Chapter 4
The Atom – Part 2
Lord Ernest Rutherford (1871 – 1937)
• Discovered the
nucleus of the atom.
• Pioneered the orbital
theory of the atom.
Rutherford’s Gold Foil Experiment
Rutherford’s Gold Foil Experiment
Rutherford’s Nuclear Model of the Atom
• The nucleus is very
small, dense, and
positively charged.
• Electrons surround
the nucleus.
• Most of the atom is
empty space
Subatomic Particles
PARTICLE
SYMBOL CHARGE
MASS
(amu)
LOCATION
electron
e-
-1
0
orbit nucleus
proton
p+
+1
1
inside nucleus
neutron
n0
0
1
inside nucleus
Alchemy “The Golden Rule”
The number of protons in the nucleus of the atom
(atomic number = Z) determines the atoms identity
Gold Atom
Lead Atom
Mass Number
• The sum of the
protons and neutrons
in the nucleus of an
atom.
What element is represented below?
Carbon - 12
• This is the symbol for
the isotope carbon-12.
• Atomic number is 6.
• Mass number is 12.
Isotopes
• Isotopes are different types of atoms of a
chemical element, having the same number of
protons in the nucleus, but having different
numbers of neutrons in the nucleus.
• Isotopes therefore have the same atomic
number but different mass numbers.
• There are 275 isotopes of the 81 stable
elements, in addition to over 800 radioactive
isotopes, and every element has known
isotopic forms.
• Isotopes of a single element possess almost
identical properties.
Isotopes
Carbon - 12
• Write the symbols for
carbon-13 and
carbon-14.
What is the average mass of a carbon atom?
12.01
Atomic Mass
• The atomic mass of
carbon is 12.01 amu.
• Atomic mass is the
average mass of all
the isotopes of an
atom. It takes into
account the different
isotopes of an
element and their
relative abundance.
Common Uses of Isotopes
• Isotope Analysis
• Radiometric Dating
• Nuclear Medicine:
– Magnetic Resonance Imaging (MRI)
– Positron Emission Tomography (PET
scans)
Importance of Isotopes
Radiometric Dating
Carbon Dating
Nuclear Medicine refers to imaging techniques that use
radioactive isotopes to detect and treat disease. Magnetic
resonance imaging (MRI) and positron emission tomography (PET
scans) are examples.
MRI
PET scan
• An atom of vanadium
contains 23 electrons.
How many protons
does it contain?
• 23 p+
• An atom of silver
contains 47 protons.
What is its atomic
number?
• 47
• An “atom” of sodium
contains 11 electrons.
What is its atomic
number?
• 11
• An atom contains 37
protons. What
element is it?
• rubidium
• How many electrons,
protons and neutrons
are in an atom of
actinium with a mass
number of 221?
• 89p+
• 89e• 132n0
• How many electrons,
protons and neutrons
are in an atom of
rhodium-105?
• 45p+
• 45e• 60n0
Homework
 Chapter 4 Worksheet 1 (due Monday)