Modern Physics course (Phys 225) Chapter III : Introduction to atomic physics 1 الدكتور : قسم الفيزياء

2 1. Basics properties of atoms Before we begin to construct a model of the atom, let us summarize some of the basic properties of atoms : 1- Atoms are very small, about 0.1nm (0.1x10 -9 m) in radius. 2- Atoms are stable, they do not spontaneously break apart into smaller pieces. 3- Atoms contains negatively electrons but are electrically neutral. 4- Atoms emit and absorb electromagnetic radiation.

2. Thomson's Atomic Model J. J. Thomson assumed that the basic body of an atom is a spherical object containing N electrons confined in homogeneous jellylike but relatively massive positive charge distribution whose total charge cancels that of the N electrons. The schematic drawing of this model is shown in the following figure. 3. Rutherford's Atomic Model Considering the results of Geiger and Marsden's experiment and the failure of Thomson's atomic model, E. Rutherford (UK, ) proposed a model in which the electric charge +Ze in an atom is not distributed over the whole area of the atom but concentrates in a small area (1911). A schematic image of the Rutherford model is shown in the following figure, where a black big ball at the center is the nucleus and small red points moving around the nucleus are electrons.

4 3. The Bohr Model Niels Bohr, coupling Rutherford's postulation with the quantum theory introduced by Max Planck, proposed that the atom consists of a dense nucleus of protons surrounded by electrons traveling in discrete orbits at fixed distances from the nucleus. An electron in one of these orbits or shells has a specific or discrete quantity of energy (quantum). When an electron moves from one allowed orbit to another allowed orbit, the energy difference between the two states is emitted or absorbed in the form of a single quantum of radiant energy called a photon. Bohr's atomic model is designed specifically to explain the hydrogen atom, his theories apply generally to the structure of all atoms.

5 An electron is held in orbit by the Coulomb force Using the Coulomb force, we can calculate the radii of the orbits:

6 4. Atomic spectra An atomic spectrum is a line spectrum –only certain frequencies appear. If white light passes through such a gas, it absorbs at those same frequencies. The wavelengths of photons emitted from hydrogen have a regular pattern: This is called the Balmer series. R is the Rydberg constant:

7 Other series: Lyman series: Paschen series: Brackett series: 5. Quantum numbers for hydrogen atom 1) Principal quantum number n It is associated with the radial function R(r). The energy of the allowed sates are: 2) Orbital quantum number l It is associated with the orbital angular momentum of the electron. 3) Orbital magnet quantum number ml It is associated with the orbital angular momentum of the electron.

8 Restrictions: The values of n can range from 1 to ∞. The values of l can range from 0 to n – 1. The values of m l can range from –l to l. Example: Summary of Quantum Numbers :

9 An arrangement of the atomic elements according to their atomic masses and chemical similarities. The chemical behavior of an element depends on the outermost shell that contains electrons. 6. The periodic table: