Atomic Structure
Standard Atomic Notation
Isotopes
Isotopic Abundance
Learning Goals
Students will be able to:
 understand standard atomic notation
 explain the relationship between the atomic number
and the mass number of an element, and the
difference between isotopes and radioisotopes of an
element
 identify the scientists who contributed to the
development of the modern Periodic Table
Success Criteria
Students will be able to:
 writing radioactive decay equations
 determining the number of protons, neutrons and
electrons in an atom.
 explaining the contributions of scientists who
developed the modern Periodic Table
 Calculate average atomic abundance using isotopic
abundance values
Atomic Theory and
The Periodic Table
Review the development of atomic theory:
 Read pages 14-16 for Mendeleev and Döbereiner
 Read pages 24-25 for Bohr and Rutherford
 Read page 27 for Soddy
Review the information on the various groups within
the periodic table.
 Read pages 10-11 for basic Chemistry term and
Information on the elements
 Read pages 17-19 on the groups within the periodic
table
Modern View of Atomic Structure
 The atom has 2 main regions
a)
b)
Nucleus - protons and neutrons
Electron cloud- region where you might find
an electron
 Rutherford’s Gold Foil experiment (remember
grade 9) proved that the atom has an extremely
dense, heavy nucleus surrounded by mostly
empty space (save for some very light electrons).
Rutherford’s Gold Foil Experiment
 The Gold Foil Experiment
The Gold Foil Experiment
 The gold foil is only
a few atoms thick.
 Most of the
positively charged
α-particles pass
straight through the
gold foil
 A few are deflected
the tiny positively
charged nuclei of
the gold atoms
Density and the Atom
 Since most of the α-particles went through the gold
atoms in the thin gold foil, it proved that the atom
was mostly composed of empty space.
 Since alpha particles are positively charged (they are
actually the nuclei of helium atoms), they must be
deflected by a positively charged “nucleus”.
 Since so few of the α-particles were deflected, the
nucleus must be very small
 This nucleus was determined to contain almost all of
the atoms mass.
Size of an atom







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Atoms are small.
Measured in picometers, 10-12 meters
Hydrogen atoms have a 32 pm radius
IF the atom was the size of a stadium, the nucleus
would be the size of a marble.
Hence most of an atom is empty space – thus you are
mostly made out of nothing.
Radius of the nucleus near 10-15m.
The density of the nucleus is near 1014 g/cm3
A dice made of a nucleus would weigh 100,000,000
tonnes (this equals 1000 supertankers).
Subatomic particles
Relative Actual
mass (g)
Name Symbol Charge mass
Electron
e-
-1
Proton
p+
+1
Neutron
n0
0
1/184 9.11 x 10-28
or ≈ 0
1
1.67 x 10-24
1
1.67 x 10-24
Standard Atomic Notation
 Contain the symbol of the element (X), the mass
number (A) and the atomic number (Z)
mass number
(A)
atomic number
(Z)
X
element
symbol
12
Standard Atomic Notation
 Remember from grade 9:
1) Symbol
• 1 or 2 letter - the first is always a capitol
• the symbols of many elements come from their latin
roots (ex. Gold (Au = aurum))
2) Atomic Number
• the number of protons in the nucleus (atom)
3) Mass Number
• the mass of the atom - since almost all of the mass
comes from the nucleus;
• mass number = number of protons + number of
neutrons
Standard Atomic Notation
 the area to the top right of
the symbol is used for
ionic charge
 the area to the bottom
right of the symbol is used
for number of atoms (ex.
H2)
Standard Atomic Notation
 FIND THE NUMBER OF PROTONS,
NEUTRONS AND ELECTRONS
FOR AN ATOM
1) Protons = atomic number
2) Electrons = atomic number unless
an ionic charge is indicated
(remember a positive charge
indicates that electrons were
LOST)
3) Neutrons = atomic mass - atomic
number (can you explain this?)
 Find the number of protons, neutrons
and electrons for the two atoms at left
Symbols
 Find the
 number of protons
 number of neutrons
 number of electrons
19
9
F
 atomic number
 mass number
16
Symbols
 Find the
 number of protons
 number of neutrons
 number of electrons
 atomic number
 mass Number
80
35
Br
17
Symbols
 If
an element has an atomic
number of 34 and a mass number
of 78 what is the
 number of protons
 number of neutrons
 number of electrons
 standard atomic notation
18
Symbols
 If
an element has 91 protons and
140 neutrons and an ionic charge
of what is the
 atomic number
 mass number
 number of electrons
 complete symbol
19
Isotopes
 Soddy determined that atoms of the same element can
have different numbers of neutrons
 This means that an element can have different mass
numbers
 Soddy called these atoms isotopes.
 Carbon has 3 isotopes, only 1 is common.
Naming Isotopes
 Put the mass number after the name of the element
 carbon- 12 or C-12
 carbon -14 or C-14
 uranium-235 or U-235
 http://www.youtube.com/watch?v=EboWeWmh5P
g (Tyler DeWitt _ Isotopes)
 http://www.youtube.com/watch?v=n4WZ0-fItt8
(Tyler DeWitt – Isotopes and Elements examples)
Isotopes and Atomic Mass
 For example magnesium has 3 naturally occurring
isotopes with masses of 24, 25 and 26.
 What atomic mass should be placed in the periodic table?
 Scientists decided that the average weight of the atoms
based on the abundance should be placed on the periodic
table as the atomic mass.
 This explains why mass values have decimals.
 http://www.youtube.com/watch?v=dRfrvpVdKGM
(Tyler DeWitt)
Determining Isotopic Abundance
The Mass Spectrometer
Determining Isotopic Abundance
of Magnesium
Determining Isotopic Abundance
The Mass Spectrometer
Mass Spectrometer scan of a protein found
in a performance enhancing drug at the
Olympics.
Mass Spectrometer read-out
of Magnesium isotopes.
Try these questions
 Calculate the relative atomic mass of gallium given that
the relative abundance of its two isotopes are: 60.5% of
Ga-69 and 39.5% of Ga-71.
 Iron has four isotopes; Fe-54 (5.82%); Fe-56 (91.66%); Fe-
57 (2.19%) and Fe-58 (0.33%). Determine the average
atomic mass for natural iron.
 Magnesium has three isotopes. 78.99% magnesium 24
with a mass of 23.9850 amu, 10.00% magnesium 25 with a
mass of 24.9858 amu, and the rest magnesium 25 with a
mass of 25.9826 amu. What is the atomic mass of
magnesium? (note the more accurate data)
 If not told otherwise, the mass of the isotope is the mass
number in amu (atomic mass units)