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Papers PHYSICS ATOM

THEORY OF ATOM Bohr

Lecturer Pengampu: Muhammad taufik

Prepared by:

Nama : Ayu Irma Kusuma Wardani

Npm : 09330094

Kelas :5C

PHYSICS DEPARTMENT

FACULTY OF MATHEMATICS AND NATURAL SCIENCES

UNIVERSITY IKIP PGRI

SEMARANG

2007

THEORY OF ATOM Bohr

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.

Bohr's atomic model Rutherford's atomic model to remedy deficiencies. To cover the weakness of the Rutherford atomic model, Bohr issued four postulates. Bohr's idea that electrons should orbit around the nucleus.

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. ISI

History

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 electrons surrounding the nucleus are negatively charged small, dense, and positively charged. Based on these experimental data, it is natural that physicists then imagine a model of planetary systems are applied to the atom, 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 radiationwhile 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 gas of low pressure 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.

o The key idea Bohr atom model

Two key ideas are:

1. The electrons move within the orbits and has a momentum that is quantized, and thus energy is quantized. This means that not every orbit, but only a few specific orbits which were probably located at a specific distance from the core.

2. The electrons do not lose energy gradually as they move in the orbit, but will remain stable in an orbit that does not decay.

o Basic postulates Bohr Atomic Model

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- EDAR surrounds the atomic nucleus; motion of electrons is influenced by the coulomb force in accordance with the rules of classical mechanics.

2. EDAR traffic electrons in solid hydrogen is priced angular momentum L which are multiples of Planck's constant divided by 2π.

$\mathbf{L} = n \cdot \hbar = n \cdot {h \over 2\pi}$

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 high energy state E _N to a lower energy state E _i, a photon with energy hυ = E _{U-E i} emitted. If a photon is absorbed, the atom will transition to a low energy state to a higher energy state.

o Bohr Atom 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 electrons , negatively charged electrons orbiting the atomic shell in a certain trajectory around the atomic nuclei are 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.

According to Bohr:

"There are rules of quantum physics which allows only a certain number of electrons in each orbit. There's only room for two electrons in the orbit closest to the core. (John Gribbin, 2005) "

Model Bohr dari atom hidrogen menggambarkan elektron-elektron bermuatan negatif mengorbit pada kulit atom dalam lintasan tertentu mengelilingi inti atom yang bermuatan positif. Ketika elektron meloncat dari satu orbit ke orbit lainnya selalu disertai dengan pemancaran atau penyerapan sejumlah energi elektromagnetik hf.

Figure 1. Bohr Atom Model

This model is the development of the plum pudding model (1904), Saturnian model (1904), and the Rutherford model (1911). Since the Bohr model is the development of the Rutherford model, many sources combine the two names in their mention the Rutherford-Bohr model.

The key to success of this model is in explaining the Rydberg formula for the line- emission line spectral atomic hydrogen , although the Rydberg formula has been known experimentally, but never get the theoretical foundation before the Bohr model was introduced. Not only because of the Bohr model to explain the reasons for the structure of the Rydberg formula, he also gives the results of empirical justification in terms of the tribes of the fundamental physical constants.

Bohr model is a primitive model of the hydrogen atom. As a theory, Bohr's model can be considered as a first-order approach of the hydrogen atom using quantum mechanics is more general and accurate, and thus can be regarded as a model that has been deprecated. However, because of its simplicity, and the exact results for a particular system, the Bohr model are still taught as an introduction to quantum mechanics.

Figure 2. Bohr model for hydrogen atom

· Which allowed for the electron trajectories are numbered n = 1, n = 2, n = 3 and so on. Numbers are called quantum numbers, the letters K, L, M, N are also used to name the track

· The radius of the orbit is expressed by 1 ^{2, 2} 2, 3 ^2, 4 ^2, ... n ^2. For a particular orbit with a minimum radius a ₀ = 0.53 Å.

