atomic structure unit 2

http://www.unit5.org/chemistry

Atomic Structure

Unit 2

Guiding QuestionsHow do we know atoms exist?

How do we know that electrons, protons, and neutrons exist?

What is radiation and what does it come from?

Is radiation safe?

Where does matter come from?

How are elements formed?

Are all atoms of an element the same?

How do we measure atoms if they are so small?

How do we know what stars are made of?

What is wrong with this picture?

Atomic Theory and Atomic Structure

You should be able to

Discuss the development of the atom from its earliest model tothe modern day atom.

Identify the correct number of subatomic particles for atoms, ions,and isotopes.

Calculate the average atomic mass of an atom from isotopic data.

Name compounds and write chemical formulas for binary compounds,ternary compounds (those with polyatomic ions), and acids.

Memorize the chemical formulas and charged of the poly-atomic ions and the most common transition metal ions.

Atomic Structure and PeriodicityYou should be able to

Identify characteristics of and perform calculations with frequency and wavelength.

Know the relationship between types of electromagnetic radiation and energy; for example, gamma rays are the most damaging.

Know what exhibits continuous and line spectra.

Know what each of the four quantum numbers n, l, m, and ms represents.

Identify the four quantum numbers for an electron in an atom.

Write complete and shorthand electron configurations as well as orbitaldiagrams for an atom or ion of an element.

Identify the number and location of the valence electrons in an atom.

Apply the trends in atomic properties such as atomic radii, ionizationenergy, electronegativity, electron affinity, and ionic size.

The Hellenic Market

Fire Water Earth Air

~~

• In fourth century B.C., ancient Greeks proposed that matter consisted of fundamental particles called atoms.

• Over the next two millennia, major advances in chemistry were achieved by alchemists. Their major goal was to convert certain elements into others by a process called transmutation.

A Brief History of Chemistry

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

The GreeksHistory of the Atom

• Not the history of atom, but the idea of the atom

• In 400 B.C the Greeks tried to understand matter (chemicals) and broke them down into earth, wind, fire, and air.

• Democritus and Leucippus Greek philosophers

Greek Model

• Greek philosopher• Idea of ‘democracy’• Idea of ‘atomos’

– Atomos = ‘indivisible’– ‘Atom’ is derived

• No experiments to support idea

• Continuous vs. discontinuous theory of matter

Democritus’s model of atom

No protons, electrons, or neutrons

Solid and INDESTRUCTABLE

Democritus

“To understand the very large,

we must understand the very small.”

DEMOCRITUS (400 BC) – First Atomic Hypothesis

Atomos: Greek for “uncuttable”. Chop up a piece of matter until you reach the atomos.

Properties of atoms:• indestructible.• changeable, however, into different forms.• an infinite number of kinds so there are an infinite number of elements.• hard substances have rough, prickly atoms that stick together.• liquids have round, smooth atoms that slide over one another.• smell is caused by atoms interacting with the nose – rough atoms hurt.• sleep is caused by atoms escaping the brain.• death – too many escaped or didn’t return.• the heart is the center of anger.• the brain is the center of thought.• the liver is the seat of desire.

“Nothing exists but atoms and space, all else is opinion”.

Democritus

Four Element Theory

• Plato was an atomist• Thought all matter was

composed of 4 elements:– Earth (cool, heavy)– Water (wet)– Fire (hot)– Air (light)– Ether (close to heaven)

‘MATTER’

FIRE

EARTHAIR

WATER

Hot

WetCold

Dry

Relation of the four elements and the four qualities

Blend these “elements” in different proportions to get all substances

http://en.wikipedia.org/wiki/Classical_element

AnaxagorasAnaxagoras (Greek, born 500 B.C.)–Suggested every substance had its own kind of “seedsseeds” that clustered together to make the substance, much as our atoms cluster to make molecules.

Some Early Ideas on Matter

O’Connor Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles 1982, page 26,

EmpedoclesEmpedocles (Greek, born in Sicily, 490 B.C.)–Suggested there were only four basic seeds – earth, air, fire, and water– earth, air, fire, and water. The elementary substances (atoms to us) combined in various ways to make everything.

