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History of the Atom

In Atomic Theory, if we were to zoom in on a copper wire, it might look like the picture below.

What is between the atoms?

What did be believe before we thought there were atoms?

What would it have looked like?

We thought matter was continuous, not made of little particles

Why would we have believed matter was continuous?

Because it looks like it. Why don’t you see atoms when you look at a piece of copper wire?

Because atoms are sooooooo small.

How big is an atom?

If a baseball was blown up to the size of the earth. An atom would be about the size of a baseball.

Picture yourself out in space trying to see someone holding a baseball in your backyard.

We could never hope to see an atom with visible light

Atom

We can “see” them with special microscope

called a scanning tunneling electron microscope.

Guess who the first to do this was?

Atoms were first proposed by 2,400 years ago by the Greek philosopher

Democritus

BUT the idea did not catch on until 2,200 years later. Why did no one believe him?

Philosopher did not experiment, they just thought. NO PROOF.

If we can’t see atoms

why do we believe in them now?

(even well before the SEM)

John Dalton (1766-1844) • English School teacher and early

meteorologist • Trying to learn what caused rain and

where it came from • Discovered that the water actually came

out of the air itself • Combined the concept of atoms with that

of elements • His conclusion came from several

experiments. (not just guessing)

How could water be in air? If matter is continuous, then there is no room in air for water.

Air

Water

How can two things occupy the same space??

Air & water mixed??

Air & water mixed

Air

Water

Atomic theory of matter

The molecules of air and water can mix because they do not occupy the same space

Experiments also showed that:

A given compound (say NaCl) had the same % of elements, no matter where he located it from. (ocean water or made from Na & Cl)

Always 1 Na & 1 Cl

by mass 39% Na & 61% Cl

Formation of NaCl from its elements

http://www.youtube.com/watch?v=2mzDwgyk6QM

http://www.youtube.com/watch?v=2mzDwgyk6QM

Why should a compound have an exact % of elements

According the the continuous theory of matter I can mix any amount of Na and Cl

Na Cl NaCl

Cl Na NaCl

atomic theory of matter: atoms of Na react with atoms of Chlorine to form a compound

Can part of a Na atom react?

Na Cl

Cl Na

Cl

Na

Cl

Na + + +

Na

Law of definite composition: a sample of a given compound will have the same composition of elements no matter where it was obtained. The composition is often expressed as % mass.

Acetic acid (active ingredient in vinegar) C2H3O2

Can be made by fermenting fruit or by chemical reactions with Crude Oil. Both are used as food ingredients. Same compound 2C : 3H : 2O

By mass this is 49% C : 1% H : 50% O

• The law of definite composition: A compound has the same composition whether you find it here or on the other side of the world, whether you find it in nature or if you synthesize the compound.

Composition of iron sulfide

Law of Multiple Proportions CO2 CO

A law proposed by Dalton which states that when elements combine, they do so in the ratio of small whole numbers. For

example carbon and oxygen react to form CO or CO2, but not CO1.8

Law of multiple proportion: when the same elements combine to form different compounds they do so in whole number ratios.

Compounds with Hydrogen and Oxygen

Mass H (g) Mass O (g)

Compound A Compound B

.125 g 1.00 g .250 g 1.00 g

What do you notice about this???

Why should there be whole numbers involved?

Because of atoms!!!

H

O

H

Compound A

H

O

H

Compound B

O

H2O H2O2

2 H : 1 O 1 H : 1 O

There is EXACTLY twice as much Oxygen in compound B

Law of Conservation of Mass

Massreactants = massproducts

If chemical reactions are rearrangements of atoms, then mass would be conserved:

If atoms are rearranged, and not created or destroyed, then the mass of the reactants must

equal the mass of the products.

Reactants Products

• In natural processes, atoms are not destroyed; they are recycled.

Atoms are conserved.

online

http://glencoe.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::550::400::/sites/dl/free/0078807239/602889/Nitrogen_Cycle_fig_2_4.swf::Nitrogen%20cycle

Dalton’s Atomic Theory

Is Dalton right on everything?????

1.) All matter is made of tiny indivisible particles called atoms. 2.) Atoms of the same element are identical, atoms of different elements are different. 3.) Atoms of different elements combine in whole number ratios to form compounds. 4.) Chemical reactions occur when atoms are separated, joined or rearranged

separated joined rearranged

Chemical reactions occur when atoms are

+

+

• Bill Nye Atomic Theory

Subatomic Parts

Parts of Atoms • J. J. Thompson - English physicist.

1897 • Made a piece of equipment called

a cathode ray tube. • It is a vacuum tube - all the air has

been pumped out.

