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Atoms, Molecules and Ions
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Chapter 2
Section 2.1 – 2.6 for Test II
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Dalton’s Atomic Theory (1808)
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1. Elements are composed of extremely small particles
called atoms.
2. All atoms of a given element are identical, having the
same size, mass and chemical properties. The atoms of
one element are different from the atoms of all other
elements.
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3. Compounds are composed of atoms of more than one
element. In any compound, the ratio of the numbers of
atoms of any two of the elements present is either an
integer or a simple fraction.
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4. A chemical reaction involves only the separation,
combination, or rearrangement of atoms; it does not
result in their creation or destruction.
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Dalton’s Atomic Theory
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Law of Multiple Proportions
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16 X
+
8Y
8 X2Y
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Law of Conservation of Mass
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Cathode Ray Tube
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J.J. Thomson, measured mass/charge of e(1906 Nobel Prize in Physics)
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Cathode Ray Tube
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Millikan’s Experiment
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Measured mass of e-
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(1923 Nobel Prize in Physics)
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e- charge = -1.60 x 10-19 C
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Thomson’s charge/mass of e- = -1.76 x 108 C/g
e- mass = 9.10 x 10-28 g
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7
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Types of Radioactivity
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(uranium compound)
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Thomson’s Model
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Rutherford’s Experiment
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(1908 Nobel Prize in Chemistry)
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 particle velocity ~ 1.4 x 107 m/s
(~5% speed of light)
1. atoms positive charge is concentrated in the nucleus
2. proton (p) has opposite (+) charge of electron (-)
3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)
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Rutherford’s Model of
the Atom
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atomic radius ~ 100 pm = 1 x 10-10 m
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nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
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“If the atom is the Houston
Astrodome, then the nucleus is a
marble on the 50-yard line.” 11
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Chadwick’s Experiment (1932)
(1935 Noble Prize in Physics)
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H atoms - 1 p; He atoms - 2 p
mass He/mass H should = 2
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measured mass He/mass H = 4
 + 9Be
1n
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+ 12C + energy
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neutron (n) is neutral (charge = 0)
n mass ~ p mass = 1.67 x 10-24 g
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mass p ≈ mass n ≈ 1840 x mass e-
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Atomic number, Mass number and Isotopes
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Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
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= atomic number (Z) + number of neutrons
Isotopes are atoms of the same element (X) with different
numbers of neutrons in their nuclei
Mass Number
1
1H
235
92
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A
ZX
Atomic Number
2
1H
U
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Element Symbol
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(D)
238
92
3
1H
(T)
U
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The Isotopes of Hydrogen
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How many protons, neutrons, and electrons are
14
in 6 C ?
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6 protons, 8 (14 – 6) neutrons, 6 electrons
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How many protons, neutrons, and electrons are
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in 6 C ?
6 protons, 5 (11 – 6) neutrons, 6 electrons
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The Modern Periodic Table
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Noble Gas
Period
Halogen
Group
Alkali Metal
Alkali Earth Metal
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A molecule is an aggregate of two or more atoms in a
definite arrangement held together by chemical forces
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H2
H2O
NH3
CH4
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A diatomic molecule contains only two atoms
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H2, N2, O2, Br2, HCl, CO
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diatomic elements
A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4
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An ion is an atom, or group of atoms, that has a net
positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
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Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl–
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17 protons
18 electrons
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A monatomic ion contains only one atom
Na+, Cl-, Ca2+, O2-, Al3+, N3-
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A polyatomic ion contains more than one atom
OH-, CN-, NH4+, NO3-
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Common Ions Shown on the Periodic Table
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How many protons and electrons are in
27 3+
13 Al
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?
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13 protons, 10 (13 – 3) electrons
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How many protons and electrons are in
78
234 Se ?
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34 protons, 36 (34 + 2) electrons
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Formulas and Models
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A molecular formula shows the exact number of
atoms of each element in the smallest unit of a
substance
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An empirical formula shows the simplest
whole-number ratio of the atoms in a substance
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molecular
empirical
H2O
H2O
C6H12O6
CH2O
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O3
O
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N2H4
NH2
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Ionic compounds consist of a combination of cations
and an anions
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• The formula is usually the same as the empirical formula
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• The sum of the charges on the cation(s) and anion(s) in
each formula unit must equal zero
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The ionic compound NaCl
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The most reactive metals (green) and the most reactive
nonmetals (blue) combine to form ionic compounds.
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Formula of Ionic Compounds
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2 x +3 = +6
3 x -2 = -6
Al2O3
Al3+
1 x +2 = +2
Ca2+
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O2-
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2 x -1 = -2
CaBr2
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Br-
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1 x +2 = +2
Na+
1 x -2 = -2
Na2CO3
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CO3227
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CH-2 HW
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Questions and Problems
Pages 55 - 56
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2.8, 2.12, 2.12, 2.14, 2.16, 2.22, 2.24.
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