Chapter One
Review of Atomic Theory of Matter
Addis Ababa Science and Technology University
College of Electrical & Mechanical Engineering
Electrical & Computer Engineering Department
Electrical Materials and Technology (EEEg-3093)
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Modern Atomic Theory, Lecture notes of Materials science
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Chapter One Review of Atomic Theory of Matter
Addis Ababa Science and Technology University
College of Electrical & Mechanical Engineering
Electrical & Computer Engineering Department
Electrical Materials and Technology (EEEg-3093)
Prepared by: Welelaw Y.
Review of Atomic Theory of Matter
Outline: Introduction Atomic Models Quantum Mechanics Electron Configuration Atomic Bonding Band Theory of Solids
Prepared by: Welelaw Y.
Introduction……
Atomic Number (Z):
This is a fundamental property of the atom and denoted by Z.
The atomic number of an element is numerically equal to the number of protons present in the nucleus.
We must note that all the atoms of the same element possess the same atomic number which identifies the position of the element in the periodic table of elements.
A normal atom is electrically neutral and hence the number of protons and electrons are equal.
Prepared by: Welelaw Y.
Introduction……
For example, an iron atom contains 26 protons ( Z = 26 ) and hence the balancing electrons are also 26. Obviously, 26 specifies the position of the iron element in the periodic table of elements. Mass Number (A): The mass number is equal to the sum of the number of protons and neutrons. The mass number is usually denoted by A. A Number ofprotons Number ofneutrons
Prepared by: Welelaw Y.
Introduction……
In most cases atoms of a given element do not all have the same mass. Atoms that have the same atomic number but different mass numbers are called isotopes. For example, there are three isotopes of hydrogen. One, simply known as hydrogen , has one proton and no neutrons. The deuterium isotope has one proton and one neutron, and tritium has one proton and two neutrons.
Prepared by: Welelaw Y.
Introduction……
The accepted way to denote the atomic number and mass number of an atom of element X is as follows:
Thus, for the isotopes of hydrogen, we write:
Prepared by: Welelaw Y.
Introduction……
Example-1: Find the number of protons, neutrons, and electrons in each of the following species:
3065 2
(^189)
(^10847)
(^19579)
.
.
.
.
d Zn
c F
b Ag
a Au
Prepared by: Welelaw Y.
Introduction……
Solution: a. The atomic number of Au (gold) is 79 , so there are 79 protons. The mass number is 195 , so the number of neutrons is 195 - 79 = 116. The number of electrons is the same as the number of protons, i. e., 79. b. The atomic number of Ag (silver) is 47 , so there are 47 protons. The mass number is 108 , so the number of neutrons is 108 - 47 = 61. The number of electrons is 47 - 1 = 46.
Prepared by: Welelaw Y.
Atomic Models
A model is a simplified approximation of reality. Scientific models are simplified but useful representations of something real. There are five basic atomic models. These are: i. Dalton’s Model (1808) ii. Thomson’s Model (1904) iii. Rutherford’s Model (1911) iv. Bohr’s Model (1913) v. Quantum Mechanical Model (present)
Prepared by: Welelaw Y.
Atomic Models……
Dalton’s Model (1808):
- An element is composed of tiny, indivisible, indestructible particles called atoms.
- All atoms of an element are identical and have the same properties.
- Atoms of different elements combine to form compounds.
- Compounds contain atoms in small whole number ratios. e.g. Each H 2 O molecule consists of one O and two H atoms
- Atoms can combine to form different compounds. e.g. carbon and oxygen can form CO 2 or CO Note: Dalton’s first two ideas were later found to be incorrect.
Prepared by: Welelaw Y.
Atomic Models……
iv. The electrons possess vibratory motion about their equilibrium position and cause emission of light.
The atom is electrically neutral.
Fig. Thomson’s plum-pudding atomic model
Prepared by: Welelaw Y.
Atomic Models……
Thomson’s proof for the existence of electrons in an atom was a major achievement and an essential prerequisite for the subsequent theories of atomic structure. By using different materials for the filament wire, Thomson established that the same value is always obtained for the charge to mass ( e/m ) ratio of the electron. Thomson’s model could account for the periodicity in elements, spectral lines existence of ions and kinetic theory of gases.
Prepared by: Welelaw Y.
Atomic Models……
Fig. Rutherford's Alpha-Scattering Experiment
Prepared by: Welelaw Y.
Atomic Models……
Rutherford’s Interpretation of the Experimental Results
- Most alpha ( α ) particles passed through foil The atom is mostly empty space with electrons moving around the space.
- Some α particles were deflected or bounced back Atom contains a small dense region and when α particles strike this region they recoil. dense region = atomic nucleus (contains atom’s protons and neutrons).