Atomic Structure
Unit 4
Expected test date: 28-Oct-08
The Atom
The smallest particle of an element that
still retains the properties of the element.
 Atoms are much too small to see
 What does it look like?

Subatomic Particles

Atoms are made of smaller particles
Protons
 Neutrons
 Electrons

The Structure of Atoms
The Philosophers
Ancient Greeks
 Formed ideas of nature based on their
experiences
 Did no experiments, just thought

The ancient idea

Everything made of :
Earth
Air
Fire
Water
Democritus
(460 – 370 B.C.)
Greek philosopher (not scientist)
 Named the atom
 Different kinds of atoms have different sizes
and shapes.
 Apparent changes in matter result from
changes in the groupings of atoms and not from
changes in the atoms themselves.

John Dalton
(1766-1844)
First atomic theory
 All matter is composed of atoms.
 All atoms of a given element are
identical and different from those of
any other element.
 Atoms combine in simple whole
number ratios to form compounds.
 In a chemical reaction, atoms are
separated, combined, or
rearranged.

Early Theories of Matter

Workbook: Page 19

Numbers 1 through 12
By the late 1800s…

Cathode ray tube invented
JJ Thomson
(1897)
Cathode rays made of
particles smaller than
atoms
 First subatomic particles
 Discovered the electron
 Plum pudding model of
the atom

Electron
Charge of negative one (-1)
 Almost no mass (1/1840 mass of proton)
 Exists in the empty space around the
nucleus

Questions raised by Thomson
If electrons are particles smaller than
atoms, are there other particles?
 What makes up most of the mass of the
atom?
 If electrons are negatively charged, why
are atoms neutral?

Plum pudding model
Atom is a sphere
 Positive charge is evenly distrubuted
 Negatively charged electrons spread
randomly through the sphere

Let’s draw that

Workbook: Page 20

Number 4
Ernest Rutherford
(1911)
Aimed a beam of “alpha particles” at
a thin gold foil
 The beam made a bright spot on a
fluorescent screen

Lead
block
Uranium
Florescent
Screen
Gold Foil
What Rutherford expected
What Rutherford expected
What Rutherford saw
What Rutherford saw
+
Rutherford concluded
There had to be a heavy central core to
the atom: the nucleus
 Nucleus contains protons

Nucleus
center of an atom
 extremely small, positively charged, dense
 contains protons, neutrons
 surrounded by empty space where
electrons move

Proton
Subatomic particle
 Exists in the nucleus
 Has a positive charge (+1)
 Has a mass of 1 atomic mass unit

James Chadwick
(1932)
Experimenting with radiation
sources
 Discovered a new particle with
no charge : the neutron

Neutron
subatomic particle
 found in an atom’s nucleus
 has a mass nearly equal to that of a proton
(1 atomic mass unit)
 Has a neutral charge (no charge)

Nuclear Atomic Model
Neutrons (n0)
 Protons (p+)
 Electrons (e-)

Workbook: Page 20, #5
Electrons, Protons and Neutrons
Workbook: Page 20
 Numbers 5

Structure of the atom
99.97% of
mass in
nucleus
 Most of
the volume
is empty
space
 Electrons
in cloud

How big is an atom?

Simulate the size of a hydrogen atom:
Nucleus : place a baseball on the 50 yard line
of Reliant Stadium
 Electron : put a grain of sand on the back row
of the highest section

Electrons, Protons and Neutrons
Workbook: Page 20
 Numbers 6, 7, 8

Sub-subatomic particles
Protons, and neutrons are made of even
smaller particles
 6 flavors of quarks


up, down, charm, strange, top, and bottom
Differences in atoms
Different atoms - different numbers of
protons and neutrons
 The number of protons determines what
the element is

Wait…
Proton mass = 1
 Neutron mass = 1
 So where did those decimal places come
from?

6
C
12.0107
Isotopes and atomic mass
Potassium has 19 protons
 How many neutrons? 20

19
K
39.0983
potassium atoms:
93.25% will have 20 neutrons,
6.7302% will have 22 neutrons,
0.0117% will have 21 neutrons
Isotopes

Atoms of the same element with the same
number of protons but different numbers of
neutrons
Video
Atomic Number
The number of protons in an atom
 Always a whole number
 Number of electrons = number of protons

Atomic number = protons = electrons
Mass Number
(whole number)

Total number of protons and neutrons in a
given isotope
6
C
12.0107
Carbon-12 has
6 protons
+ 6 neutrons
= 12 atomic
mass units
Carbon -12 is standard
Atomic Mass Unit (amu) is 1 / 12 the mass
of a carbon – 12 atom
 1 amu is nearly (not exactly) equal to the
mass of one proton or one neutron
 Because of this, an atom’s
mass is nearly equal to the
number of protons and
neutrons in its nucleus

Isotopes

Atoms of the same element with the same
number of protons but different numbers of
neutrons
Atomic Mass is an average
(decimal)
An average of all known isotope mass
numbers for an element
 The atomic mass of an element is closest
to the most common isotope found in
nature


So, since Carbon’s atomic
mass is closest to 12,
carbon-12 would be the
most common isotope
found in nature
6
C
12.0107
Atoms review

Most atoms are neutral

Protons = electrons
Number of protons determines the
element
 Atomic number = number of protons

Atoms on the periodic table
Let’s Practice!

