Structure of atom
- 1. 2. Structure Of Atom Sub-atomic particles: Relative Name Symbol Charge/C Mass/kg charge Electron e –1.6022 × 10–19 –1 9.1094 × 10–31 Proton p +1.6022 × 10–19 +1 1.6726 × 10–27 Neutron n 0 0 1.6749 × 10–27 Discovery of Electron Glass tube is partially evacuated (Low pressure inside the tube) Very high voltage is applied across the electrodes Results: Cathode rays move from the cathode to the anode. Cathode rays are not visible. These rays travel in a straight line in the absence of electric and magnetic fields. The behaviour of cathode rays is similar to negatively charged particles (electrons) in the presence of an electrical or a magnetic field. Characteristics of cathode rays do not depend upon: the material of the electrodes and the nature of the gas present in the tube Charge to Mass Ratio of Electron: e 1.7558820 1011 C kg –1 m Thomson Atomic Model
- 2. An atom possesses a spherical shape in which the positive charge is uniformly distributed. Rutherford’s atomic model: Positive charge and most of the mass of atom was densely concentrated in an extremely small region called nucleus. Electrons move around the nucleus with high speed in circular paths called orbits. Electrons and the nucleus are held together by strong electrostatic forces of attraction. Drawbacks: Cannot explain the stability of the atom Does not explain the electronic structure of atoms Atomic Number and Mass Number Atomic number (Z) = Number of protons in the nucleus of an atom = Number of electrons in a neutral atom Mass number (A) = Number of protons (Z) + Number of neutrons (n) Isobars and Isotopes Isotopes are the atoms with the same atomic number, but different mass numbers. For example, and are the isotopes of hydrogen. Isobars are the atoms with the same mass number, but different atomic numbers. For example, and ; and Bohr’s model of atom: Developments that lead to the formulation of Bohr’s model of atom: Dual character of electromagnetic radiation i.e., wave-like and particle-like Only quantized electronic energy levels in atoms can explain atomic spectra. Speed of light = 3.0 × 108 m s–1
- 3. Velocity c Frequency Wavelength 1 Wave number, (S.I unit = m–1) Particle Nature of electromagnetic radiation Plank quantum theory E h ; E = Energy = Frequency h = Planck’s constant h = 6.626 × 10–34 Js Photoelectric Effect Results: 1 h h 0 me v 2 2 h 0 Minimum energy required to eject the electron (Work function, W0) h Energy of the striking photon 1 me v 2 K.E of the photoelectron 2 Evidence for the quantized electronic energy levels:
- 4. Line spectrum of Hydrogen 1 1 1 109, 677 cm n12 2 n2 Where, n1 = 1, 2,……… n2 = n1 + 1, n1 + 2,…… 109,677 cm–1 = Rydberg constant for hydrogen The formula that describes Balmer series is 1 1 109, 677 cm 1 (n = 3, 4, 5,…..) 22 n2 Spectral lines for atomic hydrogen: Series n1 n2 Spectral region Lyman 1 2, 3,…. UV Balmer 2 3, 4,…. Visible Paschen 3 4, 5,…. IR Brackett 4 5, 6,…. IR Pfund 5 6, 7,…. IR Bohr’s model of hydrogen atom The electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy called orbit (stationary states or allowed energy states). Energy of an electron in the orbit does not change with time. Angular momentum = I × I = Moment of inertia = me r 2 v = Angular velocity = (v is linear velocity) r Bohr’s frequency rule E E2 E1 h h Angular momentum = mevr h me vr n , n = 1, 2, 3….. 2 n = 1, 2, 3……are principal quantum numbers rn n2 a0 ; a0 529 pm
- 5. rn is the radii of the stationary states. 1 En RH n 1, 2,3..... n2 RH is Rydberg’s constant. Energy associated with ions such as He+, Li2+, etc. (hydrogen-like species) is – Z2 En = 2.18 10 18 J n2 52.9 n2 And radii, rn pm Z Limitations of Bohr’s model It was unable to explain the finer details of the hydrogen atom spectrum. It was also unable to explain the splitting of spectral lines in presence of magnetic and electric field. Could not explain the ability of atoms to form molecules by chemical bonds Dual behaviour of matter (de Broglie equation): h h mv p Heisenberg’s Uncertainty Principle: h x px 4 h Or, x vx 4 m Where, x = uncertainty in position p = uncertainty in momentum Quantum mechanical model of atom: Schrodinger equation: H E
- 6. H is Hamiltonian operator. Orbitals and Quantum numbers: Principal quantum number (n) n=1 2 3 4…. Shell = K L M N…. Azimuthal quantum number (l) For a given value of n, possible values of ‘l’ are: 0, 1, 2, 3, …….(n – 1) L 0 1 2 3 4 5… Notation for sub- s p d f g h… shell Magnetic quantum number (ml) For any sub-shell, ml l, l 1, l 2 .....0,1,...... l 2 , l 1 , l Value of l 0 1 2 3 4 5 Sub-shell notation s p d f g h No. of orbitals 1 3 5 7 9 11 1 1 Electron spin: ms or 2 2 Five d-orbitals: d xy , d yz , d zx , d x 2 y2 , d z2 Three p-orbitals: px, py, pz And, there are seven f orbitals. Shapes of Atomic Orbitals
- 7. Boundary surface diagrams for 1s and 2s orbitals are: Boundary surface diagram for three 2p orbitals Boundary diagrams for the five 3d orbitals are shown in the figure below. The total number of nodes is given by (n-1) i.e, sum of l angular nodes and (n-l- 1) radial nodes.
- 8. Energies of Orbitals Energy of the orbitals in a hydrogen atom increases as 1s 2s 2 p 3s 3p 3d 4s 4p 4d 4f ... Energy of the orbitals in a multi-electron atom follows the following rules: Lower the value of (n + l) of an orbital, lower is its energy. When the two orbitals have same (n + l) value, the orbital with lower value of ‘n’ will have lower energy. Aufbau’s principle: The orbitals are filled in order of their increasing energies (in the ground state). Increasing order of the energy of the orbitals and hence, the order of the filling of orbitals: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d, 6p, 7s, … Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers. Hund’s Rule of maximum Multiplicity: Pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each i.e., it is singly occupied. Electronic configuration of different atoms can be represented as: (i) s p d c ............ notation a b (ii) Orbital diagram Fully-filled and half-filled orbitals are the stable orbitals.
- 9. Students’ Contribution to Revision Notes Exceptional cases in electronic configuration Configurations where the outer sub-shells are half-filled or completely filled provide extra stability to the atom. This is owing to the following reasons: Greater symmetry Greater exchange energy Elements such as Chromium (Cr) and Copper (Cu) deviate from the general rule of electronic configuration to attain half-filled and completely filled configuration respectively ensuring extra stability. Copper (Z=29) The expected configuration of Cu (Z=29) is 1s2 2s2 2p6 3s2 3p6 4s2 3d9 But the actual configuration is obtained by promoting one electron from 4s to 3d as shown below: The actual outer configuration of Cu then becomes: The above configuration is preferred by copper because it has fully filled‘d’ sub-shell which is more stable. Hence the actual configuration of Copper is 1s2 2s2 2p6 3s2 3p6 4s1 3d10 – Contributed by: Manish Sahu