Atomic TheoryAtomic Theory

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Suppose someone said they had a Suppose someone said they had a present for you. When they gave it present for you. When they gave it to you it was in a bag that you to you it was in a bag that you couldn’t see through. What would couldn’t see through. What would you do to figure out what was inside you do to figure out what was inside the bag without opening it? the bag without opening it?

What is an atom?What is an atom?

An An atomatom is the smallest particle is the smallest particle into which something can be into which something can be divided.divided.

Building blocks of matterBuilding blocks of matter Over time, the theory of what an Over time, the theory of what an

atom looks like has kept atom looks like has kept changing as scientists learn changing as scientists learn more.more.

Dalton Theory – late Dalton Theory – late 1700’s1700’s

Dalton proposed the theory that:Dalton proposed the theory that: All substances are made of atoms which All substances are made of atoms which

cannot be created, destroyed or dividedcannot be created, destroyed or divided Atoms of the same element are exactly Atoms of the same element are exactly

alikealike Atoms of different elements are Atoms of different elements are

differentdifferent Atoms join with other atoms to make Atoms join with other atoms to make

new substancesnew substances

Thomson Theory – late Thomson Theory – late 1800’s1800’s

Electrons Discovered!!Electrons Discovered!! Based on his observations of Based on his observations of

electron beam behavior, Thomson electron beam behavior, Thomson proposed that the particles making proposed that the particles making up an electron beam have a negative up an electron beam have a negative charge.charge.

His model of the atom was that is His model of the atom was that is was a solid structure of positive was a solid structure of positive charge with enough electrons mixed charge with enough electrons mixed in to make it neutral (no charge).in to make it neutral (no charge).

Thomson’s ModelThomson’s Model

A solid mass with the electrons A solid mass with the electrons distributed throughout it. Often distributed throughout it. Often called the “plum pudding” model.called the “plum pudding” model.

A more modern comparison would be the “chocolate chip ice cream” model.

Based on the way that positive particles Based on the way that positive particles were deflected in his gold foil were deflected in his gold foil experiments, Rutherford concluded that experiments, Rutherford concluded that there was something extremely dense there was something extremely dense and positively charged in the center of a and positively charged in the center of a particle.particle.

His model of the atom was that of a His model of the atom was that of a positively charged nucleus at the middle positively charged nucleus at the middle surrounded by tiny electrons that surrounded by tiny electrons that constantly move.constantly move.

Rutherford Model – Early Rutherford Model – Early 1900’s1900’s

Nucleus Discovered!Nucleus Discovered!

Rutherford ModelRutherford Model

Dense nucleus in center surrounded by electrons that move constantly. Equal number of protons and electrons to keep it neutral.

Bohr’s ModelBohr’s Model

Bohr proposed that while electrons Bohr proposed that while electrons are traveling around the nucleus, are traveling around the nucleus, they do so on defined paths called they do so on defined paths called levels.levels.

Each level can only hold a certain Each level can only hold a certain number of electronsnumber of electrons

Electrons can jump from one level to Electrons can jump from one level to the next, but cannot be in betweenthe next, but cannot be in between

Bohr’s ModelBohr’s Model

Neutrons and protons located in the nucleus.

Electrons were in specific orbits around the nucleus…..not moving randomly.

Electron Cloud ModelElectron Cloud Model

Current theory is that the electrons Current theory is that the electrons are located in an “electron cloud” are located in an “electron cloud” surrounding the nucleus. While they surrounding the nucleus. While they still exist in levels, it is thought that still exist in levels, it is thought that the path they travel is not quite as the path they travel is not quite as defined as Bohr suggested.defined as Bohr suggested.

Electron Cloud ModelElectron Cloud Model(current theory for what (current theory for what

an atom looks like)an atom looks like)

Evolution of Atomic Evolution of Atomic TheoryTheory

Summary QuestionsSummary Questions

1.1. In what part of an atom is most of In what part of an atom is most of the mass located?the mass located?

2.2. What are 2 differences between the What are 2 differences between the atomic theories as described by atomic theories as described by Thomson and Rutherford?Thomson and Rutherford?

3.3. What discovery demonstrated that What discovery demonstrated that atoms are not the smallest atoms are not the smallest particles?particles?

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Rewrite the following numbers in Rewrite the following numbers in standard form:standard form:

3 x 103 x 10-8-8

2 x 102 x 102222

How Small is an AtomHow Small is an Atom

An atom is so small that their An atom is so small that their diameter is three hundred-diameter is three hundred-millionths of a centimeter!millionths of a centimeter!

0.00000003 cm0.00000003 cm

This means that a single penny This means that a single penny contains twenty thousand billion contains twenty thousand billion billion atomsbillion atoms

20,000,000,000,000,000,000,00020,000,000,000,000,000,000,000

So What is Inside an So What is Inside an Atom?Atom?

Nucleus made of protons Nucleus made of protons and neutronsand neutrons

Electrons Electrons Lots of empty spaceLots of empty space

ProtonsProtons

Charge: Positive (+)Charge: Positive (+) Mass: 1.7 x 10Mass: 1.7 x 10-24-24 grams grams

0.00000000000000000000000170.0000000000000000000000017

1 atomic mass unit (amu)1 atomic mass unit (amu) Location: NucleusLocation: Nucleus

NeutronNeutron

Charge: NoneCharge: None Mass: 1.7 x 10Mass: 1.7 x 10-24-24 grams grams

0.00000000000000000000000170.0000000000000000000000017

1 atomic mass unit (amu)1 atomic mass unit (amu) Location: NucleusLocation: Nucleus

ElectronElectron

Charge: Negative (-)Charge: Negative (-) Mass: almost zeroMass: almost zero Location: Electron clouds around Location: Electron clouds around

nucleusnucleus

How Do Atoms of Different How Do Atoms of Different Elements DifferElements Differ

Each element has atoms that have a Each element has atoms that have a unique combination of protons, unique combination of protons, neutrons and electrons.neutrons and electrons.

Law of Electric ChargeLaw of Electric Charge

Opposite charges are Opposite charges are attracted to each other attracted to each other while like (same) charges while like (same) charges repel each other (push repel each other (push each other awayeach other away))

Rules for Building an Rules for Building an AtomAtom

1. The atomic number of an atom is 1. The atomic number of an atom is equal to the number of protons in the equal to the number of protons in the atom.atom.

2. All stable atoms have the same 2. All stable atoms have the same number of protons and electrons…so number of protons and electrons…so atomic number ALSO equals the number atomic number ALSO equals the number of electrons.of electrons.

3. The atomic mass of an atom is equal 3. The atomic mass of an atom is equal to the number of protons PLUS number to the number of protons PLUS number of neutrons in the nucleus of an atom.of neutrons in the nucleus of an atom.

How to Read Element How to Read Element SquaresSquares

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What is the difference What is the difference between atomic number between atomic number and atomic mass?and atomic mass?

AnswerAnswer

Atomic number is the number of Atomic number is the number of protons in the nucleus of an atom. protons in the nucleus of an atom.

(Atomic # = protons)(Atomic # = protons)

Atomic mass is the sum of the Atomic mass is the sum of the number of protons AND neutrons in number of protons AND neutrons in the nucleus.the nucleus.

(Atomic mass = protons + neutrons)(Atomic mass = protons + neutrons)

Atomic mass is always Atomic mass is always the largest numberthe largest number