Democritus (Thracian, born 470 B.C.)–Actually proposed the word atomproposed the word atom (indivisible) because he believed that all matter consisted of such tiny units with voids between, an idea quite similar to our own beliefs. It was rejected by Aristotle and thus lost for 2000 years.

AristotleAristotle (Greek, born 384 B.C.)–Added the idea of “qualities” – heat, cold, dryness, moisture – as basic elements– heat, cold, dryness, moisture – as basic elements which combined as shown in the diagram (previous page).

Hot + dry made fire; hot + wet made air, and so on.

Alchemy

• After that chemistry was ruled by alchemy.

• They believed that that could take any cheap metals and turn them into gold.

• Alchemists were almost like magicians.– elixirs, physical immortality

Alchemy

. . . . . . . . . . . .. . .

GOLD SILVER COPPER IRON SAND

Alchemical symbols for substances…

transmutation: changing one substance into another

In ordinary chemistry, we cannot transmute elements.

Contributionsof alchemists:

Information about elementsInformation about elements - the elements mercury, sulfur, and antimony were discovered- properties of some elements

Develop lab apparatus / procedures / experimental techniquesDevelop lab apparatus / procedures / experimental techniques - alchemists learned how to prepare acids. - developed several alloys - new glassware

Timeline

2000 1000 300 AD

American Independence

(1776)

Issac Newton(1642 - 1727)

400 BC

Greeks (Democritus ~450 BC)

Discontinuous theory of matter

ALCHEMY

Greeks (Aristotle ~350 BC))

Continuous theory of matter

Dalton Model of the Atom

Late 1700’s - John Dalton- England

Teacher- summarized results of his experiments and those of others

Combined ideas of elements with that of atoms in Dalton’s Atomic Theory

• In 1803, Dalton proposed that elements consist of individual particles called atoms.

• His atomic theory of matter contains four hypotheses:

1. All matter is composed of tiny particles called atoms.

2. All atoms of an element are identical in mass and fundamental chemical properties.

3. A chemical compound is a substance that always contains the same atoms in the same ratio.

4. In chemical reactions, atoms from one or more compounds or elements redistribute or rearrange in relation to other atoms to form one or more new compounds. Atoms themselves do not undergo a change of identity in chemical reactions.

The Atomic Theory of Matter


Foundations of Atomic Theory

Law of Definite Proportions

The fact that a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound.

Law of Multiple Proportions

If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first elements is always a ratio of small whole numbers.

Law of Conservation of Mass

Mass is neither destroyed nor created during ordinary chemical reactions.

Conservation of Atoms

John Dalton

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204

2 H2 + O2 2 H2O

4 atoms hydrogen2 atoms oxygen

4 atoms hydrogen2 atoms oxygen

H

H

O

O

O

O

H

H

H

H

H

H

H2

H2

O2

H2O

H2O

+

http://en.wikipedia.org/wiki/John_Dalton

45 g H2O? g H2O

Conservation of Mass


Highvoltage

Before reaction

electrodes

glasschamber

5.0 g H2

80 g O2

300 g (mass of chamber)+385 g total

H2O2

Highvoltage

After reaction

0 g H2

40 g O2

300 g (mass of chamber)+385 g total

O2

H2O

Law of Definite ProportionsJoseph Louis Proust (1754 – 1826)

• Each compound has a specific ratio of elements

• It is a ratio by mass

• Water is always 8 grams of oxygen for every one gram of hydrogen

http://en.wikipedia.org/wiki/Joseph_Proust

The Law of Multiple Proportions

• Dalton could not use his theory to determine the elemental compositions of chemical compounds because he had no reliable scale of atomic masses.

• Dalton’s data led to a general statement known as the law of multiple proportions.

• Law states that when two elements form a series of compounds, the ratios of the masses of the second element that are present per gram of the first element can almost always be expressed as the ratios of integers.


Daltons Atomic Theory

• Dalton stated that elements consisted of tiny particles called atoms

• He also called the elements pure substances because all atoms of an element were identical and that in particular they had the same mass.

http://web.lemoyne.edu/~giunta/EA/DALTONann.HTML

• Dalton’s atomic theory is essentially correct, with four minor modifications:

1. Not all atoms of an element must have precisely the same mass.

2. Atoms of one element can be transformed into another through

nuclear reactions. 3. The composition of many solid compounds are somewhat

variable. 4. Under certain circumstances, some atoms can be divided

(split into smaller particles: i.e. nuclear fission).

