Gross features of the atom – J.J. Thompson’s experiment and Bohr’s model of the atom, outline description of the Rutherford’s alpha scattering experiment
SECOND TERM E-LEARNING NOTES
SUBJECT: CHEMISTRY
CLASS: SSS 3
Gross features of the atom – J.J. Thompson’s experiment and Bohr’s model of the atom, outline description of the Rutherford’s alpha scattering experiment
WEEK 8
TOPIC: ATOMIC THEORY
CONTENT:
1. GROSS FEATURES OF THE ATOM – J.J. THOMPSON’S EXPERIMENT
2 BOHR’S MODEL OF THE ATOM,
3. OUTLINE DESCRIPTION OF THE RUTHERFORD’S ALPHA SCATTERING EXPERIMENT.
PERIOD 1: J.J. THOMPSON’S CATHODE RAYS EXPERIMENT
Experiments on discharge tubes performed by J.J Thomson (1897) led to the discovery of the electron (cathode ray) as a subatomic particle .Cathode rays:
- move in a straight line normal to the cathode and can cast the shadow of an object placed along their path.
- possess kinetic energy and so can cause the motion of a mechanical paddle wheel placed along their path.
- attract positive charge and repel negative charge, indicating that they are negatively charged.
- have a constant value of 1.76 x coulomb for the charge –to-mass ratio () no matter the gas used in the tube or the nature of the materials of the electrodes and the tube. This indicates that electrons are fundamental particles of all atoms and they are indistinguishable i.e. the universality of electros in atoms.
A typical discharge tube experiment
EVALUATION
- Write Short Note on J.J Thomson Electron Discovery
- State the properties of cathode rays
PERIOD 2: BOHR’S MODEL OF THE ATOM
Niels Bohr (1913) put forward his own model of the atom based on the quantum mechanics originally developed by plank. He assumed the Rutherford’s model and suggested that the extranuclear part consists of electrons in specific spherical orbits (shells or energy levels) around the nucleus. The orbits/energy levels are designated by letters: K, L, M, N…
The maximum number of electrons that can be accommodated by an energy level is where n is the value of the principal energy level. As one moves outwards from the nucleous the energy of the energy levels increases.
| Principal Energy Level(n) | Designated | Maximum number of Electrons() |
| 1 2 3 4 5 6 | K L M N O P | 2 8 18 32 50 72 |
Energy Levels and Maximum Number of Electrons
Bohr’s model also explaind the discrete lines in the atomic spectrum of hydrogen.This is based on the concept that the electron can move roundd the nucleous in certain permissible orbits(energy levels).the ground state (n=1) is the lowest energy state avilableto the electron.This is usually the most stable state.TThe excited state (n=2,3…) is anylevel higher than the ground state.An electron in an energy level has a definite amount of energy associated with it that level. The electron can undergo transitions from one level to another.However,when the electron changes its state,it must absorb or emit the exact amount of energy(∆E)which will bring it from the initial state to the finnal state.
∆Eelectron = E final – E initial
When the electron is excited,it moves from the ground state to an excited state,it must absorb energy.On returning from an excited state to a lower level or the ground state,energy is emitted,giving rise to a line in the spectrum. This release of energy is the basis for the hydrogen atomic spectrum.
Line series in the hydrogen spectrum
| Series Name | n1 | n2 Values | Spectral Region |
| Lyman | 1 | 2 to ∞ | Ultraviolet |
| Balmer | 2 | 3 to ∞ | Visible |
| Paschen | 3 | 4 to ∞ | Infrared |
| Brackett | 4 | 5 to ∞ | Far infrared |
| Pfund | 5 | 6 to ∞ | Far infrared |
Description of the Line Series of the Hydrogen Spectrum
EVALUATION
Describe the structure of an atom.
PERIOD 3: OUTLINE DESCRIPTION OF THE RUTHERFORD’S ALPHA SCATTERING EXPERIMENT.
Ernest Rutherford (1911) performed the gold foil experiment that gave the modeling of the atomic structure a boost. In this experiment, he bombarded a thin gold foil with alpha particles (generated from a radioactive source).He found that most of the alpha particles passed through the foil while a few of them(1 out of 8000) were deflected back.
In the bid to explain the above observation, Rutherford suggested an atomic model (the nuclear model) in which an atom has a small positively charged center (nucleus) where nearly all the mass is concentrated. Surrounding the nucleus is a large space (extranuclear part) containg the electrons. Further experiment showed that the ‘heavy’ nucleus was made up of still smaller particles called proton.
He proposed the planetary model of atom which states that Atom consists of a positively charged nucleus, where most of the mass of the atom is contained and electrons which move in orbit around the nucleus and are held in their orbits by the electrostatic force of attraction to the positively charged nucleus
EVALUATION
- Explain the sources of emission and absorption spectra line atom.
- Write the names of the scientists that contribute to the development of atomic structure.