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Subatomic Particles
What is the smallest thing in
the universe?
• We are mainly concerned with 3 subatomic
particles
• Recall Bohr model
Electrons
•
•
•
•
Discovered by _______________
Charge: ____________
Extremely light
Orbit the nucleus in fixed energy levels
Protons
•
•
•
•
Discovered by _________________in Gold Foil Experiment
Charge: ________
1000 x heavier than electrons
Sit in nucleus
A problem
• Rutherford could explain hydrogen in
terms of protons and electrons only
• This didn’t work for helium
• Mass of helium = 4x mass of hydrogen
• + charge of helium only = 2x + charge
of hydrogen
• What would you hypothesize?
A hypothesis
• Rutherford hypothesized the
existence of some other neutral
particle in the nucleus of the
atom.
• Why did he think it was neutral?
James Chadwick – 1932
(aka the original Jimmy Neutron)
• Discovered the neutral particle
Rutherford had hypothesized
• Called it the “neutron”
Summary of 3 subatomic particles
Particle
Proton
Neutron
Electron
Relative
Mass
1
1
1/1000
Charge
Location
+
Neutral
-
Nucleus
Nucleus
Orbit around
nucleus
Atomic Number
• Corresponds to the order of the periodic table
• Equal to the number of protons in the atom
Atomic Number
Mass Number
• Recall Rutherford’s model of the atom
• Mass of electrons insignificant
• The mass number of an element is the total
number of protons and neutrons in its nucleus.
Isotopes
• Sometimes an element has a number of atoms that have differing
numbers of neutrons
• Atoms with the same number of protons but different numbers of
neutrons are called isotopes.
• True or false:
• Two isotopes of the same element will have a different atomic
number.
• Two isotopes of the same element will have a different mass
number.
The sun produces energy from a nuclear
fusion reaction that uses the H-2 (deuterium)
and H-3 (tritium) isotopes of hydrogen.
Atomic Mass
• The atomic mass of an element is listed below the element
symbol on the periodic table.
• Weighted average mass of all isotopes of the element
• Ex. atomic mass of oxygen = 15.999 atomic mass units (u).
• 99.9% of oxygen atoms have a mass number of 16,
• .1% of oxygen atoms have a mass number of 15.
• You can usually determine the mass number of an element by
rounding it’s atomic mass to the nearest whole number.
Atomic Mass
Sample Problem: Atomic Mass
• How many neutrons does the most common isotope of silicon have?
• Given:
• Atomic mass = 30.974u
• Atomic number = 15
• Required: number of neutrons
• Round off the atomic mass to get the mass number.
28.09  28
• Mass number = protons + neutrons
• Protons = atomic number
• Mass number = atomic number + neutrons
• Neutrons = mass number – atomic number
• Neutrons = 28 – 14 = 14
For Neutral Atoms
• Atomic number = Number of protons = Number of
electrons
• Z = p+ = e• number of neutrons = mass number – number of protons
• n0 = M - p+
Ions
• When elements undergo chemical change to
form compounds, they often gain or lose
electrons
• This creates a charged particle called an ion
• Why are these particles charged?
Calculating the number of
particles in an ion
• How many protons, neutrons, and
electrons are in a O2- ion?
• p+ =
• n0 =
•e=
Bohr-Rutherford Diagrams
• These diagrams give us all the information we need about the
structure of a particular atom.
• p+ and n0 shown in nucleus
• e- shown in their respective orbitals around the nucleus.
• Remember: for an atom to be neutral, e- = p+
Sample Problem: B-R Diagrams
• Draw a Bohr-Rutherford diagram for a neutral C-12 atom.
Sample Problem: B-R Diagrams