Unit 2:
Atoms, Molecules, and Ions
 Atomic Structure
 Molecules and Molecular Compounds
 Ions and Ionic Compounds
 Names and Formulas of Compounds
Ionic
Atomic Structure
 Democritus (460-370 BC)
 Theorized that all matter is composed of
small indivisible particles called atomos (“not
to cut”)
 Ideas rejected by Greek thinkers of that
time in favor of fire, earth, air, water, &
heavenly ether as the basic elements
 The idea that atoms are the fundamental
building blocks of matter re-emerged in the
1800’s
Atomic Structure
Dalton’s Atomic Theory (1803 – 1807)
1. Each element is composed of
extremely small particles called
atoms.
2. All atoms of a given element are
chemically identical to each other;
Atoms of one element are
different from the atoms of all
other elements
Atomic Structure
3. During ordinary chemical reactions, atoms of
one element cannot be changed into atoms of
a different element.
 Atoms are not created or destroyed.
Atomic Structure
4. Compounds are formed when atoms of more
than one element combine.
 A given compound contains atoms combined
in definite whole number ratios.
 Fe2O3 (rust)
 H 2O
(water)
 H2O2
(hydrogen peroxide)
 These ratios are the same for all molecules
of a particular pure substance.
Atomic Structure
 Law of Conservation of Mass (or Matter)
 Matter is neither created nor destroyed.
 The mass of materials present after a
chemical reaction is the same as the total
mass before the reaction.
11.1 g H2
88.9 g O2
100.0 g H2O
Atomic Structure
 Law of Constant Composition
 A given compound always contains the same
relative numbers and kinds of atoms.
Methyl alcohol
Ethyl alcohol
Isopropyl alcohol
CH4O
C2H6O
C3H8O
Methane always contains 25% H and 75% C
Atomic Structure
 Dalton’s atomic theory was based on evidence
gained in the macroscopic world (i.e. the lab).
 Today, scientists can actually see individual
atoms using special techniques.
 See figure 2.2 of your text for a scanning
tunneling micrograph of gallium arsenide
 http://www.almaden.ibm.com/vis/stm/gallery.html
Atomic Structure
 Atoms are composed of subatomic particles
 Protons
 positively charged subatomic particles
found in the nucleus
 Neutrons
 neutral (uncharged) subatomic particles
found in the nucleus
 Electrons
 very small, negatively charged subatomic
particles found in a diffuse layer
surrounding the nucleus
Atomic Structure
Particle
Symbol
Relative
Charge
Mass
Proton
p
+ 1
1.0073 amu
Neutron
n
0
1.0087 amu
Electron
e-
-1 5.468 x 10-4 amu
amu (atomic mass unit):
 a unit used to express very small masses
 1.66054 x 10-24 g
Atomic Structure
Thompson’s “Plum
Pudding” Model
Electrons are embedded
in a positive sphere of
matter just like raisins
in plum pudding or
blueberries in a
blueberry muffin.
Atomic Structure
 Rutherford (1909) disproved Thompson’s model
with his famous scattering experiments.
Atomic Structure
 Some alpha particles
pass through the empty
space in the atom.
 Other alpha particles
are deflected by the
nucleus.
 scattered
Atomic Structure
 Rutherford’s conclusions
 Most of the mass and all of the positive
charge of the atom are contained in a small
space called the nucleus.
 Most of the volume of the atom is empty
space occupied by tiny negatively charged
electrons.
 There are as many negatively charged
electrons outside the nucleus as units of
positive charge inside the nucleus
 the atom is electrically neutral
Atomic Structure
 Atoms can be visualized as a spherical particle
with
 tiny, positively charged, center (atomic
nucleus) that contains most of the mass
 diffuse outer layer that is negatively
charged.
Atomic Structure
 Atomic nucleus
 tiny, positively charged, center of an
atom
 contains protons and neutrons
 contains most of the
mass of an atom
Atomic Structure
 If all atoms are composed of protons,
neutrons, and electrons, then why do different
types of atoms have different properties?
 The atoms of each element have a unique
number of protons in the nucleus.
 Atomic number:
the number of protons in the
nucleus
 Carbon: 6 p (atomic # = 6)
 Sulfur: 16 p (atomic # = 16)
Atomic Structure
 Each atom contains the same number of
protons and electrons.
 atoms are electrically neutral
 Helium:
 Magnesium:
 Iron:
2 p, 2 e12 p, 12 e26 p, 26 e-
 The atomic number tells you both the number
of protons and electrons in an atom.
Atomic Structure
 The number of neutrons found in atoms of
the same element can vary.
 Hydrogen atoms:
 always 1 p and 1 e may have 0, 1, or 2 n
 Oxygen atoms:
 always 8 p and 8 e may have 8, 9, or 10 n
Mass Number =
1, 2, or 3
Mass Number =
16, 17, or 18
 Mass number = # p + # n
 NOTE: This is a calculated value!
found on the periodic table!
Not
Atomic Structure
 Isotopes
 Atoms that have the same atomic number
but different mass numbers
 Atoms of the same element that have
different numbers of neutrons
 Examples
 Carbon-12
(6 p, 6 n)
 Carbon-14
(6 p, 8 n)
most common
isotope
used for carbon
dating
Atomic Structure
 Atoms are small pieces of matter and
therefore have mass.
 Atomic weight (atomic mass)
 The average atomic mass of an element
expressed in amu
 Calculated using a weighted average of each
of the naturally occurring isotopes of an
element
Ions
 Atoms can gain or lose electrons to become
charged particles called ions.
 A chemical particle that has a positive or
negative charge
 Cations are positively charged ions.
 Formed when an atom loses electrons
 Anions are negatively charged ions.
 Formed when an atom gains electrons
Ions
Formation of a cation
1p
e-
Hydrogen atom
1p, 0 n, 1 e1
H
Net charge = 0
+
1p
e-
Hydrogen ion (cation)
1p, 0 n, 0 e1
H+
Net charge = +1
Ions
Formation of an anion
8p
8n
8e-
+
Oxygen atom
8p, 8 n, 8e16
O
Net charge = 0
2e-
8p
8n
10e-
Oxygen ion (anion)
8p, 8n, 10e16
O2-
Net charge = -2
Nuclear Symbols
Mass
Number
Atomic
Number
Charge
X
Charge = # p - # e-
Nuclear Symbols
 Using nuclear symbols to determine the number
of p, n, e, and total charge
15
8
O
Mass Number =
Atomic Number =
# protons =
# neutrons =
# electrons =
Charge =
Nuclear Symbols
137
2+
Ba
56
Mass Number =
Atomic Number =
# protons =
# neutrons =
# electrons =
Charge =
Nuclear Symbols
Example: Write the nuclear symbol for an atom
that contains:
1) 50 p, 70 n
2) 17 e-, 20 n
Nuclear Symbols
Example: Write the nuclear symbol for an ion
containing:
1) 53 p, 74 n, 54 e-
2) 23 e-, 30 n, net charge = +3