THE ATOMTHE ATOM

D e v e l o p i n g o D e v e l o p i n g o u r m o u r m o d e l o f a n a t o d e l o f a n a t o

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D e m o c r i t u sD e m o c r i t u s A Greek philosopher - in the year A Greek philosopher - in the year

400 B.C. described the atom as the 400 B.C. described the atom as the smallest particle of a substance. smallest particle of a substance.

He used the word He used the word atomos (which atomos (which means not to cut) means not to cut) to describe the to describe the smallest possible particle of matter. smallest possible particle of matter.

Neither Plato nor Aristotle accepted Neither Plato nor Aristotle accepted the atomic concept. the atomic concept.

J o h n D a l t o nJ o h n D a l t o nDeveloped the Developed the first modern first modern atomic theory atomic theory in 1803. Dalton in 1803. Dalton is referred to as is referred to as the father of the father of modern atomic modern atomic theory.theory.

Dalton’s Atomic TheoryDalton’s Atomic Theory

1) All matter is made of atoms. Atoms are 1) All matter is made of atoms. Atoms are indivisible and indestructible.indivisible and indestructible.

2) All atoms of a given element are 2) All atoms of a given element are identical in mass and propertiesidentical in mass and properties

3) Compounds are formed by a 3) Compounds are formed by a combination of two or more different combination of two or more different kinds of atoms, which combine in whole kinds of atoms, which combine in whole number ratios.number ratios.

4) Atoms can neither be created or 4) Atoms can neither be created or destroyed. destroyed.

Dalton’s Atomic TheoryDalton’s Atomic Theory Modern atomic theory is, of course, a little Modern atomic theory is, of course, a little

more involved than Dalton's theory but the more involved than Dalton's theory but the basics of Dalton's atomic concept remains basics of Dalton's atomic concept remains valid. valid.

We cetainly know today that atoms can be We cetainly know today that atoms can be destroyed by nuclear reactions and by destroyed by nuclear reactions and by bombarding the nuclei of atoms in high speed bombarding the nuclei of atoms in high speed atomic accelerators (that bust the nuclei apart), atomic accelerators (that bust the nuclei apart), but not by chemical reactions. but not by chemical reactions.

There are different kinds of atoms (that have There are different kinds of atoms (that have different masses) for a particular element that different masses) for a particular element that are known as are known as isotopes,isotopes, but isotopes of an but isotopes of an element have the same chemical properties.element have the same chemical properties.

Many heretofore unexplained chemical Many heretofore unexplained chemical phenomena were quickly explained by Dalton phenomena were quickly explained by Dalton with his theory. Dalton's theory quickly became with his theory. Dalton's theory quickly became the theoretical foundation in chemistry. the theoretical foundation in chemistry.

CATHODE RAY TUBE CATHODE RAY TUBE DISCOVERED IN LATE DISCOVERED IN LATE

1800s1800s

Cathode-ray tubes contain a pair of metal plates sealed into a glass tube that has been partially evacuated.  If the residual pressure of the gas is small enough, the glass at the end of the tube across from the cathode will glow when the tube is connected to a series of batteries.

J.J. THOMSON AND THE J.J. THOMSON AND THE DISCOVERY OF THE DISCOVERY OF THE

ELECTRONELECTRON In 1897, J. J. Thomson In 1897, J. J. Thomson

found that the cathode found that the cathode rays can be deflected by rays can be deflected by an electric field, as an electric field, as shown at right. By shown at right. By balancing the effect of a balancing the effect of a magnetic field on a magnetic field on a cathode-ray beam with cathode-ray beam with an electric field, an electric field, Thomson was able to Thomson was able to show that cathode "rays" show that cathode "rays" are actually composed of are actually composed of particles. This particles. This experiment also provided experiment also provided an estimate of the ratio an estimate of the ratio of the charge to the mass of the charge to the mass of these particles. of these particles.

The cathode rays also can be deflected by an electric field in a direction which suggests they are negatively charged.

