Lecture 5 & 6

Eelo American University, Borama, Awdal, Somaliland

11/6/2008

Prepared byMd. Faysal Ahamed Khan

EELO AMERICAN UNIVERSITYEELO AMERICAN UNIVERSITYBORAMA, AWDALBORAMA, AWDAL

SOMALILANDSOMALILAND

Md. Faysal Ahamed KhanLecture 5-Structure of Atom

& Lecture 6 - Quantum theory & Atomic

structure

Welcome to the class of Chemistry ICourse No. CHEM 211

Credit hours 3

Democritus

• This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago.

– He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?

400 BC

Atomos

• His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained.

• This piece would be indivisible.

• He named the smallest piece of matter “atomos,” meaning “not divisible.”

Atomos

§ To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes.

§ Atoms were infinite in number, always moving and capable of joining together.

This theory was ignored and forgotten for more than 2000 years!


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• The eminent philosophers of the time, Aristotle and Plato, had more respect, and ultimately wrong theory.

Aristotle and Plato favored the earth,fire, air and water approach to the nature of matter. Their ideas was believed because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

Dalton’s Model

In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms. His theory of atom was based on the following laws:

1. Laws of conservation of mass

2. Laws of constant composition

3. Laws of multiple proportion

Laws of conservation of Mass

Mass can neither be created nor destroyed in Chemical reaction

2H2 + O2 = 2 H2O4x1.008 16x2 2x(2x1.008 + 16)=4.032 gm =32 gm = 2x18.016

= 36.032 gm36.032 gm


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Laws of Constant Composition

No matter what its source, a particular chemical compound is composed of the same elements in the same fraction by mass

By Mass water is: 88.8% oxygen11.2% hydrogen

Laws of multiple proportion

Same elements can combine in different ways to form different substances, whose mass ratios are small whole number multiples of each other. That is if elements A and B reacts to form two compounds, the different masses of B that combine with a fixed mass of A can be express as a ratio of small whole numbers.

Dalton’s Theory

• He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles.

• Atoms of one element cannot be converted into atoms of another elements

• Atoms of the same element are identical in mass & properties. Atoms of different elements are different.

• Compounds are formed by the joining of atoms of two or more elements by a specific ratio.

This theory became one

of the foundations of modern chemistry.

How Dalton’s theory explains the mass laws

• Mass Conservation: Since each type of atoms have a fixed mass, a chemical reaction, in which atoms are just combined differently with each other, cannot possibly result a mass change.

• Definite Composition: A compound is a combination of specific ratio of different atoms, each of which has a particular mass. Thus, each element in a compound constitutes a fixed fraction of total mass.

• Multiple Proportions: Atoms of an element have the same mass and are indivisible. Because different numbers of B atoms combine with each A atom in different compounds, the mass of element B that combine with a fixed mass of element A give a small, whole-number ratio.


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Limitation of Dalton’s model

• Atoms are divisible

• In a nuclear reaction atoms of one elements often changes to atoms of other elements

• Isotopes & Isobars

Thomson’s Plum Pudding Model

• In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smallerparticles.

He proposed a model of the atom that is sometimes called the “PlumPudding” model or Raisin Bread model.

Thomson Model

• Atoms were made from a positively chargedsubstance with negatively charged electrons scattered about, like raisins in a pudding.

• Thomson studied the passage of an electric current through a gas.

• As the current passed through the gas, it gave off rays of negatively charged particles.

Thomson Model

• This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from?

Where did they come from?

Thomson concluded that the negative charges came from within the atom.

A particle smaller than an atom had to exist.

The atom was divisible!


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Thomson called the negatively charged “corpuscles,” today known as electrons.

Since the gas was known to be neutral, having no charge, he reasoned that there must be positivelycharged particles in the atom.

But he could never find them.

Rutherford’s Gold Foil Experiment

• In 1910, after the discovery of radioactivity, emission of particles and/or radiation from atoms of certain elements, the English physicist Ernest Rutherford used one type of radioactive particle in a series of experiment that that solved the mysteries of the atomic structure.

• Rutherford’s experiment Involved firing a stream of tiny positively chargedparticles at a thin sheet of gold foil(2000 atoms thick)

– Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all.

– Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

Expect:

1. Mostly small angle scattering2. No backward scattering events

Results:

1. Some small scattering events2. Several backward scatterings!!!


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• This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material.

• Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.”

• He called the center of the atom the “nucleus”

• The nucleus is tiny compared to the atom as a whole.

Rutherford

• Rutherford reasoned that all of an atom’s positively charged particles were containedin the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge.

A new question arises. If the electron is negatively charged, won't the attraction for electrons by the nucleus cause the electron to fall into the nucleus and therefore atoms should collapse.

Rutherford attempts to explain his experimental results.

He knew about the solar system - the attraction of the planets by the sun - universal gravitation. Yet planets are not pulled into the sun. They are in motion around the sun and this motion prevents them from being pulled into the sun.

So Rutherford puts the electron in motion around the nucleus.

Limitation of Rutherford’s model

• A nucleus and an electron attract each other, so if they are to remain apart, the energy of electron’s motion (kinetic energy) must balance the energy of attraction (potential energy). According to the laws of classical physics says that a negative particle moving in a curved path around a positive one must emit radiation and thus lose energy, so orbiting electron will lose energy continuously and spiral into nucleus.

Electron crashes into the

nucleus!?


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Limitation (Cont’d)

• In solar system, the planets are electrically neutral but in atoms the electrons and protons are electrically charged

• The shape and size of electrons orbiting path is still undetermined and Rutherford did not give any idea about that also

• In case of more than one electrons atom Rutherford did not explain how they orbit around the nucleus

Quantum theory & Atomic structure

Lecture 6

From the failure of Rutherford's model

• Neils Bohr, a Danish Physicist, points out that laws of physics do not apply for the submicroscopic world of ATOM

• In 1913 Bohr proposed a new model based on the modern quantum theory of energy

• With this model he was able to explain- why orbiting e- do not collapsed to nucleus- how atomic spectra occurs

• To understand Bohr theory first we need to know

- Nature of electromagnetic radiation

- Atomic Spectra - Quantum theory of energy

Electromagnetic radiation

• Energy can be transmitted through space by Electromagnetic radiation

• Electro magnetic radiation consists of waves which have both electrical & magnetic properties

Ex: stone thrown to the pond creates wave like electromagnetic waves

• Waves convey the energy from one place to another

• They travels through empty space at the speed of light


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Wave nature of Light

• Light is electromagnetic radiation - crossed electric and magnetic waves:

Properties :

Wavelength, (nm)

Frequency, n (s-1,

Hz)

Amplitude, AMagnetic vector

wavelengthVisible light

wavelengthUltaviolet radiation

Amplitude

Node

Wavelength - distance between consecutive peaks -crests - measured in m, nm, angstroms.

frequency - (nu) - number of times per second a crest passes a given point (cycles per second)

All waves have:frequency and wavelength

symbol: ν (Greek letter “nu”) λ (Greek “lambda”)

units: “cycles per sec” = Hertz “distance” (nm)

Note: Long wavelength

→ small frequencyShort wavelength

→ high frequency

increasing wavelength

increasing frequency

•• All radiation: λ • ν = c

where c = velocity of light = 3.00 x 108 m/sec

Example:Red light has λ = 700 nm.Calculate the frequency, ν.

=3.00 x 10 8 m/s

7.00 x 10 -7 m= 4.29 x 10 14 Hzν =

cλ

Reference: See Fig:7.3 of page 257 of Silberg’s Chemistry Book

• Wave nature of light is shown by classical wave properties such as

• interference• diffraction• refraction


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Particle nature of light

Blackbody radiation & quantization of energy

When a solid body is heated it’s began to emit visible light and with the increase in temperature the light began to brighter

Observed changes in intensity & wavelength of emitted light

- This is blackbody radiation –

Attempts to explain this observed change by classical wave theory failed

- because in a continuous wave wavelength /frequency is fixed and not changing in its path

Quantum theory of electromagnetic radiation

§ In 1900 Max Planck observed the fact

§ He proposed that light radiation produced from a hot body discontinuously, in small unit of wave, each of which have a specific frequency, which increase with temperature.

§ Unit of wave = Quantum (plural, quanta)

§ The energy of Quanta is given by the following relation

E = nhν

Continuous wave

Discontinuous waves or QUANTA

- So, Energy is QUANTIZED = that is you get energy in certain no. quantity like particle/ packet, that is you can count, it is not continuous

- Where did the radiation/quantum energy came from? Ans: Hot object’s radiation is emitted by the atoms contains within it.

