Evidences for electronic energy levels of atoms

1.Variation of successive ionisation energies

The above graph illustrates all the successive ionization energies of chlorine.The number of successive ionisation energies is equal to the number of electrons in the atom. When attention is paid to sudden increase in successive ionisation energies it is proved that electrons exist as groups in various energy levels.

2.Absorption spectrum

3.Emission spectrum

Limitations of Bohr model

While the Bohr model explains the line spectrum of the hydrogen atom, it cannot explain the spectra of other atoms, except in a rather crude way. Bohr also avoided the problem why the negatively charged electrons would not just fall into the positively charged nucleus by simply assuming it would not happen. Therefore, there is a problem with describing an electron merely as a small particle circling about the nucleus.

s, p, d and f sub-energy levels

Shapes of orbitals

Wave – particle nature of electrons

• Wave properties : While passing through an ionic crystal a beam of electrons gets diffracted in the same way as a beam of X-rays does. A beam of electrons also show interference patterns.                                      • Particle properties : A beam of electrons has ability to do work (due to the momentum) and also it has a charge.

Quantization of energy

• Atoms absorb or emit energy in the form of definite small quantities.                                                             • The smallest quantity of energy is referred as ‘quantum’ or ‘photon’.                                                              • According to Planck, matter absorbs or emits energy in the form of photons or whole number multiples    of it.  eg. hυ , 2 hυ , 3 hυ , ………….                                                                                                                                   • Hence, it is considered as energy is quantized.

Quantum numbers

• Principal quantum number (n)

The main energy level to which the electron belongs is represented by this quantum number. n = 1, 2, 3, …..

• Azimuthal quantum number (l)

The sub – energy level (s,p,d,f….) to which the electron belongs is represented by this quantum number. l = 0, 1, 2, ……, (n-1)

• Magnetic quantum number (m_l )

The orbital (Eg. p _x , p _y , p _z ) in which the electron exists in a certain sub- energy level is represented by this quantum number m_l = -l, (-l+1),….,0,…..,(l-1), l

• Spin quantum number (m_s )

The direction of spin of an electron present in a certain orbital is represented by this quantum number. m_s = ± ½

Assignment of the four quantum numbers n, l, m_l  and m_s for electrons in the first four energy levels of an atom