Md. Imran Nur ManikLecture

Department of PharmacyNorthern University

Bangladesh

Structure of Atom

1. All students-Define Cathode rays-3.52. A. Even roll• Summarize

faradays experimental outcomes---5.5

2. B. Odd roll

• Write down the properties of anode & cathode rays-5.5

Quick Quiz-Time 12 minutes

What’s in the box?

Is all the mass spreadthroughout as in a box

of marshmallows”?

or is all the massconcentrated in a dense

“ball-bearing”?

Figure it out without opening (or shaking)

Shoot bullets randomly through the box. If it is filled with marshmallows, all the bullets will go straight through without (much) deflection

Figure it out without opening (or shaking)

If it contains a ball-bearing most the bullets will go straight through without deflection---but not all

Occasionally, a bullet will collide nearly head-on to the ball-bearing and be deflected by a large angle

Rutherford used a-ray “bullets” to distinguish between the plum-pudding & planetary models

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aa

a

a

no way for a-rays to scatter at wide angles

Plum-pudding:

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aa

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Occasionally, an a-rays will be pointed head-on to a nucleus & will scatter at a wide angle

distinguishing between the plum-pudding & planetary models

Rutherford saw ~1/10,000a-rays scatter at wide angles

aa

a

a

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from this he inferred a nuclearsize of about 10-14m

Rutherford atom

+10-14m

10-10m

Not to scale!!!If it were to scale,the nucleus wouldbe too small to see

Even though it has more than 99.9% of the atom’s mass

Relative scalesAloha stadium

Golf ball

Atom Nucleus99.97% of the mass

x10-4

Rutherford ’s α—particle scattering experiment

• Rutherford and his students (Hans Geiger and Ernest Marsden) bombarded very thin gold foil with α—particles. Rutherford’s famous α—particle scattering experiment is represented in Fig.

• A stream of high energy α—particles from a radioactive source was directed at a thin foil (thickness 100 nm) of gold metal. The ∼thin gold foil had a circular fluorescent zinc sulphide screen around it. Whenever α—particles struck the screen, a tiny flash of light was produced at that point.

Rutherford’s observationIt was observed that :(i) most of the α-particles passed through the gold foil undeflected.(ii) a small fraction of the α-particles was deflected by small angles.(iii) a very few α-particles ( 1 in 20,000) bounced back, that is, were deflected by nearly 180°.∼On the basis of the observations, Rutherford drew the following conclusions regarding the structure of atom :1. Most of the space in the atom is empty as most of the α-particles passed through the foil undeflected.2. A few positively charged α-particles were deflected. The deflection must be due to enormous repulsive force showing that the positive charge of the atom is not spread throughout the atom as Thomson had presumed. The positive charge has to be concentrated in a very small volume that repelled and deflected the positively charged α-particles. 3. Calculations by Rutherford showed that the volume occupied by the center is negligibly small as compared to the total volume of the atom. The radius of the atom is about 10 -10 m, while that of center is 10-15 m. One can appreciate this difference in size by realizing that if a cricket ball represents the center, then the radius of atom would be about 5 km.

Rutherford’s Nuclear Model of Atom

On the basis of above observations and conclusions, Rutherford proposed the nuclear model of atom .According to this model :(i) The positive charge and most of the mass of the

atom was densely concentrated in extremely small region. This very small portion of the atom was called nucleus by Rutherford.

(ii) As the atom is electrically neutral, so the equal number of electrons as that of protons surround the nucleus.

(iii) The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths called orbits. Thus, Rutherford’s model of atom resembles the solar system in which the nucleus plays the role of sun and the electrons that of revolving planets.

(iv) Electron and the positive nucleus are held together by electrostatic forces of attraction. This attraction force is counter-balanced by the outward force of electron.

Drawbacks of Rutherford Model Rutherford’s nuclear model of an atom is like a small scale

solar system with the nucleus playing the role of the massive sun and the electrons being similar to the lighter planets. When classical mechanics is applied to the solar system, it shows that the planets describe well-defined orbits around the sun. The theory can also calculate precisely the planetary orbits and these are in agreement with the experimental measurements. The similarity between the solar system and nuclear model suggests that electrons should move around the nucleus in well defined orbits.

