The alkali atoms consist of Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb) , Caesium

(Cs) and Francium (Fr) .The alkali elements are placed in the first column in periodic table such

as they have very high electro-positivity. In chemical reactions, they can easily exchange the

loosely bound outermost valence electron(s electron) with electro-negative element. They are

all mono-valent elements, having similar physical and chemical properties. Their spectra show

doublet structure. Their ground states are S❑21 /2

Alkali Spectra

The spectra of alkali atoms show considerable similarity to the spectra of hydrogen atoms.

However the lines in a particular series are found to have regularly decreasing separation and

converge towards a limit as hydrogen spectra. The similarity between spectra of alkali atoms

and hydrogen spectra is due to all alkali metals have one easily removable valance electron

outside a tightly bound core of electron in its orbit.

For example lithium, for which Z=3 has three electrons in its orbit. Two of them revolve in orbits

close to the nucleus and make up the core while the third revolve in outer orbit. Thus, the two

core electrons screen the potential of +3e units. Hence the electrostatic potential felt by the

outermost electron is due to effective charge of +e units as in the case of hydrogen atom for

which Z=1.

The screening effect of nuclear charge +Ze by the core electrons is not complete. The

outermost valence electron revolves in an elliptic orbit with the nucleus at one focus of the

ellipse. During its revolutions, the electron is at varying distances from the nucleus and may

penetrate into the core region. When it is closed to the nucleus, the screening is less effective

due to this penetration and the valence electron is acted upon by an effective positive greater

than +e. The degree of penetration depends upon the eccentricity of the ellipse.

From the theory of elliptical orbits eccentricity (Wilson-Sommerfeld theory) that the

eccentricity ϵ depends on the azimuthal quantum number ¿k−1 . The smaller l is more

eccentric is the ellipse. Thus the valance electron will penetrate more deeply into the core

region and comes so close to the nucleus such that the full nuclear charge +Ze may act on it,

the screening effect being practically null. In this case, the alkali spectra terms will depart from

the hydrogen term. However for the less eccentric orbits (larger l ¿ the screening will be much

effective and the spectral terms of alkali atoms will resemble more closely to the hydrogen

term.

The spectroscopy term notation of alkali atoms assigns different l value by special symbol (S, P,

D, F..etc). For the hydrogen atom, the terms with different l-values for a given principal

quantum number n have only slightly different energy values due to the relativity correction

and electron spins. In cases of alkali atoms due to the different degrees of penetration into the

core region by the valence electron, the energy values for the terms with different l for a given

n are widely different. As the azimuthal quantum number l for a given value of n increase, the

quantum defect will decreased. It is highest for the S-term (most eccentric ellipse) and very low

for the F term (eccentricity very small). The value of p also depends on n. Energy level for the

lithium atom and possible transitions between them are shown in Figure 2 below:

Figure Fig 2: Energy level lithium atoms

The energy level shown above fall into different group according to the values of l. The

transitions between the levels are governed by the selection rule:

∆ l=±1

Thus the transitions can take place from s levels to P levels only; from the P levels to the S and

D levels and so on. The spectra series originating from some of these transitions are given by

special names:

nP n0S : Principal series nSn0P: Sharp series

nDn0P: Diffuse series nFn0D: Fundamental series

From the figure 2, the integers shown in the right hand side of figure correspond to the

positions of hydrogen terms. The term of lithium depart most from the hydrogen term for l =0.

The lack of agreement with the hydrogen terms becomes progressively less as l increases. And

notice that it is the least depart for the F term.

Doublet Structure of the Alkali Spectral lines

Figure 3: Doublet structure of the energy levels of sodium.

The D lines of sodium(Na) belong to the principal series and originate from the transition 3P to

3S as shown in Figure 3 . Each spectral line of any one of the alkali spectral series should be

single. Opposite of this, it is actually found to consist of two closely lying components. They are

known as D1 and D1 which having wavelength of 5896 A and 5890 A respectively. The doublet

structure of the spectral lines is a characteristics feature of spectra of all alkali atoms. This

origin of the doublet spectra can be explained by introducing the concept of electron spin.

Since the only single valence electron outside the core region is responsible for the origin of

alkali spectra, hence the interaction between the orbital magnetic moment μl and the spin

magnetic moment μs of this electrons. This magnetic moment interaction causes a difference in

energy between the two terms of the same l having two different values of inner quantum

number j such as j =l ±12

Such splitting of the energy of the same l but different j is known as multiplicity of term or level.

If an atom having more than one electron has a resultant spin S. For the alkali atom, the

multiplicity in 2 such that S=s=12

.

The P term s with l=1 having j=5/2 or j==1/2 and are designated as P❑2

5 /2 and P❑2

3 /2 respectively.

Since all the energy levels are now split into two due to spin-orbit interaction, the transitions

between them give rise to a number of components in place of a single spectral line. These

transitions are governed by the selection rule:

∆ j=0 , ±1

For sodium lines of the principles series nP n0S with n0=3 , the following transitions are

possible:

32 P12

−→32S12

(D1line )

32 P3/2−→32S1 /2 (D2line )

Which gives the origin for the D lines.

Arrangement of the electrons in the alkali atoms

The model of the many-electron atoms which Niels Bohrn constructed in order to explain the

structure of periodic table was based on detailed spectroscopic and chemical evidence. The

principle behind the model is the Aufbau principle which implies the consideration of orbitals of

the central-field form rather than classical orbit. The orbital are filled according to the (n+l) rule:

The electron configuration of an elements may be determined by filling the orbitals after

increasing values n+l . for the fixed value n+l , orbitals with lower n-values are filled first

For electrons the fundamental symmetry principle:

A many electron wave function must be anti-symmetric that is it must change sign under the

interchange of any pair of electrons.

Interchange of a pair of electrons not only interchanges the positions and momenta of two

particles, but also their spins.

For atom, each spin-orbital may be characterized by four quantum numbers and the

formulation of the Pauli’s exclusion principle is obtained:

No two electrons can have the same set of quantum number

Lithium with Z=3, three of electrons in Li atom, two go to 1s subshell in the K shell which

thereby completed. The third electron goes to the 2s subshell in the L shell. Thus the electronic

configuration of Li is 1 s22 s1

The Sodium(Z=11), after completely filling up of the K and L shell with 2 and 8 electrons

respectively , the eleventh electrons goes to M shell with n=3. This electron goes to the 3s

subshell so that the electronic configuration of Na is1 s22 s22 p63 s1

For the potassium (Z=19) with which fourth period begins will go to the 3d subshell. This

situation does not happen. Instead it goes to the 4s subshell. This is due to the fact that this

electron is more strongly bound in the 4s subshell than in the 3d subshell.

The elements Rubidium, Rb (Z=37) located at fifth period, the additional electrons go to the 5s

subshell in the O shell outside the completed 4p subshell of the N shell. For Caesium Cs (Z=55)

the additional electrons go to the 6s subshell of the P shell. And lastly for Francium Fr(Z=87) the

extra electrons go to the 7s subshell of the Q shell.