history of atomic structure pisay version

HISTORY OF ATOMIC STRUCTURE

ANCIENT PHILOSOPHY Leucippus(490BC) and his pupil Democritus (470 –

380 BC), during one of their walks along the seashore, noted that the beach looked like one whole continuous piece from afar.

A material can be broken into smaller pieces

Atomos – a greek word which means indivisible

ARISTOTLE (384- 322 BC)

•The atomistic or particulate view was not very popular among the Greek thinkers due to rejection by the influential thinker Aristotle. •Matter is made up of 4 elements Fire, air, water, and earth•Matter was continuous all of one piece

PARTICLE THEORY

Around 17th century experimental evidence in support of particulate theory accumulated

Robert Boyle (1627-1691) – postulates that gases are composed of discrete particles separated by a void

Sir Isaac Newton (1642-1727) – “God in the beginning formed matter, in solid, massy,hard, impenetrable, movable particles…”

Dalton’s Atomic Theory

In 1808, he published a book, A New System of Chemical Philosophy in which he presented the theory in detail

1. All matter is composed of indestructible atoms.“ Matter, though divisible in an extreme degree, is nevertheless not infinitely divisible. That is, there must be some point beyond which we cannot go in the division of matter. The existence of these ultimate particles of matter can scarcely be doubted, though they are probably too small ever to be exhibited by microscopic improvements. I have chosen the word atom to signify these ultimate particles…”

2. The atoms of a given element are identical. They are different from the atoms of all other elements. They are unchageable.“the atoms never can be metamorphosed one into another by any power we can control”

Compounds are formed by the combination of the atoms of two or more elements forming compound-atoms (what we now call molecules). The atoms combine with each other in definite ratio of small whole numbers. (Law of Definite Composition by Proust)

4. Chemical reactions involve only the separation and or union of atoms. The atoms are only rearranged; none are created nor destroyed.

“Chemical analysis and synthesis go no farther that the separations of particles from another, and to their reunion. No new creation or destruction of matter is within reach of chemical agency. We might as well attempt to introduce a new planet or to manipulate one already in existence, as to create or destroy a particle of hydrogen. All the changes can produce consist in separating particles that are in state of cohesion or combination, and joining those that were previously at a distance”

Design lang ito..

•In the sixth century B.C., Thales of Miletus, accidentally found out that amber, a fossil softwood resin, when vigorously rubbed, attracted small pieces of fibers rubbed off from cloth•In England, 2000 years later, Sir William Gilbert tried similar experiments and learned that many materials when rubbed together becomes electrically charged•Greek word for amber is elektron

FOUNDATIONS OF DISCOVERY OF ELECTRON

What causes the paper bits to rise toward the comb?

A conceptual model for neutral objects always attracting charged objects is based on the fact that atoms and electrons are fairly free to move around even in solid objects. We can hypothesize that charges in the neutral object rearranges in response to the charged object. We could further extend our model by hypothesizing that after rearrangement of the negatively charged object responds by being attracted toward the surface of which has a net positive charge. This effect of neutrally charged objects interacting with charged objects is called induced polarization.

Electrostatic Attraction and Repulsion

Charging by "conduction."

Removing Electrons from Atoms

Quantitative measurements performed by Charles Coulomb on charges of equal magnitude showed the force is inversely proportional to the square of the distance between the charged bodies

The magnitude of one of the charges was decreased by one-half by touching the charged body with an uncharged body of the same size and material.

The magnitude of the force depends on the magnitude of both charges, if both charges are decreased by one half, the force decreases to one-fourth of the original.

Coulomb’s Inverse Square Law

What is the effect of magnitude of charges and the distance between them on the electrostatic forces?

q = charge and r = distance

•Coulomb ( C ) is the amount of charge that passes through a circuit if a current of one ampere passes for one second

This diagram describes the mechanisms of Coulomb's law in Physics/Electromagnetism; two equal (like) point charges repel each other, and two opposite charges attract each other, with an electrostatic force F which is directly proportional to the product of the magnitudes of each charge and inversely proportional to the square of the distance r between the charges. Regardless of attraction, repulsion, charges or distance, the magnitudes of the forces, |F| (absolute value), will always be equal. KC is Coulomb's constant.