The Atomic Theory of Matter


Dalton’s Atomic Theory1. All matter consists of tiny particles.

Dalton, like the Greeks, called these particles “atoms”.

2. Atoms of one element can neither be subdivided nor changed into atoms of any other element.

3. Atoms can neither be created nor destroyed.

4. All atoms of the same element are identical in mass, size, and other properties.

6. In compounds, atoms of different elements combine in simple, whole number ratios.

5. Atoms of one element differ in mass and other properties from atoms of other elements.

Dalton’s Atomic Theory

1. All matter is made of tiny indivisible particles called atoms.

2. Atoms of the same element are identical, those of different atoms are different.

3. Atoms of different elements combine in whole number ratios to form compounds

4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

California WEB

Structure of Atoms

• Scientist began to wonder what an atom was like.

• Was it solid throughout with no internal structure or was it made up of smaller, subatomic particles?

• It was not until the late 1800’s that evidence became available that atoms were composed of smaller parts.

History: On The Human Side

1834 Michael Faraday - electrolysis experiments

suggested electrical nature of matter

1895 Wilhelm Roentgen - discovered X-rays when

cathode rays strike anode

1896 Henri Becquerel - discovered "uranic rays" and

radioactivity

1896 Marie (Marya Sklodowska) and Pierre Curie -

discovered that radiation is a property of the

atom, and not due to chemical reaction.

(Marie named this property radioactivity.)

1897 Joseph J. Thomson - discovered the electron

through Crookes tube experiments

1898 Marie and Piere Curie - discovered the

radioactive elements polonium and radium

1899 Ernest Rutherford - discovered alpha and beta

particles

1900 Paul Villard - discovered gamma rays

1903 Ernest Rutherford and Frederick Soddy -

established laws of radioactive decay and

transformation

1910 Frederick Soddy - proposed the isotope concept

to explain the existence of more than one atomic

weight of radioelements

1911 Ernest Rutherford - used alpha particles to

explore gold foil; discovered the nucleus and the

proton; proposed the nuclear theory of the atom

1919 Ernest Rutherford - announced the first artificial

transmutation of atoms

1932 James Chadwick - discovered the neutron by

alpha particle bombardment of Beryllium

1934 Frederick Joliet and Irene Joliet Curie - produced

the first artificial radioisotope

1938 Otto Hahn, Fritz Strassmann, Lise Meitner, and

Otto Frisch - discovered nuclear fission of

uranium-235 by neutron bombardment

1940 Edwin M McMillan and Philip Abelson -

discovered the first transuranium element,

neptunium, by neutron irradiation of uranium in a

cyclotron

1941 Glenn T. Seaborg, Edwin M. McMillan, Joseph

W. Kennedy and Arthur C. Wahl - announced

discovery of plutonium from beta particle

emission of neptunium

1942 Enrico Fermi - produced the first nuclear fission

chain-reaction

1944 Glenn T. Seaborg - proposed a new format for

the periodic table to show that a new actinide series of 14

elements would fall below and be analogous to the 14

lanthanide-series elements.

1964 Murray Gell-Mann hypothesized that quarks are the

fundamental particles that make up all known subatomic

particles except leptons.

Radioactivity (1896)1. rays or particles produced by

unstable nuclei

a. Alpha Rays – helium nucleus

b. Beta Part. – high speed electron

c. Gamma ray – high energy x-ray

2. Discovered by Becquerel –

exposed photographic film

3. Further work by CuriesAntoine-Henri Becquerel

(1852 - 1908)

http://en.wikipedia.org/wiki/Henri_Becquerel

Radioactivity

• One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie CurieMarie Curie (1876 - 1934).

• She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.

http://en.wikipedia.org/wiki/Marie_Curie

The Effect of an Obstruction on Cathode Rays


Highvoltage

cathode

source ofhigh voltage

yellow-greenfluorescence

shadow

Crooke’s Tube

+-

vacuum tube

metal disks

voltage source

magnet

William Crookes

Source ofElectricalPotential

Metal Plate

Gas-filledglass tube Metal plate

Stream of negativeparticles (electrons)

A Cathode Ray Tube

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

Background Information

Cathode Rays• Form when high voltage is applied across

electrodes in a partially evacuated tube.• Originate at the cathode (negative electrode)

and move to the anode (positive electrode)• Carry energy and can do work• Travel in straight lines in the absence of an

external field

Cathode Ray Experiment

1897 Experimentation

• Using a cathode ray tube, Thomson was able to deflect cathode rays with an electrical field.