Thomson’s Experiment Voltage source

+ -

Vacuum tube

Metal Disks

Thomson’s Experiment Voltage source

+ -

Thomson’s Experiment

Voltage source

+ -

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


Voltage source

+ -

Voltage source


• By adding an electric field

Voltage source


By adding an electric field

+

-

Voltage source


By adding an electric field he found the beam was negative and that it was not just light. (light is not affected by a magnet or electric field)

+

-

Voltage source


By adding a paddlewheel

+

-

Voltage source


He added a paddlewheel

+

-

Voltage source


He added a paddlewheel and saw that the beam turned it.

+

-

Voltage source


Since the beam turned the paddlewheel, He concluded that:

+

-

the beam had mass and was likely composed of particles.

Experiments were repeated with many different gases and all found the same resulting beam of negative particles.

The conclusion was that all atoms have these negative particles in them.

It was later called the electron, but at the time Thomson called them "corpuscles”.

By using the cathode ray tube and an electric field. The deflection of the electron beam gave the charge to mass ratio of a electron.

Voltage source

+

-

1.7588028 1011 coulombs/kg = Charge of an electron?

mass of an electron?

If one of the missing numbers could be found then both would be known

Robert Millikan in 1909 set out to find the charge on 1 electron, in one of the most clever experiments to date.

Education

Millikan received a Bachelor's degree in the classics from Oberlin College in 1891 and his doctorate in physics from Columbia University in 1895 – he was the first to earn a Ph.D. from that department.

http://upload.wikimedia.org/wikipedia/en/a/a6/Robert-millikan2.jpg

http://en.wikipedia.org/wiki/Bachelor%27s_degree

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

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

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

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

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

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

– "At the close of my sophomore year [...] my Greek professor [...] asked me to teach the course in elementary physics in the preparatory department during the next year. To my reply that I did not know any physics at all, his answer was, 'Anyone who can do well in my Greek can teach physics.' 'All right,' said I, 'you will have to take the consequences, but I will try and see what I can do with it.'

http://upload.wikimedia.org/wikipedia/en/a/a6/Robert-millikan2.jpg

Millikan Oil Drop movie http://chemistry.umeche.maine.edu/~amar/fall2007/Millikan.html

http://chemistry.umeche.maine.edu/~amar/fall2007/Millikan.html

He looked at the charges on the oil drops and found that they were different by whole numbers.

The smallest amount the charge changed was the charge of 1 electron

e-

e- e-

e- e-

e-

e- e-

e-

.2 .4 .3

He found that an electron has a mass of: 9.11 x 10-28 g how “heavy” is that?

.0000000000000000000000000000911 g

If all atoms have these negative particles in them, why isn’t everything electrically charged?

Why don’t I get shocked when I touch everything??

Atoms have a neutral charge because of…

Protons

So atoms have negative particles called electrons

and some positive stuff

(we don’t know about protons yet

in 1909)

How does this fit together to make an atom?

The first accepted idea came from J.J. Thomson

It was referred to as the Plum Pudding Model

Positive charge smear

electrons

But then new evidence came to light courtesy of

Lord Ernest Rutherford

Rutherford’s experiment • Ernest Rutherford -English physicist.

(1910) • Believed in the plum pudding model of

the atom • Wanted to see how big they are. • Used radioactivity. • Alpha particles - positively charged

pieces- helium atoms minus electrons • Shot them at gold foil which can be made

a few atoms thick.

Rutherford’s experiment • When an alpha particle hits a

fluorescent screen, it glows. • Here’s what it looked like

Lead block

Uranium

Gold Foil

Fluorescent Screen

He Expected • The alpha particles to pass through

without changing direction very much.

• Because…? • …the positive charges were

thought to be spread out evenly. Alone they were not enough to stop the alpha particles.

What he expected

Because

He thought the mass was evenly distributed in the atom

Since he thought the mass was evenly distributed in the atom

What he got

How he explained it

+

• Atom is mostly empty. • Small dense,

positive piece at center.

• Alpha particles are deflected by it if they get close enough.

Density and the Atom • Since most of the particles went

through, it was mostly empty space. • Because the pieces turned so much,

the positive pieces were heavy. • Small volume, big mass, big density. • This small dense positive area is the

nucleus.

Rutherford Gold foil experiment movie

Bill Nye’s Greatest Discoveries

• Atomic Theory

• The electron

Size of nucleus, electron Start at 4:25

G:\Documents\Chemistry RJ Main\Honors\Honors 08 Unit 4\The_Structure_of_the_Atom_and_Atomic_Spectra.asf

World of Chemistry on-line video

• The Atom - first 16 minutes

Recreating Rutherford’s Experiment

Each box has a hidden shape,

Place a piece of paper on top

roll a marble into the hidden area of the box

Use a pencil to trace the marble’s path on the piece of paper

Use the path and anything you hear to help determine the shape

Repeat until you know what the hidden shape is

NO PEEKING OR FEELING to find the hidden shape

Turn in

3 pieces of paper stapled together with Names

Containing

marble paths

your guess for the actual shape

The shapes

Other particles • Proton - positively charged pieces

1840 times heavier than the electron – by E. Goldstein

• Neutron - no charge but the same mass as a proton – by J. Chadwick

• Where are the pieces?