DISCOVERY OF ELECTRONS Who: J. J. Thomson When: 1897 Where: England What: Thompson discovered that electrons were

smaller particles of an atom and were negatively charged.

Why: Thompson knew atoms were neutrally charged, but couldn’t find the positive particle.

J. J Thomson made a piece of equipment called a cathode ray

tube.

DISCOVERY OF ELECTRON..BEFORE THOMSON Sir Humphry Davy(1778-1829) – the electrical nature of

matter was revealed in his discovery that when electric current is passed through molten compounds of metals resulted in the decomposition of compounds to produce the metals. Led to the discovery of Na,K,Ca,Mg,Sr,Ba.

Michael Faraday (1791-1867) – showed that mass of the element formed is proportional the quantity of electricity that was passed. Each atom of the element was interacting with a definite amount of electricity. This further suggested that electricity was interacting with definite amount of electricity.

In 1891, George J. Stoney ( 1874-1911) – Irish physicist who suggested the name electron for the particle of electricity

DIRECT OBSERVATION AND CHARACTERIZATION OF ELECTRON

Began with the work of physicist on the discharge of electricity through a vacuum tube.

Heinrich Geissler (1814-1879) a German inventor, was able to device a method of producing a good vacuum in glass tubes.

Julius Plucker (1801-1868), a German mathematician and physicist, sealed two metal pieces into a Geissler tube and applied a high voltage across the electrodes and observe a greenish luminescence emanating from the negative electrode, the cathode (or the negatively charged plate). The cathode ray is drawn to the positively charged plate, called the anode.

Eugene Goldstein proposed that the luminescence observed by Plucker is cathode ray (later on identified as electron by Thomson).

Sir William Crookes (1832-1919) – an English physicist, showed that the cathode ray travelled in a straight line and that objects placed in its path cast a shadow at the opposite side of the tube (possibly a form of an Electromagnetic Radiation). Later he confirmed Plucker’s observation that the ray was deflected by a magnet in a way that indicated it to be unlike light but rather consist of negatively charged particles.

DIRECT OBSERVATION AND CHARACTERIZATION OF ELECTRON

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CATHODE RAY TUBE

According to electromagnetic theory, a moving charged body behaves like a magnet and can interact with electric and magnetic fields though which it passes. Because cathode ray is attracted by the plate bearing positive charges and repelled by the plate bearing negative charges, it must consist of negatively charged particles. These negatively charged particles are electrons.

A cathode ray produced in a discharge tube travelling from the cathode (left) to the anode(right). The ray itself is invisible, but the fluorescence of a zinc sulfide coating on the glass causes it to appear green

The cathode ray is bent downward when the north pole of the bar magnet is brought toward it. When the polarity of the magnet is reversed, the ray bends in the opposite direction

THOMSON’S EXPERIMENT

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Metal Disks

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Thomson’s ExperimentThomson’s Experiment

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Passing an electric current makes a Passing an electric current makes a beam appear to move from the negative beam appear to move from the negative to the positive endto the positive end


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Voltage source


By adding an electric field

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By adding an electric field By adding an electric field

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By adding an electric field he found that By adding an electric field he found that the moving pieces were negative the moving pieces were negative

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WHY THOMSON IS ATTRIBUTED FOR THE DISCOVERY OF ELECTRON?

He measured the charge mass ratio of electron thus providing a way of identifying it.

e/m = 1.76 x 108 C/g He showed that whatever metal is used as

cathode and whatever gas is present inside the tube, the cathode ray particles had the same e/m ratio.

He found that metals emit these same particles when light of appropriate wavelength shines on them

THOMSON’S MODEL Found the electron Atoms were made of small

negatively charged particles. Assumed that larger part of atom

is positively charged with small electrons scattered in it.

Said the atom was like plum pudding

HOW IS THE CHARGE OF A PARTICLE MEASURED?

•Robert Millikan devised an experiment by which to determine the charge of a particle whether positive or negative, and the magnitude of the electric charge.•When a small spherical particle such as a tiny drop of water or oil is allowed to fall through the air under influence of gravity, it will reach a steady speed of fall which depends on the friction of the air and the size and weight of the sphere. •Oil drops are sprayed into a chamber by means of an atomizer.•An oil droplet is allowed to fall between two charged plates.•The speed of an oil droplet can be measured accurately by watching the droplet falls, and measuring the time necessary for it to pass between the crosshairs of the telescope.•As oil droplet falls through the air, it may acquire a positive or negative charge due to friction•If the charged oil droplet is allowed to fall into an electric field produced by two charged metal plates, the electrical force may act on the oil drop to oppose the force of gravity.

