Ch. 3: Development of Atomic Ch. 3: Development of Atomic TheoryTheory

DemocritusJohn DaltonJ.J. ThomsonRobert A

MillikanLord KelvinRutherford,

Geiger & Marsden

or “What does an atom look like?”

Democritus, 400 BCDemocritus, 400 BC

“Everything is made up of a few simple parts called atomos.” Atomos means “uncuttable” in Greek.

He envisioned atomos as small, solid particles of many different sizes and shapes.

*His ideas were rejected later by Aristotle who supported the “earth, air, water, and fire” concept of matter.

Laws of 1790’sLaws of 1790’s

Law of conservation of mass: mass is neither created nor destroyed during ordinary chemical or physical reactions.

Law of Conservation of MassLaw of Conservation of Mass Law of Conservation of Mass In a combustion reaction, 46.0 g of ethanol reacts

with 96.0 g of oxygen to produce water and carbon dioxide. If 54.0 g of water is produced, then how much carbon dioxide is produced?

Law of Definite Proportions Law of Definite Proportions or Constant Compositionor Constant CompositionLaw of definite proportions: A

pure compound always contains the same elements in the same proportions by mass.

Ex. Law of Constant Composition  A sample of chloroform is found to contain 12.0g

of carbon, 106.4g of chlorine and 1.01g of hydrogen. If a second sample of chloroform is found to contain 30.0 g of carbon, how many g of chlorine and hydrogen does it contain?

Law of Multiple ProportionsLaw of Multiple Proportions

Ex. Law of multiple proportions:When elements combine to form more than a single compound, the ratios of the masses of the combining elements can be expressed by a ratio of small whole numbers.

Law of Multiple ProportionsLaw of Multiple ProportionsLaw of Multiple Proportions Water contains 2.02g of hydrogen and 16.0g

of oxygen. Hydrogen peroxide( H2O2) contains 2.02 g of hydrogen and 32.0g of oxygen. Show how these data illustrate the law of multiple proportions?

Animation: http://cwx.prenhall.com/petrucci/medialib/media_portfolio/02.html

John Dalton, 1808John Dalton, 1808

Atoms are indivisible in the chemical process. A chemical reaction simply changes the way the atoms are grouped together.

Dalton’s theory successfully explained ‘law of conservation of mass’ ‘ law of definite proportions’ and the ‘law of multiple proportions’. How?

The main ideas of his theory are:•Elements are made of tiny particles called atoms.All atoms of an element are identical.•Atoms of a given element are different from those of the other elements.•Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative number and types of atoms.

http://www.imageil.com/html_ver/cbt/dalton/dalton.swf

http://chemsite.lsrhs.net/c_AtomicTheory/dalton.html http://books.nap.edu/books/030907309X/html/images/p2000472eg16001.jpg

J. J. Thomson (Joseph John J. J. Thomson (Joseph John Thmompson),1897Thmompson),1897http://physics.nad.ru/Physics/Oscil2.gif

Discovery of electrons (J. J. Thomson’s cathode ray tube experiment): J. J. Thomson showed in late 1890s that atoms of any element can be made to emit tiny negative particles that he called as “electrons”. He conducted his experiments using cathode ray tube (CRT). He concluded that all types of atoms must contain this particle, which he called as ‘electron’.

On the basis of his experiments he further predicted On the basis of his experiments he further predicted that the atoms should also have a positively charged that the atoms should also have a positively charged particle to balance out the negative charge of the particle to balance out the negative charge of the electrons, since the atom is electrically neutral.electrons, since the atom is electrically neutral. He He also calculated charge to mass ratio (e/m) for also calculated charge to mass ratio (e/m) for electrons.electrons.

http://chemsite.lsrhs.net/c_AtomicTheory/Images/overheads_screen/JJThompson_lage.jpghttp://www.lip.pt/~outreach/experiments/f+0+0.jpg

Robert A. Millikan, 1909Robert A. Millikan, 1909Calculated the charge on the

electron.Electrons are present in atoms of all

elements.It was inferred that: atoms contain a positive charge to

balance the negative electrons.Atoms contain other particles that

account for most of the mass.http://www.bun.falkenberg.se/gymnasium/amnen/fysik/millikaneng.html http://physics.nad.ru/Physics/mill.gifhttp://chem100a-9.chem.lsu.edu/matter/chap26/animate1/an26_003.mov

.

http://micro.magnet.fsu.edu/optics/timeline/people/antiqueimages/millikan.jpg

Milliken’s oil drop experiment: American scientist Robert Milliken calculated the charge of electron (e) by conducting his famous oil drop experiment.

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Millikan Oil Drop ExperimentMillikan Oil Drop Experiment

Robert Millikan (University of Chicago) determined the charge on the electron in 1909.

ANIMATION: http://physics-animations.com/Physics/English/top_ref.htm

Lord Kelvin’s Plum Pudding Lord Kelvin’s Plum Pudding ModelModel

Plum Pudding Model : Lord Kelvin proposed a plum pudding model for the structure of atom in 1910. In this model the negatively charged electrons were pictured as embedded in a positively charged spherical cloud much as raisins are distributed in an old fashioned plum pudding.

http://www.nps.gov/edis/edisonia/graphics/28021003.jpg

Lord Kelvin (William Thompson),1910Lord Kelvin (William Thompson),1910

Proposed a plum pudding model

The negatively charged electrons were pictured as embedded in a positively charged spherical cloud much as raisins are distributed in a plum pudding.

Goldstein’s Canal Ray Goldstein’s Canal Ray ExperimentExperimentGoldstein discovered protons by

using the same apparatus that was used for cathode ray discovery.