• The rays bent towards the positive pole, indicating that they are negatively charged.


The Effect of an Electric Field on Cathode Rays

Highvoltage

cathode

source ofhigh voltage

positiveplate

negative plate

anode

_

+

Cathode Ray Experiment

Deflectionregion

Drift region

Displacement

+

-Anodes / collimators

Cathode

Volts

Thomson’s CalculationsCathode Ray Experiment

• Thomson used magnetic and electric fields to measure and calculate the ratio of the cathode ray’s mass to its charge.

Magneticdeflection

charge ofray particle

magneticfield

length ofdeflection region

length of drift region

mass of rayparticle

velocity ofray particle

x x x

x=

Electricdeflection

charge ofray particle

electricfield

length ofdeflection region

length of drift region

mass of rayparticle

velocity ofray particle

x x x

x=

2

magnetic deflection

electric deflection

magnetic field

electric fieldx velocity=

ThomsonPAPER

Conclusions

• He compared the value with the mass/ charge ratio for the lightest charged particle.

• By comparison, Thomson estimated that the cathode ray particle weighed 1/1000 as much as hydrogen, the lightest atom.

• He concluded that atoms do contain subatomic particles - atoms are divisible into smaller particles.

• This conclusion contradicted Dalton’s postulate and was not widely accepted by fellow physicists and chemists of his day.

• Since any electrode material produces an identical ray, cathode ray particles are present in all types of matter - a universal negatively charged subatomic particle later named the electron

J.J. Thomson

• He proved that atoms of any element can be made to emit tiny negative particles.

• From this he concluded that ALL atoms must contain these negative particles.

• He knew that atoms did not have a net negative charge and so there must be balancing the negative charge.

J.J. Thomson

http://en.wikipedia.org/wiki/J._J._Thomson

William Thomson (Lord Kelvin)

• In 1910 proposed the Plum Pudding model– Negative electrons

were embedded into a positively charged spherical cloud.


Spherical cloud ofPositive charge

Electrons

http://en.wikipedia.org/wiki/William_Thomson%2C_1st_Baron_Kelvin

Thomson Model of the Atom

• J.J. Thomson discovered the electron and knew that electrons could be emitted from matter (1897).

• William Thomson proposed that atoms consist of small, negative electrons embedded in a massive, positive sphere.

• The electrons were like currants in a plum pudding.

• This is called the ‘plum pudding’ model of the atom.

- electrons-

-

-

-

--

-

Ernest Rutherford (1871-1937)

• Learned physics in J.J. Thomson’ lab.

• Noticed that ‘alpha’ particles were sometime deflected by something in the air.

• Gold-foil experiment

Rutherford

PAPER

Rutherford

PAPER

Animation by Raymond Chang – All rights reserved.

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

http://en.wikipedia.org/wiki/Ernest_Rutherford

Rutherford ‘Scattering’

• In 1909 Rutherford undertook a series of experiments• He fired (alpha) particles at a very thin sample of gold foil• According to the Thomson model the particles would only

be slightly deflected• Rutherford discovered that they were deflected through large

angles and could even be reflected straight back to the source

particlesource

Lead collimator Gold foil

Rutherford’s Apparatus

beam of alpha particles

radioactive substance

gold foil

circular ZnS - coated

fluorescent screen


Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry.

http://www.nobel.se/laureates/chemistry-1908.html

What He Expected

• The alpha particles to pass through without changing direction (very much)

• Because

• The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

California WEB

What he expected…

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

- -

-

-

-

What he got…richochetingalpha particles

The Predicted Result:

expected path

expected marks on screen

mark onscreen

likely alphaparticle path

Observed Result:

Interpreting the Observed Deflections


.

.

.

.

.

.

.

.

.

.

.

.

.

.

gold foil

deflected particle

undeflected particles

.

.beam ofalpha particles

.

Density and the Atom

• Since most of the particles went through, the atom was mostly empty.