Subatomic particles – p.111

Electron

Proton

Neutron

Name Symbol Charge

Relative mass (amu)

Actual mass (g)

e-

p+

n0

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

Atoms contain 3 particles

+

e-

protons neutrons electrons

Charge +1 0 -1

Mass 1 amu 1 amu .0005 amu

Found nucleus nucleus outer shell

If all atoms contain just protons, neutrons, and electrons how can atoms have different flavors???

Lead

Gold

Bromine

Neon

Sulfur

Counting the Pieces • Atomic Number = number of

protons in the nucleus • # of protons determines kind of

atom (which element it is) • the same as the number of

electrons in the neutral atom. • Mass Number = the number of

protons + neutrons. • These account for most of mass

If an atom has an atomic number of 5 that means it has 5 what?

If it also has an mass number of 11 that means it has 11 what?

5 protons

11 (protons + neutrons)

How many neutrons???

- 5 protons 11 protons + neutrons Mass #

- Atomic #

# neutrons 6 neutrons

Symbols • Contain the symbol of the element (X),

the mass number and the atomic number.

X Mass number

Atomic number

If the atom is neutral then there is nothing here

Symbols • Find the

– Atomic number – Mass Number – number of protons – number of neutrons – number of electrons

F 19 9

9 19 9

10 9

Symbols Find the

–Atomic number –Mass Number –number of protons –number of neutrons –number of electrons

Br 80 35

35

45 35

35

80

Symbols if a neutral atom has an atomic

number of 34 and a mass number of 78 what is the –number of protons –number of neutrons –number of electrons –Complete symbol

34 44 34

Se 78 34

Symbols if a neutral atom has 91 protons

and 140 neutrons what is the –Atomic number –Mass number –number of electrons –Complete symbol

91 231 91 Pa 231

91

Symbols if a neutral atom has 78 electrons

and 117 neutrons what is the –number of protons –Atomic number –Mass number –Complete symbol

78 78 195 Pt 195

78

Ions

• If an atom does not have the same number of protons and electrons it has a charge

• These charged atoms are called ions • The charge is usually indicated in the top

right corner of the symbol

S 16

32 2- Does this ion have more protons or electrons??

Ions • If an atom has more electrons than

protons it has a negative charge. • If an atom has more protons than

electrons it has a positive charge –10 p+ and 11 e-

–12 p+ and 14 e-

–45 p+ and 44 e-

–150 p+ and 147 e-

Net Charge 1-

2- 1+

3+

Na 11

23 +

Cl 17

36 -

Symbol Atomic # Mass # Charge # particles in nucleus p+ n0 e-

11

17

23

36

11

17

12

19

10

18

1+

1-

23

36

Isotopes • Dalton was wrong. • Atoms of the same element can

have different numbers of neutrons. • different mass numbers. • called isotopes. • All elements have different

isotopes. • Isotopes are not necessarily

radioactive. But some are

Naming Isotopes • Another way to name an isotope is to

name the element, then the mass #. • carbon- 12 • carbon -14 • uranium-235

Comparing Isotopes

atomic # mass # protons neutrons electrons

• Carbon-12 • Carbon-14

Both carbon 12 and 14 behave the same in a chemical reactions. You have many carbon atoms in you. Carbon-12 is not radioactive but Carbon-14 is radioactive. Are you radioactive??

6

6

12

14

6

6

6

8

6

6

Other isotopes

• Hydrogen-1 “normal hydrogen” • Hydrogen-2 deuterium • Hydrogen-3 tritium

atomic # mass # protons neutrons electrons

• Hydrogen-1 • Hydrogen-2 • Hydrogen-3

1

1

1

2

1

1

0

1

1

1

1 3 1 2 1

If we have ten marbles, and each marble weighs 1 g, what is the average mass of the marbles?

1 g

If we have two marbles, and one marble weighs 1 g and the other marble weighs 6g, what is the average mass of the marbles?

3.5 g

If we have five marbles, and one marble weighs 6 g and the other four marbles weigh 1g, what is the average mass of the marbles?

(6g x 1 marble) + (1g x 4 marbles) = 10 g 5 marbles 5 marbles

If we have five marbles, and one marble weighs 6g and the other four marbles weigh 1g, what is the average mass of the marbles?

(6g x 1 marble) + (1g x 4 marbles) = 2 g/marble 5 marbles

• What is it and why is it a decimal.

