FRAUNHOFER’S LINES when white light from the sun is examined through a spectrometer, it is found to be crossed by hundreds of dark lines in the different parts of the solar spectrum. These dark lines were first observed by Wollaston in 1802, but their proper explanation was given by Fraunhofer in 1814. Fraunhofer, labelled the prominent lines by the letters of alphabet . For example, the dark line in the blue part of the spectrum was called F – line, the dark line in the yellow part as D – line and the dark line in the red part as C – line . These dark lines are called Fraunhofer’s dark lines.
The sun has a central hot core and it emits continuous visible spectrum. The Central hot core is surrounded by various elements in the vaporised state and comparatively colder than the core . When the light from the central hot core passes through the elements in the vapor state, these absorb their own characteristic wavelengths.
The best illustration of Kirchhoff ‘s law is met on the day of total solar eclipse. Just before the solar eclipse, continuous spectrum of the sun with Fraunhofer’s dark lines is obtained . The dark lines indicate that the comparatively colder elements ( in the vapour form ) in the sun’s atmosphere have absorbed their characteristic wavelengths. During total eclipse , bright lines are seen exactly in the position of the dark lines indicating that the elements in incandescent vapour state have emitted these wavelengths strongly .
The wavelengths of various dark lines in the solar spectrum have been measured completely and it has been found that they are due to the presence of various elements in the sun’s atmosphere. The study of the dark lines in the sun’s spectrum indicated the presence of hydrogen and helium in the atmosphere of the sun.
MANY ELECTRON ATOMS
The hydrogen atom contains only one proton and one electron and is simplest of all the atoms . The only electron in the hydrogen atom interacts with the only proton inside it and as such the study of hydrogen spectrum is quite simple.
In atoms having more than one electron, each electron interacts not only with the positive charged nucleus but also with the other electrons . Attempts were made to study the spectrum of many electron atoms on the basis of Bohr’s model of hydrogen atom, but no success was met . Several approximation were used to study the spectrum of many electron atoms.
Following are two important approximations:
1. Ignore the interactions between electrons completely and regard each electron interacting with the positively charged nucleus , considered to be a point charge .
2. Consider the electrons to be making a spherically symmetric charge cloud and each electron moving in the combined electric field of the positively charged nucleus and the negatively charged electron cloud. It is called central field approximations.
The central field model suggest that each electron has a lowest energy state ( n= 1 state for a hydrogen like atom) . In the ground state of a many electron atom , all the electrons are expected to be in the lowest energy state . If it is so , then there should be a gradual change in the physical and chemical properties of the elements with increasing atomic numbers . Following evidences indicate that it does not happen so:
(a) In an atom of fluorine , neon and sodium, the number of electrons are 9, 10, 11 respectively. It is known that fluorine and neon are gases , whereas sodium is a solid . Further, fluorine and sodium are chemically most active and neon is an inert gas.
(b ) If a graph is plotted between atomic number (Z) and ionisation energy of various elements, the graph has a repetitive nature . The ionisation energy curve shows peaks corresponding to the elements of group VIII of the periodic table I.e He (Z=2) ,Ne (Z=10) , Ar(Z=18), Kr(Z=36), Xe( Z= 54), and Rn(Z=86). On the other hand, it shows minimum corresponding to the elements of group I of the periodic table I.e. H (Z=1), Li(Z=3), Na(Z=11), K(Z=19), Rb(Z=37), C’s(Z=55) and Fr(Z=87). The remarkable Similarly between the chemical and physical of the elements in each group is an evidence that the atoms are made according to systematic rules and that all the electrons in a many electron atom are not in the lowest energy state.
A rule which decides the number of electrons in a given energy state was formulated by Wolfgang Pauli in 1925 . It is known as Pauli exclusion principle.
It states that not more than two electrons can occupy the same quantum state.
In Bohr’s atom model , the quantum -mechanical state of an electron is identified by a single quantum number n. But in central field model , the quantum – mechanical state of an electron is identified by a set of three quantum numbers n, l, ml. All the three quantum numbers are integers.
The first quantum number n is called principle quantum number and can have positive integral values i.e. 1,2,3………… . The energy of a state and it’s distance from nucleus increase with increase in the value of n.
The second quantum number l is called orbital quantum number.
The third quantum number ml is called magnetic quantum number.