• Because the alpha rays were deflected so much, the positive pieces it was striking were heavy.

• Small volume and big mass = big density• This small dense positive area is the

nucleus

California WEB

Rutherford Scattering (cont.)

Rutherford interpreted this result by suggesting that the particles interacted with very small and heavy particles

Particle bounces off of atom?

Particle attracts to atom?

Particle goes through atom?

Particle path is alteredas it passes through atom?

.

Case A

Case B

Case C

Case D

Table: hypothetical description of alpha particles

alpha rays don’t diffract

alpha rays deflect towards a negatively charged plate and away from a positively charged plate

alpha rays are deflected only slightly by an electric field; a cathode ray passing through the same field is deflected strongly

... alpha radiation is a stream of particles

... alpha particles have a positive charge

... alpha particles either have much lower charge or much greater mass than electrons

observation hypothesis

(based on properties of alpha radiation)

Copyright © 1997-2005 by Fred Senese

http://antoine.frostburg.edu/chem/senese/index.shtml

Explanation of Alpha-Scattering Results

Plum-pudding atom

++

+

+

+

+

+

+

-

-

-

-

-

-

-

-

Alpha particles

Nuclear atom

Nucleus

Thomson’s model Rutherford’s model

Results of foil experiment if plum-pudding had been correct.

Electrons scatteredthroughout positive

charges


++

+

+

+

+

+

+

-

-

-

-

-

-

-

-

Rutherford’sGold-Leaf Experiment

Conclusions:

Atom is mostly empty space

Nucleus has (+) charge

Electrons float around nucleus


http://en.wikipedia.org/wiki/Gold_foil_experiment

The Rutherford Atom


n +

-

--

-

-

- --

-

-

This is the modern atom model.

Electrons are in constant motion around the nucleus, protons and neutrons jiggle within the nucleus, and quarks jiggle within the protons and neutrons. This picture is quite distorted. If we drew the atom to scale and made protons and neutrons a centimeter in diameter, then the electrons and quarks would be less than the diameter of a hair and the entire atom's diameter would be greater than the length of thirty football fields! 99.999999999999% of an atom's volume is just empty space!

Website “The Particle Adventure”

http://particleadventure.org/frameless/modern_atom.html

While an atom is tiny, the nucleus is ten thousand times smaller than the atom and the quarks and electrons are at least ten thousand times smaller than that. We don't know exactly how small quarks and electrons are; they are definitely smaller than 10-18 meters, and they might literally be points, but we do not know.

It is also possible that quarks and electrons are not fundamental after all, and will turn out to be made up of other, more fundamental particles. (Oh, will this madness ever end?)

Scale of the atom.


http://particleadventure.org/frameless/scale.html

Physicists have developed a theory called The Standard Model that explains what the world is and what holds it together. It is a simple and comprehensive theory that explains all the hundreds of particles and complex interactions with only: 6 quarks. 6 leptons. The best-known lepton is the electron.

Force carrier particles, like the photon. We will talk about these particles later. All the known matter particles are composites of quarks and leptons, and they interact by exchanging force carrier particles.

The Standard Model is a good theory. Experiments have verified its predictions to incredible precision, and all the particles predicted by this theory have been found. But it does not explain everything. For example, gravity is not included in the Standard Model.


http://particleadventure.org/frameless/standard_model.html

Discovery of the electron

1807 Davy suggested that electrical forces held compound together.

1833 Faraday related atomic mass and the electricity needed to free an element

during electrolysis experiments.

1891 Stoney proposed that electricity exists in units he called electrons.

1897 Thomson first quantitatively measured the properties of electrons.

Oil Drop Experiment

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oil droplets

oil droplet under observation

Charged plate

Small hole

Charged plate

-

+

Telescope

oil atomizer

Robert Millikan

Bohr Atom

The Planetary Model of the Atom

Bohr’s Model

Nucleus

Electron

Orbit

Energy Levels

Bohr Model of Atom

The Bohr model of the atom, like many ideas in the history of science, was at first prompted by and later partially disproved by experimentation.

http://en.wikipedia.org/wiki/Category:Chemistry

Increasing energyof orbits

n = 1

n = 2

n = 3

A photon is emittedwith energy E = hf

e-e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

http://en.wikipedia.org/wiki/Category:Chemistry

http://en.wikipedia.org/wiki/Image:Bohratommodel.png

Quantum Mechanical Model

Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals).