From: http://chemed.chem.purdue.edu/genchem/history/electron.html

J.J. THOMSON AND THE J.J. THOMSON AND THE DISCOVERY OF THE DISCOVERY OF THE

ELECTRONELECTRON Thomson found the same Thomson found the same charge-to-mass charge-to-mass

ratioratio regardless of the metal used to make the regardless of the metal used to make the cathode and the anode. He also found the cathode and the anode. He also found the same charge-to-mass ratio regardless of the same charge-to-mass ratio regardless of the gas used to fill the tube. He therefore gas used to fill the tube. He therefore concluded that the particles given off by the concluded that the particles given off by the cathode in this experiment are a universal cathode in this experiment are a universal component of matter. Although Thomson component of matter. Although Thomson called these particles called these particles corpusclescorpuscles, the name , the name electronelectron, which had been proposed by , which had been proposed by George StoneyGeorge Stoney several years earlier for the several years earlier for the fundamental unit of negative electricity, was fundamental unit of negative electricity, was soon accepted. soon accepted.

From: http://chemed.chem.purdue.edu/genchem/history/electron.html

THOMSON’S PLUM THOMSON’S PLUM PUDDING MODEL OF THE PUDDING MODEL OF THE

ATOMATOM Since it was known at the time that atoms are Since it was known at the time that atoms are

neutral, there must be a positive charge to offset neutral, there must be a positive charge to offset the negative charge of the electrons. The the negative charge of the electrons. The plum plum pudding modelpudding model of the atom was proposed by J. of the atom was proposed by J. J. Thomson (the discoverer of the electron in J. Thomson (the discoverer of the electron in 1897). 1897). The plum pudding model was The plum pudding model was proposed in 1906 proposed in 1906 by Thomson before the by Thomson before the discovery of the atomic nucleus. In this model, discovery of the atomic nucleus. In this model, the atom is composed of electrons (which the atom is composed of electrons (which Thomson still called corpuscles), surrounded by Thomson still called corpuscles), surrounded by a soup of positive charge to balance the a soup of positive charge to balance the electron's negative charge, like plums electron's negative charge, like plums surrounded by pudding. The positive pudding surrounded by pudding. The positive pudding was supposedly massless, thus since the was supposedly massless, thus since the electron had a mass of 1/1840 of a hydrogen electron had a mass of 1/1840 of a hydrogen atom, there would have to be 1840 electrons atom, there would have to be 1840 electrons embedded in the positive pudding to make up a embedded in the positive pudding to make up a hydrogen atom. hydrogen atom.

Millikan’s 1909 Oil Drop Millikan’s 1909 Oil Drop ExperimentExperiment

MILLIKAN’S OIL DROP MILLIKAN’S OIL DROP EXPERIMENT AND THE CHARGE EXPERIMENT AND THE CHARGE

AND MASS OF AN ELECTRONAND MASS OF AN ELECTRON Millikan determined the Millikan determined the

fundamental charge of an electron to fundamental charge of an electron to be -1.60 x 10be -1.60 x 10-19-19 Coulombs. Coulombs.

He also determined the mass of an He also determined the mass of an electron to be 9.11 x 10electron to be 9.11 x 10-31-31 Kg (about Kg (about 1/1840 the mass of a hydrogen 1/1840 the mass of a hydrogen atom).atom).

ERNEST RUTHERFORD AND THE ERNEST RUTHERFORD AND THE DISCOVERY OF THE ATOMIC DISCOVERY OF THE ATOMIC

NUCLEUSNUCLEUS Ernest Rtherford is the Ernest Rtherford is the

British physicist who in British physicist who in 1911 1911 determined that the atom determined that the atom had a small dense nucleus.had a small dense nucleus. Starting sometime around Starting sometime around 1909, Rutherford began to 1909, Rutherford began to notice that alpha particles notice that alpha particles would not always behave in would not always behave in accordance to the accordance to the plum plum puddingpudding model of an atom model of an atom when fired at a piece of when fired at a piece of gold foil. These gold foil. These observations stimulated observations stimulated further research that was further research that was eventually published in eventually published in 1911 and has been known 1911 and has been known ever since as Rutherford's ever since as Rutherford's Gold Foil Experiment Gold Foil Experiment ((http://myweb.usf.edu/~mhight/goldfoil.htmlhttp://myweb.usf.edu/~mhight/goldfoil.html))..

RUTHERFORD (cont.)RUTHERFORD (cont.)

Rutherford bombarded gold foil with Rutherford bombarded gold foil with radioactive particles (alpha particles-radioactive particles (alpha particles-positively charged). He expected the positively charged). He expected the particles to pass right through (and most of particles to pass right through (and most of them did) and was surprised that a few of the them did) and was surprised that a few of the alpha particles were scattered at high angles.alpha particles were scattered at high angles.