•The voltage between the plates can be adjusted so that the force of gravity is exactly balanced by the electrical force•The balanced state will be shown by the state of the particle; it will remain suspended in the mid-air•This provides a means for calculating the electrical force, since the force due to gravity can be measured for the speed of fall of the particle before the electrical force was applied.•From the strength of the electrical force, the voltage and the distance between the charged metal plates, the charge carried by the particle can be calculated.•He observed that the electrical charge was always a whole-number multiple of a smallest charge, which he called the unit charge.•The value of the unit charge is 1.60 x 10-19 coulomb.•If e is the unit charge, then the charge observed to be carried by a particle may be +e, -e, +2e, etc.•Knowing the possible values of the charge of a positive ion (+e, +2e, +3e..) it is then possible to compute its mass from the charge mass ratios. The smallest positively charged particle obtained from hydrogen gas has a charge of +e and a mass of approximately 1 amu. ---proton•Each electron has a charge of -1 unit, equivalent to -1.60 x 10-19

•Each proton has a charge of +1 unit, equivalent to +1.60 x 10-19

Charged plate

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RADIOACTIVITY

In 1895, German physicist Wilhelm Rontgen noticed that cathode rays caused glass and metals to emit unusual rays. These rays could not be deflected by a magnet, they could not contain charged particles as cathode rays do ---X rays.

Antoine Becquerel professor of Physics in Paris began to study fluorescent properties of substances. He found that exposing thickly wrapped photographic plates to a certain uranium compounds caused them to darken, even without stimulation of cathode rays

Radioactivity – spontaneous emission of particles and or radiation(proposed by Marie Curie)

•Three types of ray are produced by the decay, or breakdown, of radioactivesubstances

A. Alpha rays ( α ) – consist of positively charged particles called alpha particles and therefore are deflected by the positively charged plate.

B. Beta rays ( β ) – or beta particles – are electrons and deflected by negatively charged plate

C. Gamma rays ( γ ) – have no charged and are not affected by an external electric field or magnetic field

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ERNEST RUTHERFORD’S GOLDFOIL EXPERIMENT

Who: Ernest Rutherford When: 1911 Where: England What: Conducted an experiment to isolate the

positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core.

Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

Ernest Rutherford (1871-1937), a British physicist and his associate Hans Geiger(1882-1945) a German physicist, studied the alpha particles emitted by radium which was isolated by Marie and Pierre Curie.

Alpha particles are found to be helium atoms with their electrons removed, positively charged and mass of 2500 times that of the electron.

Together with Ernest Marsden (an undergraduate student) they studied the scattering of high speed alpha particles when passed through thin metal foils.(about 2000 atoms thick)

Believed in the plum pudding model of the atom. Wanted to see how big they are

–Particleemitter

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Gold foil

Rutherford’s Experimental Design

–Particleemitter

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HE EXPECTED

The alpha particles to pass through without changing direction very much

Because most of the mass of the atom (positive charges) were spread. Alone they were not enough to stop the alpha particles

If the Thomson model were correct, all the alpha particles, travelling at high speeds and massive, would have passed through the metal foil undeflected or only slightly deflected

What he expected

Because

Because, he thought the mass was evenly distributed in the atom

What he got

They observed that although majority of the alpha particles passed through undeflected, some were only slightly deflected, some were scattered by more than 90 degrees and a few by nearly 180 degrees or almost completely turned back

Atom consists of a very small nucleus surrounded by electrons. Rutherfordestimated the radius at 10-12 to 10-13 cm compared to radius of the atom of about 10-8 cm The nucleus contains most of the mass of the atom and all of its positive charge. Alpha particles are deflected bynucleus it if they get close enough at each

other

How he explained itHow he explained it

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Ernest Rutherford’s ModelErnest Rutherford’s Model(Nuclear Model of an Atom)(Nuclear Model of an Atom)

Electron Cloud Model

history of atomic structure pisay version

Documents

charged objects

charges of equal magnitude

point charges

union of atoms

indestructible atoms

opposite charges

ultimate particles of

earth matter