Atomic Theory Atomic Theory and Nuclearand Nuclear

Honors Chemistry 6.0

Early Models of the Atom

• Democritus: c. 470-400 BC Greek Philosopher– matter is composed of tiny, discrete, indivisible

particles called atomos (Greek word meaning indivisible).

– Ideas based on philosophical speculation– Theory not accepted due to influence of Aristotle

• An atom is the smallest part of an element that retains the chemical properties of thatelement. It cannot be broken down by ordinary means.

Alchemy in the Middle Ages

• Matter composed of 4 elements– earth, air, fire and water.

• Believed that any substance could be formed: precious metals to elixirs to cure disease and prolong life

• Believed in transmutation – “turning lead into gold” cliché

• Condemned by the Catholic church– hidden practices

Contributions of Alchemists

• Produced hydrochloric acid, nitric acid, potash and sodium carbonate.

• Able to identify the elements arsenic, antimony, and bismuth.

• Invented and developed laboratory devices and procedures.

• Laid the foundation for the development of chemistry as a scientific discipline.

Antoine Laurent Lavoisier: 1780

•Law of Conservation of Matter states that matter is neither created nor destroyed, it only changes form.– 1st to announce that air was

made up of 2 gases – oxygen and azote (nitrogen)

– Work done on combustion, oxidation, and gases

• Lavoisier is known as the Father of Chemistry.

Joseph Louis Proust: 1799

•Law of Definite Proportion states that compounds always have the same elements in the same

proportion by mass. Ex) the ratio of H:O in water is always 2:16.

John Dalton 1766-1844

• English schoolteacher• Some of the original chemical symbols from his book:

John Dalton: 1803-1808

Proposed Atomic Theory of Matter:1. An element is composed of extremely

small, indivisible particles called atoms

2. All atoms of a given element have identical properties that differ from those of other elements

3. Atoms cannot be created, destroyed, or transformed into atoms of other elements

Dalton’s Atomic Theory (cont.)

4. Compounds are formed when atoms of different elements combine with one another in small whole-number ratios

5. In chemical reactions, atoms are combined, separated, or rearranged

Dalton is credited as being the Father of the Modern Atomic Theory

Dalton (Billiard Ball) Modelatoms are solid, hard,

indivisible spheres

These wooden balls, were the first models made to represent atoms and were used by John Dalton

(1766-1844) to demonstrate atomic theory. Credit: Science Museum/Science & Society Picture Library

Law of Multiple Proportionsproposed by Dalton

• If 2 or more different compounds are composed of the same two elements, then the ratios of the masses of the 2nd element is always a ratio of small whole numbers– CO (1.0 g C/1.33 g O) – CO2 (1.0 g C/2.66g O)

• 2:1 ratio of O in the compounds

– NO (1.0 g N/1.14 g O)– NO2 (1.0 g N/2.28 g O)

• 2:1 ratio of O in the compounds

Benjamin Franklin: 1706-1790American statesman/scientist

Ben’s lightning rod in the Franklin Institute

In 1752 Benjamin Franklin

• Experimented with electricity

• He found that an object can have a positive or a negative charge.

negative and negative: repelnegative and positive: attractpositive and positive: repel

Michael Faraday (1839)English scientist

• Hypothesized that atoms contain electric charge.

• Built 1st electrical motor • Introduced words such as…

– Ion, electrode, anode and cathode

• A unit of electricity was named after him = farad

• Static Electricity = electrons move and then are at rest (grounded)

William Crookes – 1875English scientist

• Cathode Ray Tube: An evacuated glass tube with gas at low pressure

• Electricity is passed through 2 electrodes: cathode (negative) and anode (positive)

• Light is cast from cathode to anode (look at the shadow)

• Magnet deflects light – this proved that particles have charge and mass.

Crookes’ ConclusionCrookes’ Conclusion• Light is composed of negatively

charged particles – Discovered based upon magnet

deflection and anode shadowYou Tube Demo

(CRT)

Applying a Magnet

Crooke’s Maltese Cross

Tutor Vista animation

Wilhelm Roentgen – 1895German scientist

• In a Crookes tube at very low gas pressure, rays of unknown origin were discovered

• Discovery called X rays

• Glass fluoresced and the air around the equipment was ionizedWilhelm Roentgen’s X ray image

of his wife’s hand

J.J. Thompson: 1897• English Physicist

who said a cathode ray is made of electrons, they

have mass (9.1 x 10-28g)

and are negatively charged particles. Thus he

is credited with “discovering” electrons.

