Atoms-2 Flashcards
3 postulates of bohr’s atomic theory
i) an electron in an atom could revolve
in certain stable orbits without the emission of radiant energy. According to
this postulate, each atom has certain definite stable states in which it can exist, and each possible state has definite total energy. These are
called the stationary states of the atom.
ii) the electron revolves around the nucleus only in those orbits for which the angular momentum is some integral multiple of h/2p where h is the Planck’s constant (= 6.6 ´ 10–34 J s). Thus the
angular momentum (L) of the orbiting electron is quantised.
iii) an electron might make a transition from one of its specified non-radiating orbits to another of lower energy. When it does so, a photon is emitted having energy equal to the energy
difference between the initial and final states. The frequency of the emitted photon is then given by
hn = Ei– Ef
where Ei and Ef are the energies of the initial and final states and Ei > Ef.
ground state
The energy of an atom is the least (largest negative value)
when its electron is revolving in an orbit closest to the
nucleus i.e., the one for which n = 1.
The lowest state
of the atom, called the ground state, is that of the lowest energy, with the electron revolving in the orbit of smallest radius, the Bohr radius, a0
. The energy of this state (n = 1),
E1 is –13.6 eV.
ionisation energy
e, the minimum energy required to
free the electron from the ground state of the hydrogen atom is 13.6 eV. It is called the ionisation energy of the hydrogen
atom.
how is atomic spectra produced
The various lines in the atomic spectra are produced when electrons
jump from higher energy state to a lower energy state and photons are
emitted. These spectral lines are called emission lines.
But when an atom
absorbs a photon that has precisely the same energy needed by the
electron in a lower energy state to make transitions to a higher energy
state, the process is called absorption
DE BROGLIE’S EXPLANATION OF BOHR’S
SECOND POSTULATE OF QUANTISATION
according to de broglie, a stationary orbit is that which contains an integral number of the De broglie waves associate witht he revovolving electrson.
Limitations of Bohr’s Atomic theory
(i) The Bohr model is applicable to hydrogenic atoms. [The formulation of Bohr model involves electrical force between
positively charged nucleus and electron. It does not include the
electrical forces between electrons which necessarily appear in
multi-electron atoms]
ii) While the Bohr’s model correctly predicts the frequencies of the light
emitted by hydrogenic atoms, the model is unable to explain the
relative intensities of the frequencies in the spectrum. In emission
spectrum of hydrogen, some of the visible frequencies have weak
intensity, others strong.
iii) Bohr’s model of the atomwas unable to explain the splitting of spectral lines under the effect of electric field ( stark effect) and magnetic field ( zeeman effect).
iv) Bohr’s model did not explain the fine structure of spectral lines.
v) did not give any explanaton regarding arrangement and distrubution of electrons in an atom.
