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Chemistry Notes
Subatomic Particles
Basic Particles in the Atom
RECALL:
• Protons – large mass – positive charge
• Neutrons – large mass – no charge
• Electrons – virtually no mass – negative
charge
Discovery of the Particles
Recall:
• Thomson and the electron
• Rutherford and the proton
• What about the neutron?
Discovery of the Particles
Discovery of the Neutron
• Scientist since Rutherford realized that
protons alone could not account for the
mass of the nucleus.
– Mass of H+ = 1amu (H + is only a proton)
– Mass of He2+ = 4 amu
• Discovered in 1932 by James Chadwick
– Cambridge, England
Discovery of the Particles
• Chadwick's experiment
– Shot alpha particles at Be, this ejected radiation from
the Be in the form of particles
• The ejected particles (neutrons) were not effected by an
electric or magnetic field. Inference  particles are neutral
– The ejected particles dislodged protons from the wax.
• Chadwick counted the number of protons and reasoned that
it was the same number of particles coming from the Be.
– Inference  the size of protons and neutrons are nearly the same
Discovery of the Particles
Purpose of the Neutron
• Recall: protons are particles with a positive
charge; they want to repel. This makes the
nucleus unstable.
• The nucleus being comprised (in part) of protons
needs a force to hold it together – the strong
force
– Only powerful at a very short range
• The strong force is more powerful than the electrostatic force
at a close range
Discovery of the Particles
Purpose of the Neutron (cont.)
• Neutrons also possess this strong force but with
no charge.
• This allows them to hold the nucleus together
more tightly than the protons alone can
accomplish.
• As a nucleus gets larger, it gets more electric
force pushing out the protons.
– A larger nucleus needs more neutrons.
– The lighter elements are about half proton and half neutron.
The heaviest elements have many more neutrons than protons.
Summary
• Do we know each of the three elementary
particles of the atom?
– How discovered
• With what experiments / person?
– Their charge
– Their location in the atoms
Particles of the Atoms
Different numbers of subatomic particles
alter the behavior of atoms.
• Protons = define the atom!!!
– If Z = 4 you always have Be
• Different #’s of neutrons create isotopes
– C – 12 and C -14
• Different #’s of electrons than protons
create ions
– Na and Na+
Atoms and the Periodic Table
Each atom is represented on
the PT
• Atomic # - represents the
atom’s protons
– Smaller of the two #’s next to
the symbol
– Protons define the atom
• EX: all carbon atoms have 6
protons no matter how many
neutrons or electrons they have
ATOMIC #
Atoms and the Periodic Table
• Mass # - mass of atom
– Tells you the # of protons and neutrons
– Larger of the two numbers next to the symbol
Atomic masses are weighed
averages of isotopes
Ex: naturally occurring
lithium is composed of 2
isotopes: (7.4%) Li-6 and
92.6% Li-7
Averaging masses 6.941
Mass #
Atoms and the Periodic Table
The units for the mass # are amu’s
Atomic Mass Units
• Defined as 1/12 the mass of a carbon-12
atom
• 1 amu is approximately equal to the mass
of 1 proton (1.0073) or 1 neutron (1.0087)
Determining the #
of Particles in an Atom
Determining Protons
• Atomic # = # of Protons
Determining Electrons
• In a neutral atom the # of protons = the # of
electrons
– Therefore the Atomic # also tells the # of electrons
– Ions will have a symbol w/ a “+” or “-” sign. This
indicates how many electrons are lost or gained.
• EX: Al3+ aluminum’s has 13 electrons minus the 3 it lost, so
it now has 10 electrons total.
Determining the #
of Particles in an Atom
Determining the number of neutrons
• Subtract the atomic # (# of protons) from the
mass # (# of protons and neutrons)
• EX: What is the number of neutrons in a carbon atom?
Mass # of C = 12.011 (round to 12)
Atomic # of C = 6
# of neutrons in C = 6
• The number of neutrons do not always equal the
number of protons (ex U  238 – 92 = 146 neutrons)
Atoms and Subatomic Particles
Recall:
• Protons define the
atom!
Know:
• Atoms with the same #
of protons but different
# of neutrons are called
isotopes.
– Ex: Carbon 12 and
Carbon 14
Atoms and Subatomic Particles
• Know:
– Atoms that are
not neutral due to
losing or gaining
electrons are
called ions.
Atoms and Subatomic Particles
Ions: Charged Atoms
• Positive ions are called cations
– Sodium loses 1 e- and
becomes Na+1
• Negative ions are called anions
– Chlorine gains 1 e- and
becomes Cl-
Atoms and Subatomic Particles
Valance Shell Electrons
Electrons of the outermost
energy level
• Most likely involved on bonding
• Most likely to leave atom
• Elements in the same group on the PT have the
same # of valance shell electrons
– Group 1 elements all have 1 electron in their valance
shell
Atoms and Subatomic Particles
Valance electrons can be represented with
Lewis Dot Structures (symbols)
Subatomic Particles
Summary
• Subatomic particles
– Protons / neutrons / electrons
• Atoms and the PT
– Atomic # / mass #
• Determining protons / neutrons / electrons
• Atoms can have isotopes and ions
• Valance electrons can be represented by
Lewis Dot Structures