Complete 1-5 in study guide on page 21.
1.
2.
3.
4.
5.
False
True
False (can be, but not always)
True
False
More Practice!

Questions 6-12 on page 21.
6. 82
protons and 82 electrons
7. 8 protons
8. 30
9. 85
10.104 protons and electrons
11.84 protons and electrons
12.102 protons and electrons
Proton, neutron & electron relationships

Mass number = protons + neutrons
An isotope has an atomic number of 6 and a
mass number of 13
 Number of protons, neutrons and electrons?
Protons = 6
7 neutrons
13 amu = 6 protons + _____
Electrons = 6

Relationship Practice

On pages 21-22, questions 13-17.
13.Protons
= 19, Electrons = 19, Neutrons = 20
14.Protons
= 14, Electrons = 14, Neutrons = 14
15.Protons
= 19, Electrons = 19, Neutrons = 21
16.Protons
= 51, Electrons = 51, Neutrons = 72
17.13
and 15 are both isotopes of Potassium
Atomic Symbols
• There are two ways to represent elements:
•
Symbol:
Mass Number
atomic #
(# of p+)
• Name:
• Ex...
#
#
X
element symbol
Mass Number =?
# of p+ and n0
_____________
name of element followed by mass number.
Aluminum - 27
Nitrogen - 14
Carbon - 14
Atomic number =13
Atomic number =7
Atomic number =6
What are the Atomic Numbers for the above names?
For Example
• There are two ways to represent nitrogen isotopes:
Symbol
14
N
Nitrogen - 14
N
Nitrogen - 15
7
15
Name
7
Isotope Calculations
Symbol
Name
#P+ and n0
#P+
31
15
P
element = _______________ Phosphorus
Mass Number= ___________
atomic # = ______________
# of p+ = _______________
# of n0 = _______________
# of e- = _______________
31
15
15
16
15
Manganese - 55
symbol = ______________
Mass Number = __________
atomic # = ____________
# of p+ = ______________
# of n0 = ______________
# of e- = ______________
Mn
55
25
25
30
25
Let’s Practice!

Questions 18-23 on page 22.
Some More Practice!

Page 23, questions 27-31
27.76
28.Nb
29.190.2
30.Amu
31.Osmium
has 76 protons and electrons;
Niobium has 41 protons and electrons
Test Tomorrow
Unit 4
 Requiz on elements available afterwards

Review
The Atom

The smallest particle of an element that
still retains the properties of the element.
Democritus
(460 – 370 B.C.)
Greek philosopher (not scientist)
 Named the atom

First atomic theory
 All atoms of a given element are
identical and different from those of
any other element.
 In a chemical reaction, atoms are
separated, combined, or
rearranged.

JJ Thomson
(1897)
Cathode rays made of
particles smaller than
atoms
 First subatomic particles
 Discovered the electron
 Plum pudding model of
the atom

Electron
Charge of negative one (-1)
 Almost no mass (1/1840 mass of proton)
 Exists in the empty space around the
nucleus

Ernest Rutherford
(1911)
There had to be a heavy central
core to the atom: the nucleus
 Nucleus contains protons

Proton
 Subatomic particle
 Exists in the nucleus
 Has a positive charge (+1)
 Has a mass of 1 atomic mass
unit
James Chadwick
(1932)

Discovered a new particle with
no charge : the neutron
Neutron
 subatomic particle
 found in an atom’s nucleus
 has a mass nearly equal to that of
a proton (1 atomic mass unit)
 Has a neutral charge (no charge)
Nuclear Atomic Model
Neutrons (n0)
 Protons (p+)
 Electrons (e-)

Isotopes

Atoms of the same element with the same
number of protons but different numbers of
neutrons
Mass Number
(whole number)

Total number of protons and neutrons in a
given isotope
Atomic Mass
(decimal)

An average of all known isotope mass
numbers for an element
For Example
• There are two ways to represent nitrogen isotopes:
Symbol
14
N
Nitrogen - 14
N
Nitrogen - 15
7
15
Name
7