From this work Rutherford determined that From this work Rutherford determined that the alpha particles were deflected by a the alpha particles were deflected by a positively charged dense nucleus within a positively charged dense nucleus within a gold atom. So atoms must have a small, gold atom. So atoms must have a small, dense, positively charged nucleus.dense, positively charged nucleus.

RUTHERFORD’S GOLD RUTHERFORD’S GOLD FOIL EXPERIMENTFOIL EXPERIMENT

RUTHERFORD’S RUTHERFORD’S INTERPRETATION OF INTERPRETATION OF

RESULTSRESULTS

Expected based on Plum Pudding Model.

Actual Results.

RUTHERFORD (CONT.)RUTHERFORD (CONT.) Based on the scattering of alpha particles, Based on the scattering of alpha particles,

Rutherford estimated that the positive nucleus Rutherford estimated that the positive nucleus must be orbited by electrons at a distance of must be orbited by electrons at a distance of 100,000 times the radius of the nucleus (so, 100,000 times the radius of the nucleus (so, atoms are composed mostly of empty atoms are composed mostly of empty spacespace).).

Within a few years Rutherford was able to Within a few years Rutherford was able to isolate and define the positive particles in the isolate and define the positive particles in the nucleus, which we now call nucleus, which we now call protonsprotons. He . He speculated on the existence of neutral particles speculated on the existence of neutral particles in the nucleus, in the nucleus, neutronsneutrons..

James ChadwickJames Chadwick isolated and identified the isolated and identified the neutron in 1932.neutron in 1932.

MODERN MODEL OF THE MODERN MODEL OF THE ATOMATOM

Protons – Red. Positive Charge.

Neutrons – Green.Neutral Charge.

Electrons – Tan.Negative Charge.

Nucleus

Electron Orbits

MODERN MODERN UNDERSTANDING OF THE UNDERSTANDING OF THE

ATOMATOM ATOMIC NUMBERATOMIC NUMBER ISOTOPESISOTOPES (EXAMPLES H, C, Cl) (EXAMPLES H, C, Cl) CARBON-12 AND ATOMIC MASS CARBON-12 AND ATOMIC MASS

UNITSUNITS ATOMIC MASSATOMIC MASS OF A PARTICULAR OF A PARTICULAR

ISOTOPE (IN ATOMIC MASS UNITS)ISOTOPE (IN ATOMIC MASS UNITS) ATOMIC MASS NUMBERATOMIC MASS NUMBER ATOMIC WEIGHTATOMIC WEIGHT (IN ATOMIC MASS (IN ATOMIC MASS

UNITS)UNITS)

ATOMIC SPECTRAATOMIC SPECTRA During the 1800s and into the early 1900s During the 1800s and into the early 1900s

scientists (such as Max Planck) were studying scientists (such as Max Planck) were studying the nature of electromagnetic radiation and its the nature of electromagnetic radiation and its relationship to matter. Around 1900, Max relationship to matter. Around 1900, Max Planck was studying the electromagnetic Planck was studying the electromagnetic radiation given off by hot solids. This radiation radiation given off by hot solids. This radiation is referred to as is referred to as black body radiationblack body radiation and is and is dependent on the temperature of the body dependent on the temperature of the body emitting electromagnetic radiation.emitting electromagnetic radiation.

Black body radiation emitted from a hot solid, Black body radiation emitted from a hot solid, hot liquid, or hot dense gas and viewed hot liquid, or hot dense gas and viewed through a prism or through a prism or spectroscopespectroscope (or (or spectrographspectrograph) forms a ) forms a continuous continuous spectrumspectrum in the visible light region of the in the visible light region of the electromagnetic spectrumelectromagnetic spectrum. .