Cathode Rays and Electrons

• Cathode Ray Tube: An evacuated glass tube where a beam of electrons flows from the

cathode (negative electrode) to the anode (positive electrode.)

J.J. Thomson• Used Crookes tube (gas discharge tube)• Applied positive and negative field to a beam

of cathode rays. The deflection was the same for all gases.

• Experimentally proved the existence of the electron (e-)

http://www.aip.org/history/electron/jjappara.htm

Cathode Ray Tube (McGraw Hill)

Experimented with hydrogen gas at low pressure• 2nd beam of particles was moving towards the

cathode, therefore, positive particles• Deflection of positive ions varied with different

gases• Hydrogen ions had the greatest deflection,

therefore, the smallest positive mass• Hydrogen ion deflection was smaller than that

of the electron, therefore more massive than an electron – Hydrogen ion = proton

Thomson

J.J. Thomson• Calculated the charge to mass ratio using different

cathode metals and different gases (e/m= 1.76x108 C/g)

• Measured how much they were deflected by a magnetic field and how much energy they carried.

• He found that the charge to mass ratio was over a thousand times higher than that of a hydrogen ion, suggesting either that the particles were very light or very highly charged.

Credit:Science Museum/Science & Society Picture Library

J.J. Thomson• Made a bold conclusion:

– Cathode rays were indeed made of particles which he called “corpuscles," and these corpuscles came from within the atoms of the electrodes themselves, meaning the atoms were, in fact, divisible.

• Won a Nobel Prize in Physics in 1906.

J.J. Thomson: 1897• Thought the atom was made up of these

corpuscles (negative charges) distributed in a sea of positive charge

• Related it to “plum pudding”

Different models of the plum

pudding model

Robert Millikan:1909American scientist

1. Oil drop experiment2. Measured voltage to

determine the charge on one electron = -1.60 x 10-19 coulomb/e-

3. Used Thomson’s charge to mass ratio to calculated the mass of an electron

Mass of 1 electron = 9.11 x 10-28g

• An atomizer sprayed a fine mist of oil droplets into the upper chamber. Some of these tiny droplets fell through a hole in the upper floor into the lower chamber of the apparatus.

• Next, Millikan applied a charge to the falling drops by irradiating the bottom chamber with x-rays. This caused the air to become ionized - meaning the air particles lost electrons.

• A part of the oil droplets captured one or more of those extra electrons and became negatively charged

• By attaching a battery to the plates of the lower chamber he created an electric field between the plates that would act on the charged oil drops

• He adjusted the voltage till the electric field force would just balance the force of gravity on a drop, and the drop would hang suspended in mid-air.

• Some drops have captured more electrons than others, so they will require a higher electrical field to stop

• Particles that did not capture any of that extra electrons were not affected by the electrical field and fell to the bottom plate due to gravity.

• When a drop is suspended, its weight  m · g  is exactly equal to the electric force applied, the product of the electric field and the charge q · E.

• The values of E (the applied electric field), m (the mass of a drop which was already calculated by Millikan), and g (the acceleration due to gravity), are all known values. Unknown charge on the drop, q

m · g = q · E• Millikan repeated the experiment numerous times varying

the strength of the x-rays ionizing the air so that differing numbers of electrons would jump onto the oil molecules each time.

• He obtained various values for q. The charge q on a drop was always a multiple of 1.59 x 101.59 x 10-19-19 Coulombs Coulombs.

• This is less than 1% lower than the value accepted today: 1.602 x 101.602 x 10-19-19 C C

Ernest Rutherford: 1903

• Rutherford studies under Thomson.

• He discovered 3 types of natural radiation or radioactive decay.α - Alpha Particlesβ - Beta Particles

γ - Gamma Rays

high energy X-rays

Rutherford’s Gold Foil Experiment 1909

• This experiment showed the atom has a small, central positive nucleus and that most of the atom is empty space.