Niels Bohr &Albert Einstein

Development of Atomic Models

Rutherford modelIn the early twentieth century, Rutherfordshowed that most of an atom's mass isconcentrated in a small, positively chargedregion called the nucleus.

Bohr modelAfter Rutherford's discovery, Bohr proposedthat electrons travel in definite orbits aroundthe nucleus.

Thomson modelIn the nineteenth century, Thomson describedthe atom as a ball of positive charge containinga number of electrons.

Quantum mechanical modelModern atomic theory described theelectronic structure of the atom as theprobability of finding electrons withincertain regions of space.

Modern View

• The atom is mostly empty space• Two regions

– Nucleus • protons and neutrons

– Electron cloud• region where you might find an electron

Review Models of the Atom

Dalton proposes theindivisible unit of anelement is the atom.


Thomson discoverselectrons, believed toreside within a sphere ofuniform positive charge(the “plum-pudding model).

Rutherford demonstrates the existence of a positivelycharged nucleus thatcontains nearly all themass of an atom.

Bohr proposes fixedcircular orbits aroundthe nucleus for electrons.

In the current model of the atom, electrons occupy regions of space (orbitals) around the nucleus determined by their energies.

Models of the Atom

Dalton’s model (1803)

Thomson’s plum-pudding model (1897)

Rutherford’s model (1909)

Bohr’s model (1913)

Charge-cloud model (present)


Greek model(400 B.C.)

+--

--

-e

e

e

+

+ +

+

++

++

e

ee

e

e

ee

"In science, a wrong theory can be valuable and better than no theory at all."- Sir William L. Bragg

Models of the Atom

Dalton’s model (1803)

Thomson’s plum-pudding model (1897)

Rutherford’s model (1909)

Bohr’s model (1913)

Charge-cloud model (present)


Greek model(400 B.C.)

1800 1805 ..................... 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945

1803 John Dalton pictures atoms astiny, indestructible particles, with no internal structure.

1897 J.J. Thomson, a Britishscientist, discovers the electron,leading to his "plum-pudding" model. He pictures electronsembedded in a sphere ofpositive electric charge.

1904 Hantaro Nagaoka, aJapanese physicist, suggests that an atom has a centralnucleus. Electrons move in orbits like the rings around Saturn.

1911 New Zealander Ernest Rutherford statesthat an atom has a dense,positively charged nucleus. Electrons move randomly in the space around the nucleus.

1913 In Niels Bohr'smodel, the electrons move in spherical orbits at fixed distances from the nucleus.

1924 Frenchman Louis de Broglie proposes thatmoving particles like electronshave some properties of waves. Within a few years evidence is collected to support his idea.

1926 Erwin Schrödinger develops mathematicalequations to describe the motion of electrons in atoms. His work leads to the electron cloud model.

1932 James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons.

+--

--

-e

e

e

+

+ +

+

++

++

e

ee

e

e

ee

Bohr Model

After Rutherford’s discovery, Bohr proposed that electrons travel in definite orbits around the nucleus.

Planetary model

Neils Bohr

Bohr’s contributions• Bohr’s contributions to the understanding of

atomic structure: 1. Electrons can occupy only certain regions of space,

called orbits.

2. Orbits closer to the nucleus are more stable —

they are at lower energy levels.

3. Electrons can move from one orbit to another by absorbing or emitting energy, giving rise to characteristic spectra.

• Bohr’s model could not explain

the spectra of atoms heavier

than hydrogen.


ElectronsElectrons (-) charge no mass located outside the nucleus

ProtonsProtons (+) charge 1 amu located inside the nucleus

NeutronsNeutrons no charge 1 amu located inside the nucleus

Particles in the Atom

Subatomic particles

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

Actual mass (g)

e-

p+

no

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

Discovery of the Neutron

James Chadwick bombarded beryllium-9 with alpha particles, carbon-12 atoms were formed, and neutrons were emitted.

n10

+He42

+Be94 C12

6

Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 764 *Walter Boethe

http://en.wikipedia.org/wiki/James_Chadwick

http://en.wikipedia.org/wiki/Walter_Bothe

Humor

Two atoms are walking down the street.One atom says to the other, “Hey! I think I lost an electron!”The other says, “Are you sure??”“Yes, I’m positive!”