ELECTROMAGNETIC ELECTROMAGNETIC SPECTRUMSPECTRUM

The Electromagnetic The Electromagnetic SpectrumSpectrum

DISPERSION OF LIGHT INTO DISPERSION OF LIGHT INTO DISCRETE WAVELENGTHS DISCRETE WAVELENGTHS

THROUGH A PRISM OR THROUGH A PRISM OR SPECTROSCOPE (OR SPECTROSCOPE (OR

SPECTROGRAPH)SPECTROGRAPH)

CONTINUOUS CONTINUOUS SPECTRUMSPECTRUM

EMISSION (BRIGHT LINE EMISSION (BRIGHT LINE SPECTRA)SPECTRA)

However, light from a low density hot However, light from a low density hot gas is dispersed into an emission (line) gas is dispersed into an emission (line) spectrum at discrete wavelengths. The spectrum at discrete wavelengths. The wavelengths of light that are emitted wavelengths of light that are emitted are characteristic for the particular are characteristic for the particular elements that are present in the gas. elements that are present in the gas. Thus the emission (bright line) Thus the emission (bright line) spectrum is a fingerprint of the gas (or spectrum is a fingerprint of the gas (or gases) that is (are) emitting the light.gases) that is (are) emitting the light.

EXAMPLES OF EMISSION EXAMPLES OF EMISSION (LINE) SPECTRA(LINE) SPECTRA

Hydrogen:                                                                  Helium:                                                                  Carbon:                                                                 

ABSORPTION (DARK LINE ABSORPTION (DARK LINE SPECTRA)SPECTRA)

If light from a dense hot gas, hot liquid, or hot If light from a dense hot gas, hot liquid, or hot solid passes through a region that contains a solid passes through a region that contains a cool, diffuse gas, then energy from the cool, diffuse gas, then energy from the continuous spectrum will be absorbed by the continuous spectrum will be absorbed by the atoms in the cool gas. The wavelengths that are atoms in the cool gas. The wavelengths that are absorbed by the gas (or gases) present are absorbed by the gas (or gases) present are exactly the same as the wavelengths these gases exactly the same as the wavelengths these gases give off when they emit light. Since light is being give off when they emit light. Since light is being absorbed from the continuous spectrum, dark absorbed from the continuous spectrum, dark lines appear superimposed on the continusous lines appear superimposed on the continusous spectrum. This is called an spectrum. This is called an absorption (dark absorption (dark line) spectrum).line) spectrum).

ABSORPTION (DARK LINE) ABSORPTION (DARK LINE) SPECTRASPECTRA

SUMMARY – THREE TYPES SUMMARY – THREE TYPES OF SPECTRAOF SPECTRA

THE BOHR MODEL OF THE THE BOHR MODEL OF THE HYDROGEN ATOMHYDROGEN ATOM

In 1913, the Danish In 1913, the Danish physicist Niels Bohr physicist Niels Bohr (1885 - 1962) managed to (1885 - 1962) managed to explain the spectrum of explain the spectrum of atomic hydrogen by an atomic hydrogen by an extension of Rutherford's extension of Rutherford's description of the atom. description of the atom. In that model, the In that model, the negatively charged negatively charged electrons revolve about electrons revolve about the positively charged the positively charged atomic nucleus because atomic nucleus because of the attractive of the attractive electrostatic force electrostatic force according to Coulomb's according to Coulomb's law.law.

THE BOHR MODEL OF THE THE BOHR MODEL OF THE HYDROGEN ATOMHYDROGEN ATOM

The Bohr model consists of four principles:The Bohr model consists of four principles: 1)Electrons assume only certain orbits around the 1)Electrons assume only certain orbits around the

nucleus. These orbits are stable and called nucleus. These orbits are stable and called "stationary" orbits."stationary" orbits.

2)Each orbit has an energy associated with it. For 2)Each orbit has an energy associated with it. For example the orbit closest to the nucleus has the example the orbit closest to the nucleus has the lowest energy (the lowest energy (the ground stateground state), the next closest ), the next closest orbit (the orbit (the first exicted statefirst exicted state) has a certain higher ) has a certain higher energy, and so on. (the electron is defined to have energy, and so on. (the electron is defined to have zero energy when it escapes from the hydrogen atom, zero energy when it escapes from the hydrogen atom, thus the energies of the electron positions in the thus the energies of the electron positions in the hydrogen atom are negative energies)hydrogen atom are negative energies)

3)Light is emitted as a photon when an electron 3)Light is emitted as a photon when an electron jumps from a higher orbit to a lower orbit and jumps from a higher orbit to a lower orbit and absorbed (as a photon) when it jumps from a lower to absorbed (as a photon) when it jumps from a lower to higher orbit.higher orbit.

4)The energy and frequency of light emitted or 4)The energy and frequency of light emitted or absorbed is given by the difference between the two absorbed is given by the difference between the two orbit energies.orbit energies.