He used a perforated anode in the CRT and found that there were these positively charged rays (canal rays), which consisted on positively charged particles called as protons.

Discovery of ProtonsDiscovery of Protons

In 1886, Goldstein discovered positively charged particles in atom, which he called as “protons”.

http://images.google.com/images?hl=en&q=Goldstein's%20and%20discovery%20of%20protons&um=1&ie=UTF-8&sa=N&tab=wi

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

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In 1911, Ernset Rutherford carried out his famous gold foil experiment changing the idea about the structure of atom dramatically. This experiment included bombarding a very thin layer of gold foil with alpha particles that are positively charged. Surrounding the foil was alpha particle detector that glowed each time an alpha particle hit it. Although most of the alpha particle passed straight through the foil, very few particles deflected and a very small number bounced back. Based on his experiment Rutherford concluded that•Most of the space in the atom is empty. •In the center of the atom is a very small and positive nucleus.Thus, Rutherford came up with the idea of a ‘nuclear atom’.

Rutherford’s Gold Foil Experiment Rutherford’s Gold Foil Experiment Contd.Contd.

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Rutherford, Geiger & Rutherford, Geiger & Marsden,1911Marsden,1911

Discoved that the mass of the atom is contained in a tiny positive nucleus

Inferred most of the atom is empty space.

Discovered nucleus by shooting positively charged alpha particles at a thin piece of gold foil and observing how some, but very few, of the particles were deflected.

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

Animation: http://cwx.prenhall.com/petrucci/medialib/media_portfolio/02.html

Discovery of Neutrons Discovery of Neutrons (James Chadwick) 1932(James Chadwick) 1932

In 1932, James Chadwick discovered that most nuclei also contain another neutral particle called neutron, which is slightly more massive than proton but has no charge. His experiment involved bombarding Beryllium atoms with alpha particles, which produced a strong beam of particles that was not deflected by electrical field.

http://www.google.com/search?hl=en&lr=&q=picture+of+James+chadwick

James Chadwick, 1932James Chadwick, 1932

In the nucleus, there is also a particle without charge, called a neutron, which is about the size of a proton and has a slightly greater mass.

Chadwick bombarded Beryllium atoms with alpha particles which produced a strong beam of particles that were not deflected by an electrical field.

Bohr’s Model : Planetary Model of Atom

Niels Bohr’s theory stated that the electrons move around the nucleus in circular orbits with certain energy, just as the planets move around the sun. Bohr’s most important contribution was the concept that the energy of these orbits is quantized.

http://web.gc.cuny.edu/ashp/nml/copenhagen/Bohr.jpg

Niels Bohr, 1913Niels Bohr, 1913

His model of the hydrogen atom consists of electrons circling the nucleus but are restricted to particular orbits, like the planets around the Sun.

The electron is in its lowest energy state when it is in the orbit closest to the nucleus.The energy is higher when its in orbits successively farther from the nucleus.

It can move to a higher energy orbit by gaining an amount of energy equal to the difference between the higher energy orbit and the initial energy orbit.

When an electron drops back to a lower energy orbit a photon is emitted that has energy equal to the energy difference between the the two orbits.

Bohr’s ModelBohr’s ModelThe orbits next to the nucleus have the least energy and the one farthest from the nucleus has the most energy. When an electron jumps from lower energy orbit to a higher energy one, it absorbs energy and when it moves form a higher to lower energy orbit it gives out energy. This energy exchange also is quantized.

The shortcoming of Bohr’s model was that it could only explain the atomic spectrum of hydrogen, but could not explain the atomic spectrum of other elements.

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The Nature of EnergyThe Nature of Energy

Electrons in an atom can only occupy certain orbits (corresponding to certain energies).

Modern Concept of Atomic Modern Concept of Atomic Structure(Wave Mechanical Model) by Structure(Wave Mechanical Model) by Schrodinger:Schrodinger: Atom is divisible and has three subatomic

particles- electrons, protons and neutrons, which further are made up of sub sub atomic particle (such as mesons, antineutrino etc.). Protons and neutrons are present in the nucleus while the electrons move around the nucleus in an electron cloud. Electron cloud is the place around the atom where the probability of finding the electron is the most. (>90%). 

Atom has mostly empty space, with a small positive nucleus in the center. (If nucleus is the size of the grape, the electrons would be about a mile away from the nucleus.) 

If all the atoms are made of three basic subatomic particles, then why are atoms different?

http://www.msu.edu/~russe153/schrodinger(young).jpg http://www.epa.gov/radiation/graphics/shrodinger.jpg 

http://www.msu.edu/~russe153/schrodinger(young).jpg

Atomic VocabularyAtomic Vocabulary

Atomic number = # p (=#e-)◦Elements are put in order of increasing atomic

number on the periodic table, identifies an element.

Mass number = p + n (in the nucleus)

EAZ

Element symbol

Ex: An atom of carbon with 7 neutrons:

An atom of lead with 125 neutrons:

13C6

207Pb82

Relative Atomic MassRelative Atomic MassRelative Atomic Mass: It is

convenient to use relative atomic mass because the atoms are too small. (Oxygen atom weighs 2.657X 10-23g).

The atomic mass of any nucleide is determined by comparing it with carbon-12 atom.

One atomic mass unit (amu) is exactly 1/12 of the mass of one C-12 atom. (1.660 540 X 10-27 kg)

Average Atomic MassAverage Atomic MassAverage atomic mass: It is the

weighted average of the atomic masses of naturally occurring isotopes of an element.

Formula for calculating average atomic mass

(mass of nucleide-1 X decimal fraction for its % in the mixture) + (mass of nucleide-2 X decimal fraction for its % in the mixture)