A neutron walks into a restaurant and orders a couple of drinks. As she is about to leave, she asks the waiter how much she owes. The waiter replies, “For you, No Charge!!!”

Subatomic Particles

POSIT IVECHARG E

PROT ONS

NEUT RALCHARG E

NEUT RONS

NUCLEUS

NEG AT IVE CHARG E

ELECT RONS

AT OM

Most of the atom’s mass.

NUCLEUSNUCLEUS ELECTRONSELECTRONS

PROTONSPROTONS NEUTRONSNEUTRONS Negative Charge

PositiveCharge

NeutralCharge

ATOM

QUARKSAtomic Numberequals the # of...

equal in a neutral atom

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Subatomic Particles

• Quarks– component of

protons & neutrons

– 6 types

– 3 quarks = 1 proton or 1 neutron

He


3 p+

4 n0 2e– 1e–

Li shorthand

Bohr - Rutherford diagrams• Putting all this together, we get B-R diagrams• To draw them you must know the # of protons, neutrons,

and electrons (2,8,8,2 filling order)• Draw protons (p+), (n0) in circle (i.e. “nucleus”)• Draw electrons around in shells

2 p+

2 n0

He

3 p+

4 n0

Li

Draw Be, B, Al and shorthand diagrams for O, Na

11 p+12 n°

2e– 8e– 1e–

Na

8 p+8 n°

2e– 6e–

O

4 p+5 n°

Be

5 p+6 n°

B

13 p+14 n°

Al

Mass Number

• mass # = protons + neutrons

• always a whole number

• NOT on the Periodic Table!

+

+

+

+

+

+

Nucleus

Electrons

Nucleus

Neutron

Proton

Carbon-12Neutrons 6Protons 6Electrons 6

e-

e-

e-

e-

e-

e-

Isotopes

• Atoms of the same element with different mass numbers.

Mass #

Atomic #

• Nuclear symbol:

• Hyphen notation: carbon-12carbon-12Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

12

6 C

Isotopes

+

+

+

+

+

+

Nucleus

Electrons

Nucleus

Neutron

Proton


Nucleus

Electrons


+

+

+

+

+

+

Nucleus

Neutron

Proton

3 p+

3 n02e– 1e– 3 p+

4 n02e– 1e–

6Li 7Li

+

+

+Nucleus

Electrons

Nucleus

Neutron

Proton

Lithium-6Neutrons 3Protons 3Electrons 3

Nucleus

Electrons

Nucleus

Neutron

Proton

Lithium-7Neutrons 4Protons 3Electrons 3

+

+

+

Isotopes

• Chlorine-37

– atomic #:

– mass #:

– # of protons:

– # of electrons:

– # of neutrons:

17

37

17

17

20


Cl37

17

37

17 Cl

Relative Atomic Mass

• 12C atom = 1.992 × 10-23 g

• 1 p = 1.007276 amu

1 n = 1.008665 amu

1 e- = 0.0005486 amu

• atomic mass unit (amu)

• 1 amu = 1/12 the mass of a 12C atom+

+

+

+

+

+

Nucleus

Electrons

Nucleus

Neutron

Proton


Average Atomic Mass

• weighted average of all isotopes• on the Periodic Table• round to 2 decimal places


Avg.AtomicMass

= (mass)(%) + (mass)(%)

100

Average Atomic Mass

• EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.


Avg.AtomicMass

= (16)(99.76) + (17)(0.04) + (18)(0.20)

100= 16.00

amu

Average Atomic Mass

• EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.


Avg.AtomicMass

= (35)(8) + (37)(2)

10= 35.40 amu

Mass Spectrophotometer

electron beam

magnetic field

gas

stream of ions of differentmasses lightest

ions

heaviest ions

Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138

.