- It means atoms contains only certain quantity of energy

- So the energy of atom is also QUANTIZED

- So, change in atoms energy means gain or loss of one or more “packets” of energy

- So, an atom changes its energy state by emitting one or more quanta of energy

Photoelectric effect & photon theory of light

When a beam of light of sufficiently high frequency is allowed to strike a metal surface in vacuum, e- ejected from the surface- this is Photoelectric effect


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They are only ejected when the frequency of the light exceeds a certain threshold value for each particular metal.

Violet light will cause potassium to eject electrons, no amount of red light, no matter how bright (intense) it is will not have any effect. Einstein was surprised that the threshold energy was related to color rather than the intensity.

More electrons are ejected with brighter light of a certain color, but the energy of each electron is the same.

Observation 1

Observation 2

Einstein takes Planck's theory and extends it to the structure of light itself. He doesn't think of it as a

continuous wave, but as chunks/parts of 1wavelength. He proposed light itself like energy is also particulate, occurring as quanta. He calls

these chunks of light, photons.Explanation of observation 1:In order to release an e from metal surface, the incident photon has first to overcome the attractive force exerted by the positive ion of the metalThe energy of photon proportional to its frequency of incident light

Explanation of observation 2:The more the intensity = the more no. of photon = each photon releasing one e but the energy of each e is fixed as frequency of each photon is fixed

Planck’s quantum theory & Einstein’s photon theory proved that

- Energy/electromagnetic radiation/light has fixed quantity & it is discrete particle

But those were only true for matter only previously.

• Visible light/white light is radiant energy coming from sun or from incandescent lamps.

• It composed of light waves in the range of 400-800nm. Each wave has a characteristic color

• When a beam of white light from an incandescent light bulb pass through a prism, diff wavelengths are

refracted at diff angles, when received on a screen these form series of color bands: VIBGYOR, Shorter λ's are

bent more than longer λ 's.

• Thus a continuous spectrum results

Spectrum


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Pass a current through a hydrogen gas in a discharge tube at low pressure. The gas glows. When examined through a spectroscope, a line spectrum is observed, not a continuous one as seen with the light bulb.

The wave theory of light could not explain the line spectra of excited gases

Bohr Model• In 1913, the

Danish scientist Niels Bohr proposed a model for H atom and explain the line spectra of Hydrogen.

• In his model, Bohr used Planck’s and Einstein’s idea about quantized energy and proposed three postulates:

Bohr Model

1. Electrons travel around the nucleus in specific orbits at fixed distance from the nucleus and e- in each orbit have a definite energy.

Bohr model cont’d

2. While in these orbits, an e does not radiate (or lose) energy. These orbit is called energy levels.

3. The electron can moves to another orbit/energy level only by absorbing or emitting a photon/quantum, whose energy equals the difference in energy between the two orbits/levels.

Ephoton = Estate A – Estate B = ΔE = hν


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Bohr explanation for line spectrum

• Bohr ‘s model explain that an atomic spectrum is not continuous because a atom has only certain discrete energy levels or states

• A spectrum line results when a photon of specific energy (and thus specific energy) is emitted as electron moves from a higher energy states to a lower energy states

LIMITATION of BOHR MODEL

But problems existed with Bohr theory —

– theory only successful for the H atom, that is it failed for more that one electron atom

– Electrons do not travels in fixed orbits

THE WAVE MODEL

• Today’s atomic model is based on the principles of wavemechanics.

• According to the theory of wave mechanics, electrons do not move about an atom in a definite path/ORBIT.

The Wave Model

• In fact, it is impossible to determine the exact location of an electron. The probablelocation of an electron is based on how much energy the electron has.

• According to the modern atomic model, at atom has a small positively charged nucleussurrounded by a large region in which there are enough electrons to make an atom neutral.


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Electron Cloud:

• ELECTRON CLOUD is a space in which electrons are likely to be found.

• Electrons whirl about the nucleus billions of times in one second

• They are not moving around in random patterns.

• Location of electrons depends upon how much energy the electron has.

• Depending on their energy they are locked into a certain area in the cloud.

• Electrons with the lowest energy are found in the energy level closest to the nucleus

• Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

Lecture 5 & 6

Technology

Transcript of Lecture 5 & 6