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The Modern View of the
Atom
Chemistry
Fall 2013
History
 Democritus:
atoms as indivisible units
 Thomson: discovery of electrons
 Rutherford: small, massive, and positivelycharged nucleus
 Bohr: special orbits for electrons
Quantum Mechanical Atom

Nucleus: protons and neutrons




Positively (+) charged
Heavy
Small
Dense
Question: What are the properties of the nucleus?
Quantum Mechanical Atom
 Electron
cloud: where an
electron could be



Heisenberg Uncertainty
Principle: can’t be sure where
an electron is
Cloud is much bigger than
nucleus
Orbitals: various shapes of
electron clouds
Question: Why do we represent electrons as
clouds instead of dots?
Subatomic Particles
Particle
Charge
Mass
Location
Proton
+1
Heavy
Nucleus
Neutron
0
Heavy
Nucleus
Electron
-1
Light
Electron
cloud
Question: What are the properties of the three subatomic
particles?
Atomic Number (Z)

Number of protons
 Identifies which element an atom is
 Example: A carbon atom has 6 protons while an
iron atom has 26 protons
Carbon
Iron
Question: Which subatomic particle determines the
identity of an element?
Mass Number (A)
 Mass
number = #protons + #neutrons
 Example: If a lithium atom has 3 protons
and 4 neutrons, its mass number is 7.
 Question:
What two numbers do we add
together to get mass number?
Checking for Understanding
 Where
do you find the atomic number on
the periodic table?
 What is the atomic number of Oxygen?
 Where do you find the mass number on
the periodic table?
 What is the mass number of Oxygen?
Isotopes
 Same
# of protons but different # of
neutrons
 Same atomic number but different mass
number
 Question:
How are isotopes the same?
How are they different?
Element
atomic number
6
C
Carbon
average
atomic mass
12.01
Element
symbol
Element
name
Atomic Symbol
charge
mass number
201
80
atomic
number
2+
2
number of
atoms
Electrons
 For
neutral atoms (no charge): # electrons
= # protons
 Ions: atoms with charges (i.e. Ca2+ or Br-)


Extra electrons for negative (-) charge
Fewer electrons for positive (+) charge
Question: Why do atoms with negative charges
have extra electrons?
Summary Questions
 Why
is the proton responsible for the
identity of the element? (Hint: what
happens when you change the # of
protons)
 What would happen to the atom if you
change the number of electrons? Or
neutrons