# Atoms, molecules and ionsPage 1

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Chapter 2

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## Dalton’s Atomic Theory (1808)

Elements are composed of extremely small particles called atoms. All atoms of a given element are identical. The atoms of one element are different from the atoms of all other elements.

Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same.

Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions.

2.1

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2

Law of Multiple Proportions

2.1

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8 X2Y

Law of Conservation of Mass

2.1

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J.J. Thomson, measured mass/charge of e-

(1906 Nobel Prize in Physics)

2.2

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e- charge = -1.60 x 10-19 C

Thomson’s charge/mass of e- = -1.76 x 108 C/g

e- mass = 9.10 x 10-28 g

Measured mass of e-

(1923 Nobel Prize in Physics)

2.2

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(Uranium compound)

2.2

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2.2

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atoms positive charge is concentrated in the nucleus

proton (p) has opposite (+) charge of electron

mass of p is 1840 x mass of e- (1.67 x 10-24 g)

particle velocity ~ 1.4 x 107 m/s

(~5% speed of light)

(1908 Nobel Prize in Chemistry)

2.2

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atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Rutherford’s Model of

the Atom

If the atom is the Houston Astrodome

Then the nucleus is a marble on the 50 yard line

2.2

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H atoms - 1 p; He atoms - 2 p

mass He/mass H should = 2

measured mass He/mass H = 4

neutron (n) is neutral (charge = 0)

n mass ~ p mass = 1.67 x 10-24 g

2.2

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Subatomic Particles (Table 2.1)

mass p = mass n = 1840 x mass e-

2.2

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