Atomic Excitation and De-Atomic Excitation and De-excitation excitation

Atoms can make transitions between Atoms can make transitions between the orbits allowed by the orbits allowed by Bohr ModelBohr Model (also by the (also by the QuantumQuantum Mechanics Mechanics Model) by absorbing or emitting Model) by absorbing or emitting exactly the energy difference between exactly the energy difference between the orbits. The following figure shows the orbits. The following figure shows an atomic excitation caused by an atomic excitation caused by absorption of a photon and an atomic absorption of a photon and an atomic de-excitation caused by emission of a de-excitation caused by emission of a photon. photon.

Atomic Excitation and De-Atomic Excitation and De-excitation (Decay)excitation (Decay)

Atomic Excitation and De-Atomic Excitation and De-excitationexcitation

In each case the wavelength of the In each case the wavelength of the emitted or absorbed light is exactly such emitted or absorbed light is exactly such that the photon carries the energy that the photon carries the energy difference between the two orbits. This difference between the two orbits. This energy may be calculated by dividing the energy may be calculated by dividing the product of the Planck constant and the product of the Planck constant and the speed of light speed of light hchc by the wavelength of the by the wavelength of the light). Thus, an atom can absorb or emit light). Thus, an atom can absorb or emit only certain discrete wavelengths (or only certain discrete wavelengths (or equivalently, frequencies or energies). equivalently, frequencies or energies).

THE BALMER SERIESTHE BALMER SERIES Bohr determined that the line spectrum Bohr determined that the line spectrum

from hydrogen gas gave four lines in the from hydrogen gas gave four lines in the visible portion of the spectrum that visible portion of the spectrum that corresponded to jumps from n=3 to n=2, n corresponded to jumps from n=3 to n=2, n = 4 to n = 2, n =5 to n = 2, and n=6 to n = 4 to n = 2, n =5 to n = 2, and n=6 to n =2 (corresponding to the emission lines =2 (corresponding to the emission lines red, blue-green, violet, and violet, red, blue-green, violet, and violet, respectively). Remember n = 1 is the respectively). Remember n = 1 is the ground state orbital, n = 2 is the first ground state orbital, n = 2 is the first excited state orbital, etc. This sequence of excited state orbital, etc. This sequence of lines is called the lines is called the Balmer SeriesBalmer Series (named (named for J. J. Balmer who first studied the for J. J. Balmer who first studied the hydrogen spectrum).hydrogen spectrum).

THE BALMER SERIES – THE BALMER SERIES – ALSO See figure 8.11 in textALSO See figure 8.11 in text

BOHR’S MODEL BOHR’S MODEL INCOMPLETEINCOMPLETE Although Bohr’s solar system model of the Although Bohr’s solar system model of the

atom made excellent predictions for the atom made excellent predictions for the energy of light emitted and absorbed from energy of light emitted and absorbed from the hydrogen atom, it did not work well for the hydrogen atom, it did not work well for higher atomic numbered elements. higher atomic numbered elements. Although Bohr’s model did predict Although Bohr’s model did predict correctly that photons for other elements correctly that photons for other elements would be absorbed or emitted as electrons would be absorbed or emitted as electrons jumped to higher or lower orbits.jumped to higher or lower orbits.

Bohr’s model considered the electron as a Bohr’s model considered the electron as a discrete particle at a certain distance from discrete particle at a certain distance from the nucleus and that it was contained in a the nucleus and that it was contained in a radiationless orbit. He could not radiationless orbit. He could not demonstrate that this was the case.demonstrate that this was the case.

Quantum Mechanical ModelQuantum Mechanical Model

Soon after Bohr’s work, a more complex and Soon after Bohr’s work, a more complex and complete model of the atom emerged that complete model of the atom emerged that considered both the particle nature and wave considered both the particle nature and wave nature of the electron (and also of light). nature of the electron (and also of light). Electrons were demonstrated to behave as Electrons were demonstrated to behave as standing waves in allowed orbits. This more standing waves in allowed orbits. This more complex and more correct model of the atom is complex and more correct model of the atom is called the called the Quantum Mechanical ModelQuantum Mechanical Model..

However, for the work that we will do in this However, for the work that we will do in this class and for many problems in chemistry, the class and for many problems in chemistry, the simple solar system model works fine and simple solar system model works fine and makes valid predictions.makes valid predictions.