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•Chapter 4

•Atomic Structure

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History of Atom

• Democritus (2500 YEARS AGO) Greek philosopher

– Atmos – “uncut” or “indivisible”

• Dalton (1800’s) British Scientist

– Proposed the first Atomic Theory

• All elements are composed of atoms

• All atoms of the same element are the same

• Compounds contain atoms of more than one element

• In a compound, atoms combine in the same way

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History of Atom

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History of Atom

• J.J. Thompson (early 1900’s)

– Cathode Ray Tube Experiment

– Discovered the first sub-atomic particle, the electron

• A particle smaller than the atom

• Has a negative charge

– Change: Proposed the “plum pudding” model

• Milikan did the oil-drop experiment

– Determined the charge of the electron

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Thomson’s Experiments Figure 5

Cathode Ray Tube Experiment

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History of Atom

• Rutherford (1900’s)

– Gold-foil experiment

• Alpha particles shot at a thin gold foil

• Particles were expected to go right through

• Some were deflected and even bounced back

• Conclusion – there was a nucleus in the middle where the + charge is found

• Model changed again

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The Gold Foil Experiment

Figure 5

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History of the Atom

• From Dalton to Rutherford

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A Scientific Method Figure 12

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Structure of Atom

• Atomic number: the number of protons found in the nucleus

– Unique to each element

– A whole number (count the # of protons)

– Will be found on the periodic chart

• Mass number: the number of protons AND neutrons in the nucleus.

– This will change because it can gain or lose neutrons

• # of neutrons = Mass # - atomic #

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a. 16 – 8 = 8

b. 32 – 16 = 16

c. 108 – 47 = 61

d. 80 – 35 = 45

e. 207 – 82 = 125

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Structure of Atom

• Isotopes: Will have the same atomic number ( # of P+)

• But different mass number

– This is because there will be more or less neutrons

– Isotopes are still neutral because the neutrons are neutral

• Atomic mass it the average mass of the commonly occurring isotopes.

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History of Atom

• Bohr’s Model (1900’s)

• He focused on the electrons

– Found that they had certain energy levels

– Proposed that the have fixed, specific orbits

• Change: Orbit model

• Evidence for this – gaining and releasing of energy from an atom

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History of Atom

• Schrodinger – mathematician (1900’s)

• Electron Cloud Model

– Found that they had certain energy levels

– Instead of fixed, specific orbits, there are orbitals

• Orbital is a proposed area where the electron is likely to be found

• Change: Electron cloud model

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History of Atom

• Modern Theories: Bohr Model to Electron Cloud Model

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Structure of Atom

• In the quantum mechanical modelquantum mechanical model, the probability of finding an electron within a certain volume of space surrounding the nucleus can be represented as a fuzzy cloud. The cloud is more dense where the probability of finding the electron is high.

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• The propeller blade has the same probability of being anywhere in the blurry region, but you cannot tell its location at any instant. The electron cloud of an atom can be compared to a spinning airplane propeller.

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Reading Strategy

a. Electron moves to higher energy level.

b. Electron moves to lower energy level.

Section 4.3

Sequencing

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Number of Orbitals and Electrons in Energy Levels

Figure 15

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Structure of Atom

• Different atomic orbitals are denoted by letters. The s orbitals are spherical, and p orbitals are dumbbell-shaped.

These are probable locations where an electron is most likely to be found.

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Structure of Atom

• Four of the five d orbitals have the same shape but different orientations in space.

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Reading Strategy

Summarizing

Section 4.1

a. Dalton b. Indivisible, solid spheres

c. Thomson d. Negative charges evenly scattered through a positively charged mass of matter

(plum pudding model)

e. Deflection of alpha particles passing through gold foil