Rutherford Video Clip

Gold Foil Experiment on

You Tube

Rutherford’s Gold Foil ExperimentUsed a narrow beam of particles to bombard

targets made of thin sheets of gold. Metal foil was surrounded by a fluorescent screen.Results:•most of the particles passed through the foil•some were deflected at small angles•few were deflected at large angles

View of the atoms in the Gold Foil Experiment

• Rutherford's Gold Foil Experiment

Conclusions: •atom must contain a very small, dense center of positive charge

•NUCLEUS•all the positive charge and 99.9% of the mass is in the nucleus•electrons define the space of an atom•electrons move at high speeds around the nucleus•atom does not have uniform density

Gold Foil Experiment on

You Tube

Rutherford: 1909• After his Gold Foil

Experiment, Rutherford modifies his model of the atom to contain 2 basic regions: a small dense positive nucleus (protons) with electrons outside.

• Proposed a neutral part of the nucleus

Neils Bohr: 1913• Thought the atom was like the solar system (planetary

model). Electrons orbit the nucleus with a fixed energy.

• Energy Levels - analogous to rungs of a ladder• He wins the Nobel Prize for this model in 1922. It was

eventually shown to be inaccurate and too simplistic.

Henry Moseley: 1913

• Worked under Rutherford.

• Using CRT’s he bombarded metals with electrons and observed the emitted X rays by the metals

• Results: each metal produced X rays of unique frequencies or wavelengths (X ray spectral lines)

• Conclusions: He determined that each element has a unique nuclear charge. Hence, a different number of protons (Atomic Number).

• Each atom is electrically neutral and therefore has an equal number of electrons.

• Killed by a sniper in WW in 1915

Moseley cont.

James Chadwick: 1932

• Studied under Rutherford. • 1st isolated a neutron by

bombarding beryllium atoms with alpha particles

• He determined that the atom also contained a neutron which had approximately the same mass as a proton– Mass of proton =

1.673x10-24g– Mass of neutron =

1.675x10-24g• He proposed that the neutron

had a neutral chargeChadwick won the Nobel

Prize for his work in 1935.

Wave (electron cloud) Model:1924 to Present

• Using Quantum Mechanics, the electron can be found in a probability region.

Therefore: • There are 3 subatomic particles: protons, neutrons

and electrons. These are measured in “atomic mass units” (amu) as their mass is so small.

SubatomicParticle

Mass (amu) Location Charge

Proton ( p+ )1.673 x 10-27 kg

(1.0073 amu or 1 amu)

Neutron ( n0 )1.675 x 10-27 kg

(1.0087 amu or 1 amu)

Electron ( e- )9.1x 10-31 kg

(0.0005 amu or 0 amu)

Atomic Number and Mass Number

• Atomic Number = the number of protons– Unique to each element– In a neutral atom, the number of

protons equal the number of electrons. • Mass Number equal to the total number

of protons + neutrons in the nucleus of an atom.Ex) carbon-12

IsotopesAtoms that have the same number of protons but a different number of neutrons (mass.)

Isotopic NotationShorthand way of representing an isotope of an

element.

Ex) top number is the mass number (#p + #n)

bottom number is the atomic number (#p)

May also be written: chlorine-37 or Cl-37

The actual average atomic mass for all chlorine isotopes is 35.45 amu

3717 Cl

Isotopes of Hydrogena. hydrogen (hydrogen – 1) 1p+ 0n0

b. deuterium (hydrogen – 2) 1p+ 1n0

c. tritium (hydrogen – 3) 1p+ 2n0

11H21H31H

Isotope ProtonsNeutro

nsMass

NumberElectro

ns

Isotopic Notatio

n

Carbon-12 6 6 12 6

Carbon-13 6 7 13 6

Carbon-14 6 8 14 6

Ions• Formed when an atom gains or loses an electron

a. Charge = # of protons - # of electrons Ex) Mg +2 = lost 2 electrons

# of protons: 12 # of electrons: 10 Charge: +2

Positively Charged ion - CATION

Ex) N-3 = gained 3 electrons # of protons: 7 # of electrons: 10

Charge: -3Negatively Charged ion - ANION

CATION“cat”ion

ca+ion

ANION“ant”ion

Isotope ProtonsNeutro

nsMass

NumberElectro

ns

Isotopic Notatio

nCharge

Mg-25 12 13 25 10 +2

N-14 7 7 14 10 -3

Br-79

35 44 79 36 -179 135 Br

14 37 N

25 212 Mg

Atomic Mass:• The mass of an atom expressed in amu (atomic mass units.)

