Chapter 4
Atomic Structure
•Early Models of the Atom
•
•
•
•
Democritus’s Atomic Philosophy
Greek philosopher (460 B.C.-370 B.C.)
1st to suggest the existence of atoms
Said that atoms were indivisible and indestructible
•Early Models of the Atom
•
Dalton’s Atomic Theory
•
English chemist and school teacher (1766-1844).
1.
2.
3.
4.
1.All elements are composed of indivisible
‘solid sphere’ atoms.
All atoms of a given element are identical.
Atoms are different elements differ in their
masses.
Different atoms combine in simple whole
number ratios to form compounds.
•Defining the Atom
• All
• All
elements are composed of atoms
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
•Structure of the Nuclear Atom
•
•
1.
2.
3.
Atoms are divisible
Three kinds of subatomic particles
are:
Electrons
Protons
Neutrons
1. Electrons
• Orbit
the nucleus of the atom
• Have a negative (-1) charge
• Mass ~0 atomic mass unit (amu)
• Shorthand symbol (e-)
• J.J. Thomson discovered the
electron using the cathode-ray
tube
•CATHODE RAY
A cathode ray
produced in a
discharge
tube travels
from left to
right. The ray
itself is invisible,
but the
fluorescence
of a zinc
sulfide coating
on the glass
causes it to
appear green
•CATHODE RAY
When the
polarity
of the
magnet is
reversed,
the ray
bends in
the
opposite
direction
•CATHODE RAY
The
cathode
ray is bent
downwar
d when
the south
pole of
the bar
magnet is
brought
toward it
•JJ Thomson’s Atom
THE PLUM PUDDING MODEL
•2. Protons
• Charge
of +1 (positive)
• Mass of 1 amu (atomic mass unit)
• Located inside the nucleus
• Shorthand symbol (p+)
•3. Neutrons
• Consists
of charge of 0 (neutral)
• Mass of 1 amu
• Located inside the nucleus
• Shorthand symbol (n0)
•Discovering the Atomic Nucleus
• When
the particles were discovered,
scientists wondered how these particles
were put together.
• Ernest Rutherford made it possible to
understand how the subatomic
particles were assembled
•Rutherford’s Gold-Foil Experiment
• Proposed
that the atom is made up of
empty space
• The mass and positive charge of the
atom is located in the nucleus (a dense
region)
•The Nuclear Atom
1.
•
•
•
•
Nucleus
Contains positive electrical charge
Makes up the most of the mass of the atom
There are two subatomic particles: protons
and neutrons
The electrons are distributed around the
nucleus and occupy almost all the volume
of the atom
•Atomic Number
• Elements
are different because they
contain different number of protons
• Atomic number is the number of
protons in the nucleus of an atom of
that element
• Atomic Number = protons or electrons
• Is usually written as a subscript
• Ex: 12Mg or Mg12
• What
element has 11 protons?
Sodium
• How many protons does potassium
have?
19
•Complete the Table
Element
K
Atomic
Number
Protons
19
Electrons
19
5
S
V
16
23
•Mass Number
Mass Number = Protons + Neutrons
•
•
•
•
Mass number is also known as atomic mass
The number of neutrons in an atom is the difference
between the mass number and the atomic number
# of neutrons = mass number – atomic number
Usually located at the top of the element symbol
Ex: 12C
C12
C-12
•Sample Problems
• How
many protons, electrons, and
neutrons are in each atom?
Beryllium (Be) -
4, 4, 5
Neon (Ne) -
10, 10, 10
Sodium(Na) -
11, 11, 12
• How
many neutrons are in each
atom?
16 – 8 =
16
O
8
8
108 – 47 =
61
207 – 82 =
125
108
Ag
47
207
Pb
82
• 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
+ = 15 , n0 = 16, e- = 15
p
Phosphorus -31
Atomic Mass
#
#
# of
proton
9
# of
# of
Neutron e10
14
47
55
15
22
25
Symbol
•Isotopes
•
•Sample Exercise
• Determine
the number of protons,
neutrons and electrons in the
following:
1
H
1
p+ = 1, n0 = 0, e- = 1
2
H
1
p+ = 1, n0 = 1, e- = 1
3
H
1
p+ = 1, n0 = 2, e- = 1
•Natural Abundance of Stable Isotopes
of Some Elements
Name
Symbol
Natural
Percent
Abundance
Mass
(amu)
Average
Atomic
Mass
Nitrogen
7N
14
15
14.003
15.000
14.007
7N
99.63
0.37
Chlorine
35
17Cl
37
17Cl
75.77
24.23
34.969
36.966
35.453
Helium
3
2He
4
2He
0.0001
99.9999
3.0160
4.0026
4.0026
Sulfur
32
16S
33
16S
34
16S
36
16S
95.002
0.76
4.22
0.014
31.972
32.971
33.967
35.967
32.06
•Uncovering 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. Which is more
abundant?
Si -28
•Practice Exercise
• Boron
has two isotopes: boron – 10 and
boron – 11. Which is more abundant,
given that the atomic mass of boron is
10.81?
• Answer: B-11
•Calculating Atomic Mass
• Multiply
the mass of each isotope by its
natural abundance, expressed as a
decimal, and then add the products.
•Sample Exercise
• An
element consists of two isotopes.
Isotope A has an abundance of 75%,
and its mass is 14.000 a.m.u. Isotope B
has an abundance of 25% and its mass
is 15.000 a.m.u. What is the atomic
mass of the element?
• Ans: 14.25
•Sample Exercise
• Boron
in nature has two isotopes. Boron
-10 has a mass of 10.0130 a.m.u. and
abundance is 19.9%. B – 11 has a mass
of 11.0093 a.m.u., and abundance is
80.1%. Calculate the atomic mass of
boron.
• Ans: 10.8109 a.m.u
•Sample Exercise
• There
are four isotopes of Lead: Pb –
204, Pb – 206, Pb – 207, Pb – 208. Their
weights are 1.37%, 26.26%, 20.82%, and
51.55% respectively. Using this data
calculate the atomic mass of lead.
207.2118
• Calculate
the atomic mass of bromine.
The two isotopes of bromine have atomic
masses and relative abundance of 78.92
amu (50.69%) and 80.92 amu (49.31%).
•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+
• Cl• As3• Mg2+
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
Ions & Subatomic Particles
•Review and Assessment
• Would
you expect two electrons to
attract or repel each other?
• What charge does the atom become
when it loses/gains an electron?
• What were the results of Rutherford’s
experiment?
• What is the charge – positive or
negative – of the nucleus of every
atom?
•Review and Assessment
• Which
subatomic particles are located
in the nucleus?
• Name two ways that isotopes of an
element can differ?
• What is the atomic mass of an
element?
• How are the elements arranged in the
modern periodic table?