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History and Structure of the Atom

From Democritus to...

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History of Atomic Theory

Democritus (from about 440 BC)coined the term atom which means uncuttableHe felt that if you kept cutting matter smaller and

smaller eventually you will no longer be able to cut any further.

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John Dalton (1766-1844)3

Felt that an atom was indivisiblespherical in shapeModel: SphereAnalogy: Billiard ball

Dalton’s Postulates

Atoms are tiny, indivisible particles.Atoms of the same element are identical.Atoms of different elements can chemically

combine.Chemical reactions occur when atoms are

separated, joined or rearranged. Atoms of one element can never be changed to

atoms of another element.

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Size of an atom

Pure copper coin the size of a penny = 2.4X1022 atoms

Earth’s population is about 6 X 109 people

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JJ Thomson (1856-1940)

Discovered electronsFelt that an atom was negatively charged

particles floating in a positive soupModel: Charges floating aroundAnalogy: Raisin bun or Plum Pudding

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77

Ernest Rutherford (1871-1937)

Discovered the nucleus, the proton and first split an atom

Model: An atom was a small positively charged nucleus surrounded by electrons orbiting around it.

Analogy: A beehive

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Rutherfords Famous Experiment

Called the Gold Foil Experiment Alpha particles were fired at thin gold foil A detector encircled the foil and lit up when hit with

alpha particles. If the plum pudding model were true it was expected

most particles would go straight through the foil with only slight deflection.

However, in the experiment, some particles were deflected back at a sharp angle proving the existence of a small, dense, and positively charged nucleus.

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Text

Plum pudding expected results. Particles go through.

Actual results. Some are deflected back due to a nucleus

Rutherfords results

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Another view of the experiment10

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Niels Bohr

Worked out details of atomic structure. Notably orbital layers. Solved problems related to Rutherfords model.

Model: Electrons orbit in rings at different distances from the nucleus.

Analogy: Planets orbiting the sun

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Erwin Schroedinger (1887-1961)

Major discoveries in quantum mechanics. Nature of electrons in atoms.

Model: Electrons exist in a probability distribution around the atom. Kind of like a cloud.

Analogy: A spinning fan blade.

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Quick Quiz: Historical Atom

1. What did Dalton say an atom was like?

2. What did JJ Thomson say an atom was like?

3. What did Rutherford say an atom was like?

4. What are the 3 parts of an atom?

1. A billiard ball, or a pool table ball

2. Raisin bun

3. A beehive

4. Proton, Neutron, Electron

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Parts of the Atom14

The Nucleus

Electrons

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Parts of the Atom15

The Nucleus

ElectronsMass=0Charge=-1

ProtonsMass=1Charge=+1

NeutronsMass=1Charge=0

Atomic Structure

An atom is considered electrically neutral.

Electrically neutral means the number of protons (+) = the number of electrons (-)

4 red protons4 red protons = = 4 blue electrons4 blue electrons

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Atoms on the Periodic Table

3Be7.0

Atomic # = Number of protons

neutrons+ protons Atomic mass

You must know how to find:A. # of protons = atomic number

B. mass # = # of n0 + # of p+ (atomic #) What’s in the nucleus of the atom

C. # of electrons = # of protons (in a neutral atom)

Boron

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B10.811

atomic number

atomic mass(Not the same as the mass #)

Nuclear Symbol

16. What is the mass number?

The mass of the entire atom!The sum of the protons and neutrons in an atom.Electrons are not included (too small)

17. What is the atomic mass?

The weighted average of the masses of all naturally occurring isotopes.

How to write a Nuclear Symbol

B11

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Mass Number

= p+ + n0

Atomic Number

= p+

-3

Charge if ionElement Symbol

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To Determine Other Numbers 22

To find Do this

Proton # same as atomic number

Electron #same as atomic number for

neutral atoms

Neutron # =atomic mass-atomic number

Mass =Prot # + Neut#

Mg12

27 +2

Nuclear Symbol Examples

Atomic Number Mass NumberNumber of

ProtonsNumber of Neutrons

Number of Electrons

12 1227 15 10

Cl17

35

Atomic Number Mass NumberNumber of

ProtonsNumber of Neutrons

Number of Electrons

17 1735 18 17

+2

Nuclear Symbol Examples

Ca20

40

Atomic Number Mass NumberNumber of

ProtonsNumber of Neutrons

Number of Electrons

20 2040 20 18

O8

17 -2

Atomic Number Mass NumberNumber of

ProtonsNumber of Neutrons

Number of Electrons

8 817 9 10

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Try This25

Atomic # Element Symbol Atomic Mass

Be

Na

Fe

I

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Element

Atomic # e- # Mass # Neut #

12

P 15

16 8

26

Fill in the blanks

Foldable27

Ions - an atom that has lost or gained an electron(s)

The charge on an ion indicates an imbalance between protons and electrons.

If the atom GAINS electrons it will have a negativenegative charge. (more e- than p+)

If the atom LOSES electrons it will have a positivepositive charge. (more p+ than e-)

BProtons = Electrons =

B+2

Protons =

Electrons =

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Isotopes are atoms of the same element that have the same # p+, but different mass numbers.

Isotopes have different masses because they have a different number of neutrons.

Boron

5B10.811

atomic number

(average) atomic mass

Isotopes

19. How are isotopes written?

The element name with its mass number after it:Carbon-12Carbon-13Carbon-14

Learning Check

Naturally occurring carbon consists of three isotopes, C-12, C-13, and C-14. State the number of protons, neutrons, and electrons in each of these carbon atoms.

12C 13C 14C 6 6 6

# p _______ _______ _______

#n _______ _______ _______

#e _______ _______ _______ #

The average atomic mass - weighted average of all the naturally occurring isotopes of an element.

Example: A sample of cesium is 75% 133Cs, 20% 132Cs and 5% 134Cs. What is the average atomic mass?

Answer:.75 x 133 = 99.75.20 x 132 = 26.4.05 x 134 = 6.7

132.85 = average atomic mass