• One amu is equal to 1/12 the mass of a carbon-12 atom.

Average Atomic Mass:• The weighted average of all an element’s isotopes.

• Mass Spectrometers are instruments used to measure masses of isotopes as well as their isotopic abundance.

• This is the number shown in the box on the Periodic Table.

• It is calculated by: (mass1 x %1) + (mass2 x %2) + …

Calculation of atomic mass

Magnesium has 3 naturally occurring isotopes:

78.99% Mg-24, 10.00% Mg-25, and11.01% Mg-26

Calculate the atomic mass of magnesium. (24 x 0.7899) = 18.9576 = 18.96+ (25 x 0.1000) = 2.500 =

2.500+ (26 x 0.1101) = 2.8626 = 2.863

24.323 24.32 amu

Calculation of atomic mass

Magnesium has 3 naturally occurring isotopes:

78.99% is 23.98504 amu10.00% is 24.98584 amu11.01% is 25.98259 amu

Calculate the atomic mass of magnesium. (23.98504 amu x 0.7899) + (24.98584 amu x 0.1000) + (25.98259 amu x 0.1101)

24.31 amu

Atomic Mass - the mass of an atom, based on a C-12 atom, in atomic mass units (amu)

1 amu = 1.66 x 10-24g = 1/12 the mass of a C-12 atom

Example: atomic mass of Na = 23.0 amu

Atoms are too small to count or mass individually. It is easier to count many or mass many.

amu gram (atomic scale) (macroscopic scale)mol

e

Mole = amount of substance that contains 6.02 x 1023 particles (abbreviated: molmol)

Avogadro’s Number = number of particles in a mole = 6.02 x 106.02 x 102323 particles particles

Particles can be atoms, ions, molecules, or formula units

Molar Mass = mass, in grams, per 1 mole of a substance

units = grams/mole (g/mol)

1 Mole = 6.02x1023 particles of substance

1 Mole = mass (g) of substance from PT

Change the composition of an atom’s nucleus.

Protons & neutrons are called nucleons;

atom is called the nuclide.

1. Elements may be converted from one to another

2. Particles within the nucleus are involved.

3. Tremendous amounts of energy are released (million times that of chemical)

4. Rate of reaction is not influenced by external factors.

1. No new elements can be produced

2. Only electrons participate

3. Relatively small amounts of energy are absorbed or released

4. Rate of reaction depends upon factors such as temperature and pressure

Ionizing Radiation is radiation with sufficient energy to change atoms and molecules into ions (can damage living tissue).

Nonionizing Radiation is radiation that does not have sufficient energy to ionize matter.

A. Alpha Decay – spontaneous emission of alpha particle from the nucleus; from neutron-poor heavy nuclei

B. Beta Decay – spontaneous emission of beta particle from the nucleus; from neutron-rich nuclei

18177Ir226 222 4

88 86 2Ra Rn α 185 479 2Au _________ α

228 228 088 89 -1Ra Ac + β

131 0 13153 1 54I β Xe 14

714 06 1N C β

204 200 482 80 2Pb Hg + α

ParticleMass (amu)

Charge

SymbolStopped

byProton(Hydrogen nucleus)

1.00727647

Neutron1.00866490

Beta Particle (electron)

0.0005486

Alpha Particle

(Helium nucleus)

4.00150617

Gamma Ray (high energy

EMR)0

Positron(positively

charged electron)0.0005486

0

0

14 4 18 17 17 2 9 8 1[ ]N He F O H

target nucleus

projectile

new isotope (element)

ejected particleUnstable compound

nucleus

10 14 244 7 12 _____Be N Mg

2 3 41 1 2 _____H H He

238 247 192 99 0_____ 5U Es n

9 4 14 2 0_____Be He n

What is the ½ Life of Strontium-90???

How long until no more Strontium-90 remains?