Lecture 7
• The Atom and
Elements
• Subatomic Particles
• Isotopes
• Radioactivity
Atom: the fundamental unit of an element.
Mostly empty space: if an atom were expanded to a
diameter of 3 km, the nucleus would be the size of a
basketball.
• Element: a substance that
cannot be broken down into
simpler substances by
chemical or physical means.
– Composed entirely of one
type of atom.
Periodic Table: arranges elements into rows so that those
with the same properties are found in the same column.
Subatomic Particles
• Protons:
• Atomic number: number of protons in the nucleus
of an atom.
• Elements are defined by their atomic number.
• Number of protons = Number of electrons in
an electrically neutral atom.
Subatomic Particles
Neutrons:
• Accompany protons in the nucleus.
• Atomic Mass Number: total number of
protons and neutrons in the nucleus.
• Number of neutrons = atomic mass number –
atomic number.
1 atomic mass unit = 1.661 X 10-24 grams
Subatomic Particles
Electrons:
• Behave as both a wave and a particle.
• Travel at speeds of 2 million m/s within an atom.
• Electrical repulsion with electrons of neighboring
atoms prevents one atom from passing through
another.
Isotopes
• Isotopes: atoms of the same element that
contain different numbers of neutrons in
the nucleus.
– Different isotopes = different mass numbers.
– Isotopes of an element have similar chemical
properties – same electron structure.
Isotopes
Radioactivity
• Radioactivity: results from radioactive decay the process whereby unstable atomic nuclei
break down and emit radiation.
– An imbalance of neutrons to protons can result in
radioactivity.
• A large amount of Earth’s internal heat is
maintained by the presence of abundant
radioactive isotopes.
Origins of radiation exposure for an average individual in the
United States.
Note: lethal radiation doses begin at 500 rems.
Types of Radiation
α = Alpha Particle
β = Beta Particle
γ = Gamma Ray
Radiation and Energy
• Energy is released during radioactive
decay.
– Kinetic Energy of particles released from
atom.
– Kinetic Energy of atom when it recoils.
– Gamma rays.
Forms of Electromagnetic Radiation
Low Frequency
(Low Energy)
High Frequency
(High Energy)
The Atomic Nucleus
• Strong Nuclear Force: attractive force
between neutrons and protons within the
nucleus.
– Very strong over extremely short distances
(within the diameter of a typical atomic
nucleus).
– Easily overcomes the repulsive electrical
force of protons with same charge.
The strong nuclear force is a very
distance-sensitive attraction.
The presence of neutrons helps hold
the nucleus together.
In large nuclei, the nucleons are farther apart
which weakens the strong nuclear force.
Larger nuclei require more
neutrons than protons in order
to overcome the repulsive
electrical forces.
Exanple: Pb has 1.5 times as
many neutrons as protons.
All nuclei with more than 83
protons are radioactive.
Farther apart neutrons are unstable - require the
presence of nearby protons to remain stable.
Decay of a Neutron to a Proton by emission of an
electron (Beta particle)
Unstable nucleus undergoing
radioactive decay
Transmutation
• Transmutation: changing of one chemical
element into another.
– Occurs both naturally and artificially.
Transmutation
Alpha emission from a nucleus:
• mass number decreases by 4
• atomic number decreases by 2
Transmutation
Beta emission from a nucleus:
• One neutron transforms to a proton but no
change in mass number.
• Atomic number increases by 1
Transmutation
Gamma emission from a nucleus:
• No change in mass number
• No change in atomic number