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Chapter 4
Atomic Structure
Defining the Atom
 All elements are composed of particles
called atoms
 All atoms of the same element are
identical
 Atoms of different elements combine to
form compounds
 Atoms of one element can never be
changed into another element
History of the Atom
 Democritus (Greek 460-370 BC)
• 1st to use the word atom
• Believed that atoms were
indivisible & indestructible
• No scientific support
 Aristotle (Greek 384-322 BC)
• Rejected the theory of atoms
• Believed in 4 core elements
fire, air, water, earth
 Antoine Lavoisier (French 1743-1794)
• Law of Conservation of Matter –
Matter is not created or destroyed
during a chemical reaction
 Joseph Proust (French 1754-1826)
• Law of Definite Proportions A compound is always composed of
the same elements, in the same
proportion by mass
 John Dalton (English 1766-1844)
• Studied the ratio in which elements
combine
• Quantitative data – mass
• Law of Multiple Proportions When 2 elements form more than one
compound, they do so in a ratio of whole
numbers
Dalton’s Atomic Theory
1. All elements are composed of
indivisible ‘solid sphere’ atoms.
2. All atoms of a given element are
identical.
3. Atoms of different elements differ in
their masses.
4. Different atoms combine in simple
whole number ratios to form
compounds.
Structure of the
Atom
Subatomic Particles
 Today… Dalton’s Atomic Theory has one
important change
 Atoms are divisible…
 3 types of subatomic particles
• Protons
• Neutrons
• Electrons
Electrons
 J.J. Thomson (English 1897) –
• Identified the 1st subatomic particle
• Cathode-ray tube
Thomson’s Electron Model
 Named electrons
 Symbol: e Charge: (-1)
 Mass: ~ 0 amu
 Plum Pudding/
Chocolate Chip Cookie
What about the protons &
neutrons?
 Protons
• Symbol: p+
• Charge: (+1)
• Mass: 1 amu
 Neutons
• Symbol: n0
• Charge: (0) neutral
• Mass: 1 amu
The Nucleus
 Ernest Rutherford (English 1911) • Discovered the ‘structure’ of the atom
• Gold foil experiment
Rutherford’s Gold Foil Experiment
Rutherford’s Nuclear
Model





Atom is mostly empty space
Atoms have a dense center – nucleus
Most of the mass is in the nucleus
Nucleus has a positive charge
Protons & neutrons are located in the
nucleus
 Electrons are outside of the nucleus &
occupy most of the atom’s volume
Distinguishing
Among Atoms
What makes atoms different
from one another?
 # of protons
 Atomic number = # of protons
 Usually written as a subscript
Mg12 or 12Mg
What element has 11 protons? Sodium
How many protons does potassium have?
19
How do you determine the
# of neutrons?
 Atomic mass = # protons + # neutrons
 Usually written as a superscript
12C or C12 or C -12
 # of neutrons is the difference between
the atomic mass & the atomic number
# of neutrons = atomic mass - atomic #
What is the atomic mass of helium?
4.0026… round to 4
What element has an atomic mass of 27?
Aluminum
How many protons does this element
contain?
13
How many neutrons does this element
contain?
27 – 13 = 14
What about electrons?
 # electrons = # of protons
 Why?
• Atoms are electrically neutral
How many electrons does sulfur contain?
16
Which element contains 10 electrons?
Neon
Sample Problems
 How many protons, neutrons and
electrons are in each atom?
Beryllium (Be) -
4, 5, 4
Fluorine (F) -
9, 10, 9
Hydrogen (H) -
1, 0, 1
 How many neutrons are in each atom?
16
O
8
16 – 8 = 8
108
Ag
47
108 – 47 = 61
207
Pb
82
207 – 82 = 125
 Express the composition of each atom in
shorthand form.
Nitrogen -14
p+ = 7, n0 = 7, e- = 7
Sodium - 23
p+ = 11, n0 = 12, e- = 11
Phosphorus -31
p+ = 15 , n0 = 16, e- = 15
Isotopes
 Atoms that contain the same number of
protons but have different numbers of
neutrons
 Different atomic masses
 Does not change the atom’s identity
 Are naturally occurring
 Symbols:
23 or
Na
11
24
Na
11
12 or C13 or
C
6
6
14
C
6
 Determine the number of protons,
neutrons and electrons in the following:
1
H
1
p+ = 1, n0 = 0, e- = 1
1
H2
p+ = 1, n0 = 1, e- = 1
1
H3
p+ = 1, n0 = 2, e- = 1
Determining the Atomic Mass
 Atomic mass # is an average of atom’s
naturally occurring isotopes
Copper has 2 isotopes Cu-63 and Cu-65
Which isotope is most abundant?
63
There are 3 isotopes of Silicon with mass
numbers of 28, 29, and 30.
Si -28
Which is more abundant?
Calculating the Atomic Mass
 Multiply the mass of each isotope by its
natural abundance (expressed as a
decimal), then add the products
 Practice…
Ions
 An atom with an electrical charge
 Occurs 2 ways:
• Oxidation – loss of an eresults in a (+) charge
• Reduction – gain of an eresults in a (-) charge
Mnemonic: ‘Leo says Ger’
Loss of electrons = oxidation
Gain of electrons = reduction
Examples of Ions




H+
ClAs3Mg2+
Lost 1eGained 1eGained 3eLost 2e-
 Oxidation State –
• Number found on the periodic table
• Shows the number of e- gained or lost
• Many elements have more than one number