According to the electromagnetic theory of Maxwell, charged particles when move around another charged particle, then they should emit electromagnetic radiation. Therefore, an electron in an orbit will emit radiation, the energy carried out by radiation comes from electronic motion. The orbit will, thus, continue to shrink. But this does not happen. Hence, the Rutherford model cannot explain the stability of an atom. Another serious drawback of the Rutherford model is that it says nothing about the electronic structure of atoms i.e. how the electrons are distributed around the nucleus and what are the energies of these electrons.It could not explain how the spectral lines are produced by the simplest atoms like hydrogen atoms when an electron jumps from one orbit to another.

SpectrumA spectrum is an array of waves or particles which is spread out according to the increasing or decreasing of some properties such as wavelength or frequency.

Types of SpectraDepending on the nature of the source emitting the radiation there are two types of spectra.(1) Emission spectra

-Emission spectra are further of two types:(a) Continuous spectrum(b) Discontinuous spectrum which may be -

- Band spectrum or, - Line spectrum

(2) Absorption spectra

Spectra

Emission spectrum: It is due to the emission of white light by gas or vapor, consisting of bright lines on dark background.

Absorption spectra: It is due to the absorption of white light by gas or transmitted white light, consisting of dark lines on bright back-ground.

Discontinuous spectrumContinuous spectrum

Line spectrum(atomic spectrum)

Band spectrum(molecular spectrum)

Emission SpectraEmission spectra can be obtained from the substances which emit light on excitation. Their

excitation can be done as follows:1. By heating the liquid or solid substances in a flame at high temperature. These substances

become incandescent at high temperature i.e. when these substances are heated at high temperature they emit light. Electric light, lime light, arc light are the examples of such solid substances.

2. By passing an electric discharge through a gas at low pressure.3. By passing electric current through thin filament of a high melting point metal like tungsten.

Emission spectra are of the following two types:a) Continuous Spectrum

When a narrow beam of sunlight or any white light is allowed to pass through a prism and then allowed to fall on a screen, it gets resolved into seven colours i.e. violet, indigo, blue, green, yellow, orange and red (VIBGYOR). This phenomenon is called dispersion and the band of seven colours observed on the screen is called visible spectrum.In this spectrum, since one colour merges into another without any break or discontinuity (i.e. one colour changes gradually into another), the spectrum thus obtained is called continuous spectrum.

The Wave Nature of LightRefraction is the bending of light when it passes from one medium to another of different density.

– Speed of light changes.– Light bends at an angle depending on its wavelength.– Light separates into its component colors.

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b) Discontinuous spectrumDiscontinuous spectrum may be band or line spectrum.

Band spectrum:The band spectrum consists of a number of bright bands separated by dark spaces. Each band is sharp at one end and fades gradually in the other end. Band spectrum is the property of molecules and generally given by the compounds or gases like nitrogen and oxygen molecules at low temperature and pressure. Since this spectrum is characteristics of molecules it is also called molecular spectrum.

Line spectrum:When an atom of an element absorbs energy, it gets excited. The excited atom emits light radiations of a characteristic colour. When this emitted radiation is allowed to pass through a prism, it is resolved into several individual lines. The pattern of different lines of the radiation is called line spectrum. It is also called atomic spectrum because this spectrum is given by the atoms of an element. - Helium was first discovered in the sun in 1868 with the help of its spectrum.

2. Absorption spectra If, in the arrangement to get a continuous spectrum of white light, a substance is placed between the white light source and the prism, we get an absorption spectrum. This spectrum appears in the form of dark lines in a particular region of the continuous spectrum of white light. For example, if a solution of NaCl is placed between the white light source and prism, the continuous spectrum of white light is found to be crossed by two dark lines in the yellow region of the spectrum. If the white light source is removed, the whole of the continuous spectrum disappears leaving behind only two bright yellow light. These two dark lines produced by NaCl solution in the continuous spectrum of white light are known as absorption line spectrum.