colour bands : violet, indigo,

blue, green, yellow, orange and red (VIBGYOR). This series of bands that form a continuous

rainbow of colours, is called a Continuous Spectrum

   The violet component of the spectrum has shorter wavelengths (4000 – 4250 Å) and higher

frequencies. The red component has longer wavelengths (6500 – 7500 Å) and lower frequencies.

The invisible region beyond the violet is called ultraviolet region and the one below the red is

called infrared region

Significance of the Henderson-Hasselbalch equation

Significance of the Henderson-Hasselbalch equation

With its help :

(1) The pH of a buffer solution can be calculated from the initial concentrations of the weak acid

and the salt provided Ka is given.

However, the Henderson-Hasselbalch equation for a basic buffer will give pOH and its pH can be

calculated as (14 – pOH).

(2) The dissociation constant of a weak acid (or weak base) can be determined by measuring the

pH of a buffer solution containing equimolar concentrations of the acid (or base) and the salt.

[salt] pH = p + log [acid]

Ka

Since

[salt] [salt] = [acid], log log1 0 [acid] = =

∴ pKa = pH

The measured pH, therefore, gives the value of pKa of the weak acid.

Likewise we can find the pKb of a weak base by determining the pH of equimolar basic buffer.

(3) A buffer solution of desired pH can be prepared by adjusting the concentrations of the salt

and the acid added for the buffer.

It is noteworthy that buffer solution are most effective when the concentrations of the weak acid

(or weak base) and the salt are about equal. This means that pH is close to the value of pKa of the acid

(or pKb of the base).