Free Powerpoint Templates Page 1 History of the Atom (Chapter 4) Why Study Atoms? First ideas of the Atom, The Greeks Robert Boyle, Joseph Proust, and

Free Powerpoint TemplatesPage 1

History of the Atom(Chapter 4)

•Why Study Atoms?•First ideas of the Atom, The Greeks•Robert Boyle, Joseph Proust, and

Antoine Lavoisier •Dalton’s Atomic Theory

•Thomson’s Model•Rutherford’s Atomic Theory

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There is a Problem

• Atoms are extremely small

• You can not see them.

• Because of this, most evidence of their existence is through indirect evidence.



But That Doesn’t Stop Science

• However, the idea that atoms exist is one of the most strongly supported of all scientific theories.

• Scientists and engineers spend millions of man-hours each year studying atomic processes

• Governments and Companies spend 100’s of billions of dollars each year studying Atomic processes



Why Study About Atoms?

• Everything made of matter is made of atoms.

• To understand atoms is necessary to understand most of science and the world around us.

• Almost all new technologies requires the understanding of atoms or their subatomic parts.



Think About

• The more you know about atoms the The more you know about atoms the more you know about yourself.more you know about yourself.

• The more you know about atoms the The more you know about atoms the more you know about the world around more you know about the world around you.you.



Who Thought of Atoms First?

• The Ancient GreeksDemocritus and Leucippus

Greek philosophers (~400BC) reasoned you couldn't grind something forever into smaller pieces.

• Would be no shape to objects• Believed there must be an

ultimate small particle things were made of

• Called the “Atomos” for indivisible


• Leucippus is named by most sources as the originator of the theory that the universe consists of two different elements, which he called ‘the full’ or ‘solid,’ and ‘the empty’ or ‘void’. Both the void and the solid atoms within it are thought to be infinite, and between them to constitute the elements of everything.

• To Democritus, atoms were completely solid, homogeneous, indestructible objects.



• Democritus – He said that there was only so many times that something could be broken down into smaller pieces than the previous one. It had to end at some point with the smallest thing you could get, the atom. He was the founder of the atomic hypothesis.

• He thought that everything was formed up of atoms like the objects itself, such as all trees were lots of small trees put together.



Thales (around 600 B.C.)There was someone earlier – kind of

• Thales of Miletus discovered that after rubbing a piece of amber with fur it would attract other light objects. He suggested that this force came from inside the amber but did not completely develop his idea and connect it with the atom

• Thought everything was made of one element, water.

.



Aristotle (384-322 B.C.) • Thought all substances were

built up of four elements– Earth (cool & heavy)– air (light)– water (wet)– fire (hot)

• Blend these in different proportions to get all substances


Who Was Right?

• Greek society was slave based

• It was beneath the Famous to work with their hands

• Typically didn't experiment

• Greeks settled disagreements by argument & logic

• Aristotle was more famous

• He won

• His ideas carried through middle ages.



• Aristotle's Ideas about the elements was Aristotle's Ideas about the elements was accepted through the dark ages. But the accepted through the dark ages. But the idea of the “atomos” was largely forgotten.idea of the “atomos” was largely forgotten.

• However, alchemists by the end of the dark ages realized there must be more than 4 elements

• i.e., silversilver, goldgold, ironiron, coppercopper, carbon, etc.

• It was time for a change.


Robert Boyle (1600’s)

• 11stst true “chemist” true “chemist”• First to . . .

– develop modern concepts of element and compound;

– to distinguish between acids, bases, and neutral substances;

– conduct and publish experiments along the lines of the scientific method.

– Discovered a relationship between pressure and volume (Boyle’s Law)

Antoine Lavoisier (1743-1794)

• He burnt phosphorus and sulfur in air, and proved that the products weighed more than he original. Nevertheless, the weight gained was lost from the air. Thus he established the

• ”Law of Conservation of Mass”.

• Matter cannot be

• created or destroyed

Joseph Proust (1754 - 1826)

• Found that a given compound always contains exactly the same proportion of elements by mass

“Law of Definite Proportion”i.e., if one sample of water breaks down into 2

grams of hydrogen and 16 grams of water, then twice the amount of water would contain 4 grams of hydrogen and 32 grams of water. The proportion of hydrogen to oxygen is "definite" or fixed.


Dalton’s Atomic Theory The Father of Modern Chemistry (born 1766)

Reintroduced the Idea of the atom along with 4 rules concerning atoms, compounds and chemical reactions.

1)All matter is made of atoms.

- Atoms are indivisible and indestructible.

- can not be created or destroyed


Dalton’s Atomic Theory (cont.)• 2) All atoms of a given element

are identical in mass and properties



Dalton’s Atomic Theory (cont.)

A compound made of 3 elements. Which ones?

- Hydrogen

- Oxygen

- Carbon

3) Compounds are formed by a combination of two or more different kinds of atoms.



Dalton’s Atomic Theory (cont.)

• 4) A chemical reaction is a 4) A chemical reaction is a rearrangementrearrangement of atoms. of atoms.

Weee got ya carbon

Hay, we have the oxygen


Thomson’s Model of the Atom (1897)

• Discovered the atom was made of smaller parts.

• How did he do it?

• Thomson used a CRT Thomson used a CRT (Cathode Ray Tube)(Cathode Ray Tube)


The CRT (Cathode Ray Tube)

• The green beam emitted by the cathodes atoms could be deflected by a charged plate

Why?• The beam must be made of charged particles.• Like charges ________, Unlike ________• Beam attracted to positive plate so what

charge is it? Negative

repel attract


Moving the Electron Beam

● Can use magnets or charged metal plates to move around the beam. ● Means the beam is charged.