• Protons and neutrons both weigh 1 amu. (mass of electrons is almost nothing)

• Chlorine’s atomic mass = 35.453 amu

Does Chlorine have a half a proton or half a neutron???

Atomic mass

Most elements have more than 1 isotope in nature

Atomic Mass

Protactinium-216 217

222 223 224 225 226 227 228 229 230 231

234 235

232 233

236 237 238

•Different isotopes have different masses because they have different #’s of _______

+ + no

no

Hydrogen - 3 Hydrogen - 1

You need to gain some weight

•Some of these isotopes are very rare. (partially because some isotopes are very unstable)

Some isotopes will only last a fraction of a second

•The atomic mass is an average based on the % abundance of these isotopes in nature

If there are 10 boys and 20 girls in a room

What is the % abundance of boys?

10 / 30 *100% = 33%

Atomic Mass • Chlorine has two common isotopes

Cl 17

35 Cl 17

37

• Notice that the actual mass (amu) of the

isotope is very close to the mass number

Isotope mass =34.9689 amu Isotope mass =36.9659 amu

Atomic Mass

Cl 17

35 Cl 17

37

Isotope mass =34.9689 amu Isotope mass =36.9659 amu

•If you were to go out and collect a sample of chlorine atoms (say in a bucket of chlorine bleach). You would find that there are some of each isotope. More specifically it is always:

75.77 % Cl-35

24.23 % Cl-37

Cl 17

35

Cl 17

37

No matter where you collected the sample, you would find that: 75.77 % are Cl-35 & 24.23 % are Cl-37

What is the average mass?

Finding the atomic mass

(34.9689 amu*.7577 + 36.9659 amu*.2423)

26.50 amu + 8.957 amu

= 35.457 amu

% abundance as a decimal

Cl 17

35

Cl 17

37

Notice the atomic mass is “weighted” towards the more common isotopes

Atomic Mass • Is not a whole number because it is

an average. • The atomic mass is closer to the

mass of the most common isotope • BUT NO chlorine atoms have a

mass of 35.45 because it is an average.

If you have 1 marble that has a mass of 1 g

and another that has a mass of 2 g.

The average mass is 1.5 grams

How many marbles have a mass of 1.5 grams?

Cu 29 63

Copper has 2 Isotopes

The atomic mass of copper is 63.546.

Which isotope is more common?

How many copper atoms have a mass of 63.546 amu?

Cu 29 65

62.93 amu 64.93 amu

C 6 12

Carbon has 2 (primary) Isotopes

The atomic mass of Carbon is 12.011 amu

Which isotope is more common?

How common is the other?

C 6 13

12.000 amu 13.003 amu

Magnesium has 3 isotopes Mg-24 23.99 amu 78.9% abundant Mg-25 24.99 amu 10.0% abundant Mg-26 25.98 amu The rest

What is the atomic mass of Mg?

Mg-24 23.99 amu 78.9% Mg-25 24.99 amu 10.0% Mg-26 25.98 amu 11.1%

23.99 amu *.789 +

Homework / Class work

SHOW ALL WORK

Gallium has 3 isotopes

Isotope Isotope Mass Abundance

Gallium-69 68.926 60.1%

Gallium-71 70.924 The rest

What is its atomic mass?

Silicon has 3 isotopes

Isotope Isotope Mass Abundance

Silicon-28 27.98 amu 92.23%

Silicon-29 28.98 amu 4.67%

Silicon-30 29.97 amu remainder

What is its atomic mass?

It’s the number!! The atomic number is the number of protons. This is what determines the element. It is often listed on the bottom left of the symbol.

The elements are listed in order of increasing atomic number.

He 2 All atoms of helium have 2 protons

N How many protons does a nitrogen atom have.

Most atoms are electrically neutral.

• If the atomic number of magnesium is 12. How many electrons does it have.

Mg

What 2 particles make 99.98% of the mass in an atom?

• The mass number is the number of protons AND neutrons.

• It is often indicated on the top left of the symbol

Mg 12

24 Mass #- protons and neutrons

atomic #- protons

# neutrons

12

Two ways to find the average mass

Simple Average

Does this average reflect how many of each isotope would be found in nature?

(34.969 amu + 36.966 amu) / 2 =

35.9677378 amu

A recipe

1 Scoop Potassium Iodide (KI)

5 mL Soap

10 mL H2O2

H2O

H2O2

%H %O

11% 89%

6% 94%

Distinguishing Between Atoms

• OBJECTIVES: – Explain how the atomic number

identifies an element.


• OBJECTIVES: – Use the atomic number and mass

number of an element to find the numbers of protons, electrons, and neutrons.


• OBJECTIVES: – Explain how isotopes differ, and why

the atomic masses of elements are not whole numbers.