• mass spectrometry is used to experimentally determine isotopic masses and abundances • interpreting mass spectra • average atomic weights

- computed from isotopic masses and abundances - significant figures of tabulated atomic weights gives some idea of natural variation in isotopic abundances

Weighing atomsgas sampleenters here

filament currentionizes the gas

ions acceleratetowards chargedslit

magnetic fielddeflects lightest ionsmost

ions separated by massexpose film

The first mass spectrograph was built in 1919 by F. W. Aston, who received the 1922 Nobel Prize for this accomplishment

Copyright © 1997-2005 by Fred Senese

http://pl.nobel.se/laureates/chemistry-1922-1-bio.html

http://pl.nobel.se/laureates/chemistry-1922.html

http://antoine.frostburg.edu/chem/senese/index.shtml

Mass Spectrometry

- +

Photographic plate

196 199 201 204

198 200 202

Mass spectrum of mercury vaporMass spectrum of mercury vapor

Hill, Petrucci, General Chemistry An Integrated Approach1999, page 320

Stream of positive ionsStream of positive ions

Mass Spectrum for Mercury

196 197 198 199 200 201 202 203 204

Mass numberMass number

Rel

ativ

e n

umb

er o

f at

oms

Rel

ativ

e n

umb

er o

f at

oms

30

25

20

15

10

5

196 199 201 204

198 200 202

Mass spectrum of mercury vaporMass spectrum of mercury vapor

The percent natural abundances The percent natural abundances for mercury isotopes are:for mercury isotopes are:

Hg-196 0.146%Hg-196 0.146% Hg-198 10.02%Hg-198 10.02% Hg-199 16.84%Hg-199 16.84% Hg-200 23.13%Hg-200 23.13% Hg-201 13.22%Hg-201 13.22% Hg-202 29.80%Hg-202 29.80% Hg-204 6.85%Hg-204 6.85%

(The photographic record has been converted to a scale of relative number of atoms)

Mercury IsotopesThe percent natural abundances The percent natural abundances for mercury isotopes are:for mercury isotopes are:

Hg-196 0.146%Hg-196 0.146% Hg-198 10.02%Hg-198 10.02% Hg-199 16.84%Hg-199 16.84% Hg-200 23.13%Hg-200 23.13% Hg-201 13.22%Hg-201 13.22% Hg-202 29.80%Hg-202 29.80% Hg-204 6.85%Hg-204 6.85%

(0.00146)(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x

0.28616 + 19.8396 + 33.5116 + 46.2600 + 26.5722 + 60.1960 + 13.974 = x

x = 200.63956 amu

Hg200.59

80

(% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM

ABCDEFG

Natural uranium, atomic weight = 238.029 g/molDensity is 19 g/cm3. Melting point 1000oC.

Two main isotopes:

U238 92

U235 92

99.3%

0.7%

Because isotopes are chemically identical(same electronic structure), they cannot beseparated by chemistry.

So Physics separates them by diffusion orcentrifuge (mass spectrograph is too slow)…

Separation of Isotopes

(238 amu) x (0.993) + (235 amu) x (0.007)

236.334 amu + 1.645 amu

237.979 amu

U238

92

Average Atomic Mass

• Assume you have only two atoms of chlorine.• One atom has a mass of 35 amu (Cl-35)• The other atom has a mass of 36 amu (Cl-36)

• What is the average mass of these two isotopes?

35.5 amu

• Looking at the average atomic mass printed on the periodic table...approximately what percentage is Cl-35 and Cl-36?

55% Cl-35 and 45% Cl-36 is a good approximation

Cl35.453

17

Naming Isotopes

• Put the mass number after the name of the element

• carbon- 12

• carbon -14

• uranium-235

California WEB

Calculating averages

• You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks?

• Total mass = (4 x 50) + (1 x 60) = 260 g

• Average mass = (4 x 50) + (1 x 60) = 260 g 5 5

• Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5

California WEB

Calculating averages

• Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5

• Average mass = .8 x 50 + .2 x 60

• 80% of the rocks were 50 grams

• 20% of the rocks were 60 grams

• Average = % as decimal x mass + % as decimal x mass + % as decimal x mass +

California WEB

Isotopes

• Because of the existence of isotopes, the mass of a collection of atoms has an average value.

• Average mass = ATOMIC WEIGHT

• Boron is 20% B-10 and 80% B-11. That is, B-11 is 80 percent abundant on earth.

• For boron atomic weight= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

atomic structure unit 2

Documents

properties of atoms

prickly atoms

smooth atoms

atomic theory

atomic properties

history of atom

nose rough atoms

atomic structureyou