Plum pudding model of atomPlum pudding model of atom: :

electrons embedded in positive charged gelelectrons embedded in positive charged gel


Plum Pudding Model


Is there a Problem with Thomson’s Model????

• If opposite charges attract and likes repel.If opposite charges attract and likes repel.

• Why don’t the electrons get repelled to the surface of the gel???

• They would end up like sprinkles• on top of a cupcake. Not inside it.

??

In 1911, Robert Millikan Robert Millikan devised one of the classic experiments in atomic physics and chemistry,

the “oil-drop experiment”“oil-drop experiment”..

He constructed a chamber with a graduated view lens.

The chamber had oppositely-charged plates. He used an atomizer to spray tiny oil drops between the plates.

By adjusting the charge on the plates until a drop was suspended, he determined the chargecharge and massmass of the electron.




The SolutionRutherford’s Atomic model

• In 1911 Ernest Rutherford determined the true nature of atom.

• New Zealand born English physicist.

• Believed in the plum pudding model of the atom.

• (worked for Thomson at one point)


Shot things at Atoms

• His experiment used alpha particlesalpha particles as atomic bullets.

• Wanted to see what happened.

• Did they

- Go through

- Bounce

- Break things?

Side view


Rutherford’s experiment• Wanted to see how big atoms were

• Used radioactivity• Alpha particles - positively charged pieces

given off by uranium • His experiment used alpha particlesalpha particles as

atomic bullets.

• Shot them at gold foil gold foil which can be made a few atoms thick



What did he do?

Rutherford beamed alpha particles through gold foil

• Detected them as flashes of light on a florescent screen

• The gold foil was only – -0.00004 centimeter thick.

(1/400,000 th)– a few hundred atoms a few hundred atoms

thick.thick.

Top View

Lead block

Uranium

Gold Foil

Florescent Screen


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



What he expected


Because

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

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


What he observed



Most alpha particles passed straight through the gold foil

A few were scattered

Some even bounced back

He observed the following:

How he explained it

+

• Atom is mostly empty

• Small dense,Small dense, positive piecepositive piece

at centerat center

• Alpha particles

are deflected by

it if they get close enough

+


Inferred

• Only a very small, heavy, positively charged center could cause these results.

• Reasoned there must be a very small hard center to the atom.

• Demonstrated the atom had a nucleus.



The Nucleus

• Tiny center of the atom, 1/10,000th the size of the atom.

• The nucleus positively charged.

• Contains almost all the mass of the atom.

• Electrons (negatively charge) circulate at a distance around the nucleus.



To Summarize

• By the early 1900’s scientists knew that the atom consisted of a small dense nucleus with little electrons in orbit around the nucleus

• But . . .They didn’t know how the electrons were arranged



Where are the Electrons?In the Electron Cloud

• The electrons are held in orbit by their attraction to the protons

• Due to the

• electromagnetic force

• But, this force also makes electrons repel each other

- -

00 -+



What does this mean?

•This force causes the “electron Cloud” to be

10,000 times larger than the nucleus

•Thus, the“electron cloud” is mostly empty space.


Modern View• The atom is

mostly empty space

• Two regions

• Nucleus• Protons (+)• Neutrons (o)

• Electron cloud• region where you might

find the electrons (-)


Now know Atoms Are Made of Smaller “Subatomic” Parts


Subatomic particles

Electron

Proton

Neutron

Name Symbol Charge

Relative mass

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

What Makes The Atoms of Different Elements Different

From Each Other?

• The number of electrons, protons and neutrons they contain.

• The atoms of each element contain a unique combination of each of these parts.


How do you ID the Element the Atom Represents?

• By the number of protons in the atom or atomic number.

• The atoms of each element are unique.


For Each Element There Is a Unique Atom

•


It is All Recorded on the Periodic Table


Atomic number

• The number of protons


Atomic Mass The number of protons The number of protons

++

Neutrons in an atomNeutrons in an atom

(the mass of the nucleus)


Figuring Parts

# Protons = Atomic Number

# Electrons = Protons, unless the atom is an ion (ionized).

# Neutrons = Atomic Mass - Atomic Number (rounded)

The #neutrons and #protons do not have to equal


Isotope• Atoms with the same number

of protons but a different number of neutrons

• They are radioactive atoms

(will decay)

Unstable - nucleus falls apart


Ion• Atoms with the same

number of protons and neutrons but a different number of electrons

• Are “Ionized”


Zero Charge (neutral) is Normal

• Atoms are normally neutral

– charges add up to zero (0).

• Thus,

# protons (+) = # electrons (-)

unless ionized


Atoms which have a charge, have been ionized.

• By adding or removing electrons.

• A superscripted number is used to show the charge on the ion.

i.e.) Ca+2 and F-1


The Anion – a Negative Ion

• If add electron(s) the atom becomes more negative.

• Called an Anion.

• i.e.) Carbon becoming an Anion

C + 2e- → C-2

(anion)


The Cation – a Positive Ion

• If remove electron(s) the atom becomes more positive.

• Called a Cation.

• i.e., Sodium becomes an Cation

Na+2 if 2 electrons are removed.


What About Protons?

• Protons are never removed or added to ionize an atom.

• They are stuck in the nucleus– can’t be moved – strong force holds them

• Electrons are easy to remove or add – located on the outside of the atom.

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Free Powerpoint Templates Page 1 History of the Atom (Chapter 4) Why Study Atoms? First ideas of the Atom, The Greeks Robert Boyle, Joseph Proust, and