If the electrons are attracted to the nucleus and is owned by the orbit of n, the energy emitted and the electron energy becomes lower by

Figure 3. Atomic energy levels of Hydrogen

· Energy levels of electrons in hydrogen atoms

Bohr model is only accurate for one electron systems such as atomic hydrogen or helium 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:

E $=E_{kinetik} + E_{potensial} \quad \quad \quad \quad \quad \quad (1) \,$

$= \begin{matrix} \frac{1}{2} \end{matrix}m_e v^2 - \frac{k q_e^2}{r}$

	$= E_ {kinetic} + E_ {potential} \ quad \ quad \ quad \ quad \ quad \ quad (1) \,$

with k = 1 / (4πε _0), and q _e is the electron charge.

2) Angular momentum electrons can only have certain discrete prices:

$L = m_e v r = n \frac{h}{2 \pi} = n \hbar \quad \quad \quad \quad \quad (2) \,$

with n = 1,2,3, ... and called the principal quantum number , h is Planck's constant , and $\ Hbar = h / (2 \ pi)$ $\hbar=h/(2\pi)$

3) Electrons are in orbit governed by the coulomb force . This means the coulomb force equal to the centripetal force :

$\frac{kq_e^2}{r^2} = \frac{m_e v^2}{r} \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad (3) \,$

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

$\frac{kq_e^2}{r} = m_e v^2. \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad (4) \,$

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

$E = \begin{matrix} \frac{1}{2} \end{matrix}m_e v^2 - \frac{k q_e^2}{r} = -\begin{matrix} \frac{1}{2} \end{matrix} m_e v^2 \quad \quad \quad \quad (5) \,$

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

$r = \frac{n \hbar}{m_e v}. \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad (6) \,$

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

$k q_e^2 \frac{m_e v}{n\hbar} = m_e v^2 \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad (7) \,$

By dividing both sides of equation (7) with m _e v obtained

$\frac{k q_e^2}{n \hbar} = v \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad (8) \,$

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

$E _n \,$ $= \frac{-1}{2} m_e \left( \frac{k q_e^2}{n \hbar} \right)^2 \,$

$= \frac{-1}{2} m_e \left(\frac{1}{4 \pi \epsilon_0} q_e^2 \frac{2 \pi}{n h} \right)^2 \,$

	$= \frac{-m_e q_e^4}{8 h^2 \epsilon_{0}^2} \frac{1}{n^2} \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad (9) \,$

	$= \ Frac {-m_e q_e 8 ^ 4} {h ^ 2 \ epsilon_ {0} ^ 2} \ frac {1} {n ^ 2} \ quad \ quad \ quad \ quad \ quad \ quad \ quad \ quad \ quad \ quad (9) \,$

By entering the price of all constants, obtained,

$E_n = (-13.6 \ \mathrm{eV}) \frac {1}{n^2} \,$

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 of ter ionization when electrons are no longer bound, but in a dispersed state.

With quantum theory, Bohr also discovered a mathematical formula that can be used to calculate the wavelength of all lines that appear in the spectrum of the hydrogen atom. The value calculated was very suited to that obtained from direct experiments. However for the more complex elements of hydrogen, Bohr's theory is just not suitable to predict the wavelengths of spectral lines. Nonetheless, this theory is recognized as a step forward in explaining the phenomena of physics that occur in the atomic level. Planck's quantum theory of truth is recognized as it can be used to describe various physical phenomena which at that time can not be explained by classical theory.

· Advantages and Disadvantages of Bohr's theory

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

o One of the discoveries is a collection of fine lines, especially if the atoms are placed on the magnetic field dieksitasikan

Weakness

o 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

o 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

o Have not been able to explain complex atomic spectra

o Itensitas relative importance of each spectral line emission.

o Zeeman effect, namely the splitting of spectral lines when an atom is in a magnetic field.

III. CONCLUSION

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

o Bohr's model was welcomed as an important step forward because by giving the distance to the electron orbit, can explain the spectrum of light from an atom.

o 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.

o 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. Concepts of Modern Physics. New York: Erlangga

Gribbin, John. 2003. Quantum Physics. New York: Erlangga

------. 2005. Science Workshop: Modern Physics. New York: Erlangga

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