General Chemistry I Lecture Notes PDF

General Chemistry I Dr. Mahmoud A. A. Ibrahim Chemistry Department, Faculty of Science, Minia University Email: [email protected] 1 Mahmoud A. A. Ibrahim...

General Chemistry I Dr. Mahmoud A. A. Ibrahim Chemistry Department, Faculty of Science, Minia University Email: [email protected] 1 Mahmoud A. A. Ibrahim CH101  Lecture No. 1 Models of Atom 2 Mahmoud A. A. Ibrahim CH101 Models of atom 1. Thomson’s model 2. Rutherford’s model 3. Bohr’s model 3 Mahmoud A. A. Ibrahim CH101 1. Models of atom Thomson’s model By 1911 the components of the atom had been discovered. The atom consisted of subatomic particles called protons and electrons. However, it was not clear how these protons and electrons were arranged within the atom. J.J. Thomson suggested a model in which the electrons and protons are uniformly mixed throughout the atom. 4 Mahmoud A. A. Ibrahim CH101 1. Models of atom Rutherford’s model Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Rutherford reasoned that if Thomson's model was correct then the mass of the atom was spread out throughout the atom. In gold foil experiment, Rutherford bombarded a gold foil with alpha particles (Helium nuclei). 5 Mahmoud A. A. Ibrahim CH101 1. Models of atom Rutherford’s model A source which undergoes alpha decay is placed in a lead box with a small hole in it. Any of the alpha particles which hit the inside of the box are simply stopped by the box. Only those which pass through the opening are allowed to escape, and they follow a straight line to the gold foil. 6 Mahmoud A. A. Ibrahim CH101 1. Models of atom Rutherford’s model Observations: Most of the alpha particles pass straight through the gold foil. Some of the alpha particles get deflected by very small amounts. A very few get deflected greatly. Even fewer get bounced of the foil and back to the left. 7 Mahmoud A. A. Ibrahim CH101 1. Models of atom Rutherford’s model Conclusions The atom is 99.99% empty space. The nucleus contains a positive charge and most of the mass of the atom. The nucleus is approximately 100,000 times smaller than the atom. 8 Mahmoud A. A. Ibrahim CH101 1. Models of atom Rutherford’s model Difficulty The model was not able to explain the stability of atoms and the atomic spectra standing on the viewpoint of the classical theory. 9 Mahmoud A. A. Ibrahim CH101 1. Models of atom According to all the principles of physics known in 1911, the nuclear atom should have been unstable. If the electron were stationary, there was nothing to keep them from being drawn into the nucleus; if they were in circular motion, the well-documented laws of electromagnetic predicted that the atom should radiate light until all electronic motions ceased. Only two years after Rutherford’s model, Bohr attempted to resolve this paradox by analyzing the atomic structure in terms of the quantum theory of energy. 10 Mahmoud A. A. Ibrahim CH101 1. Models of atom Bohr’s model Bohr postulates that: The electron in an atom has only a certain definite stationary states of motion allowed to it; each of these stationary states has a definite, fixed energy. When an atom is in one of these states, it does not radiate, but when it changes from a high-energy state to a state of lower energy, the atom emits a quantum of radiation whose energy is equal to the difference in the energy of the two states. In any of these states the electron moves in a circular orbit about the nucleus. 11 Mahmoud A. A. Ibrahim CH101 1. Models of atom Bohr’s model The states of allowed electronic motion are those in which the angular momentum of the electron is integral multiplies of h/2π. The first two postulates are correct and are retained in the modern quantum theory. The fourth postulate is partially correct, the angular momentum of an electron is fixed, but not quite the way Bohr proposed. The third postulate is entirely incorrect and doesn’t appear in the modern quantum theory. 12 Mahmoud A. A. Ibrahim CH101 1. Models of atom Bohr’s model The derivation of the expression giving the energies of the allowed states of an atom is very simple: First, mechanical stability of the electron orbit requires that the Coulombic force between the electron and nucleus be balanced by the centrifugal force due to the circular motion: i.e. Coulombic force = centrifugal force Ze2 mn 2 = 4pe 0 r 2 r where m and v are the mass and velocity of the electron; Z is the number of units of elementary charge on the atomic nucleus; r is the electron-nucleus separation. 13 Mahmoud A. A. Ibrahim CH101  End of Lecture 1 14 Mahmoud A. A. Ibrahim CH101