makalah

INTERNATIONAL PHYSICS

 “Bohr's atomic theory”

Arranged for meeting semester final task

Lecturer: Muhamad Taufiq, S.Pd,M.Pd

By:

Name : Zaenal Abidin

Class  : 5C

Npm   : 09330289

STUDI'S PROGRAM PHYSICS EDUCATION

MATHEMATICS EDUCATION FACULTY

AND NATURAL SCIENCES

IKIP PGRI SEMARANG

2012

I. INTRODUCTION

Atom is the smallest unit of an element that has certain basic properties. Each atom consists of a small nucleus composed of protons and neutrons and the number of electrons over long distances.
In 1913 Neils Bohr first proposed the quantum theory to atomic hydrogen. This model is a transition between the models of classical mechanics and wave mechanics. Due to the principle of classical physics does not correspond to the observed stability of the hydrogen atom.
However, the atomic theory proposed by Neils Bohr also has many drawbacks. Bohr's model is useful for atom-atom containing one electron, but not for many electron atoms.

II. Contents

At the beginning of the 20th century, experiments by Ernest Rutherford has been able to show that the atom consists of a form of a diffuse cloud of negatively charged electrons surrounding the nucleus of small, dense, and positively charged. Rutherford model 1911, with electrons orbiting the nucleus like planets orbit the sun. However, the atomic model planetary systems to meet some difficulties. For example, the laws of classical mechanics (Newtonian) predicts that the electron will release electromagnetic radiation while orbiting the nucleus. Since the release of the electron loses energy, it will eventually fall in spirals toward the core. When this occurs, the frequency of electromagnetic radiation emitted will change. However, experiments in the late 19th century shows that the electric spark jumps which is passed in a low-pressure gas inside a vacuum tube will make the gas atoms emit light (which means electromagnetic radiation) in the fixed frequencies are discrete.
In 1913, Niels Bohr, Swedish physicist, Einstein followed to apply quantum theory to explain the results of her study of the hydrogen atom spectrum. Bohr put forward a new theory about the structure and properties of atoms. Bohr's atomic theory is in principle combine the Planck quantum theory and atomic theory of Ernest Rutherford proposed in 1911. Bohr argued that if the electrons in orbit atom absorbs a quantum of energy, electrons will jump out to a higher orbit. Conversely, if the electrons that emit a quantum of energy, the electrons will fall into orbits closer to the atomic nucleus.

Bohr Atomic Model Basic Postulates
There are four postulates are used to cover the weaknesses of Rutherford's atomic model, among others:

1.      The hydrogen atom consists of an electron moving in a circular cross-orbit surrounds the atomic nucleus; motion of electrons is influenced by the coulomb force in accordance with the rules of classical mechanics.

2.      Orbit traffic electrons in solid hydrogen is priced angular momentum L which are multiples of Planck's constant divided by 2π.
where n = 1,2,3, ... and called the principal quantum number, and h is Planck's constant.

3.      In the cross-shelf-stable electrons surrounding the atomic nucleus does not radiate electromagnetic energy, in this case the total energy E is not changed.

4.      If an atom makes the transition from the EU high energy state to a state of lower energy EI, a photon with energy EI hυ = EU-emitted. If a photon is absorbed, the atom will transition to a low energy state to a higher energy state.

  Bohr Atomic Model
"Bohr states that electrons occupy only certain orbits around the nucleus of atoms, each of which related to a number of energy multiples of a fundamental quantum value. (John Gribbin, 2002) "

Bohr model of the hydrogen atom describes the negatively charged electrons orbiting the atomic shell in a certain trajectory around the atomic nucleus is positively charged. When the electrons jump from one orbit to another orbit is always accompanied by emission or absorption of a number of electromagnetic energy hf.
The energy level of electrons in hydrogen atoms
Bohr model is only accurate for one electron systems such as hydrogen or helium atoms are ionized once. Decrease in the formulation of the energy levels of hydrogen atom using the Bohr model.
Decrease in the formula is based on three simple assumptions:

1. The energy of an electron in orbit is the sum of kinetic energy and potential energy:
    with k = 1 / (4πε0), and qe is the electron charge.
2. Angular momentum electrons can only have certain discrete prices:
   with n = 1,2,3, ... and called the principal quantum number, h is Planck's constant, and.
3. Electrons are in orbit governed by the coulomb force. This means the coulomb force equal to the centripetal force:

By multiplying the two sides of equation (3) with r obtained:

Interest on the left side of the potential energy states, so the energy equation to be:

By solving the equation (2) for r, the radius of the price obtained is allowed:

By inserting equation (6) into equation (4), is obtained:

By dividing both sides of equation (7) MeV obtained with

By entering the price of v in the energy equation (equation (5)), and then substituting for k and prices, then the orbital energy at different levels of the hydrogen atom can be determined as follows:

By entering the price of all constants, obtained,

Thus, the lowest energy levels for atomic hydrogen (n = 1) is -13.6 eV. The next energy level (n = 2) is -3.4 eV. The third energy level (n = 3) is -1.51 eV, and so on. The prices of this energy is negative, stating that the electrons are in a bound state with the proton. Energy prices are positively associated with atoms that are in a state that is ionized when electrons are no longer bound, but in a dispersed state.

Pros and Cons Bohr Theory

a)      The success of Bohr's theory lies in its ability to meeramalkan lines in the spectrum of hydrogen atom

b)      One of the discoveries is a collection of fine lines, especially if the atoms are placed on the magnetic field Weakness

Fine line structure is explained by modification of the Bohr theory but this theory has never succeeded in describing the spectrum in addition to atoms of hydrogen Have not been able to explain the presence of fine structure (fine structure) in the spectrum, ie 2 or more lines are very close together. Have not been able to explain complex atomic spectra. Itensitas relative importance of each spectral line emission.

III. CONCLUSION

Ø Bohr's atomic theory states that electrons should orbit around the nucleus like planets orbit the Sun.

Ø Electrons can move from one orbit to another orbit by means of quantum leaps, and jumps always involve emission or absorption of quantum intact with the amount of energy equivalent to hf or multiples thereof, but never a value in between.

Ø Bohr was still wearing newton law in addition to several other postulates, Bohr theory of value is not the predictions that can be produced but in understanding and new laws were dictated.

IV. REFERENCES

Beiser, Arthur. 1999. Konsep Fisika Modern. Jakarta : Erlangga

Gribbin, John. 2003. Fisika Kuantum. Jakarta : Erlangga

Krane, Kenneth. 1988. Fisika Modern. Jakarta : UI Press