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Early Atomic Theory and Structure
Dalton’s Model of the
Atom
1. Elements composed of small, indivisible particles
(atoms).
2. Atoms the same element are identical (mass & size).
3. Atoms of different elements have different mass and
size.
Dalton’s Model of the
Atom
4. Atoms of different elements combine in simple,
whole number ratios to form compounds.
5. Atoms of 2 elements may combine in different
ratios to form different compounds.
The Law of Constant
Composition
Every compound contains 2 or more elements
combined in a definite proportion by mass.
This proportion is the same for every example of that
compound.
The Law of Constant
Composition
For example, every molecule of ibuprofen
(C13H18O2) is 75.69% C, 8.79% H, and 15.52% O, by
mass.
These can be used as conversion factors:
75.69 g C = 100 g ibuprofen
8.79 g H = 100 g ibuprofen
15.52 g O = 100 g ibuprofen
The Law of Multiple
Proportions
Atoms of 2 or more elements may combine in
different ratios to produce more than one
compound.
The Nature of Electric
Charge
Positive and Negative
Opposites attract, likes repel.
May be transferred
q1q 2
F= k 2
r
Subatomic Particles
Proton
Neutron
Electron
Proton
Mass = 1.673 x 10-24 g
Charge = +1
Symbol = p (or p+)
Neutron
Mass = 1.675 x 10-24 g
Charge = 0
Symbol = n (or n0)
Electron
Mass = 9.110 x 10-28 g
Charge = -1
Symbol = e—
Thomson’s Plum Pudding
Model of the Atom
Rutherford’s Gold Foil
Experiment
Thomson’s Model vs.
Rutherford’s Experiment
The Nuclear Atom
Most of the mass and all of the positive charge is in
a very small region in the center of the atom, the
nucleus
Very little of the mass, and all of the negative
charge occupies most of the volume of the atom,
outside of the nucleus.
The Nuclear Atom
The nucleus contains protons and neutrons
The electrons surround the nucleus
In a neutral atom, the number of electrons is equal
to the number of protons.
The Atomic Number (Z)
The atomic number of an element is the number of
protons in the nucleus.
The number of protons in the nucleus is what
determines which element any given atom is.
Isotopes
Have the same number of protons, but different
number of neutrons.
This means they are the same element, but have
different masses.
Mass number = p + n
A
Z
Atomic number = p
E
Elemental symbol
Atomic Mass
Use the atomic mass unit (amu)
The amu is defined as exactly 1/12 the mass of a
12C atom.
1 amu = 1.6606 x 10-24 g
The average atomic mass of the elements is on most
periodic tables.
Average Atomic Mass
(AAM)
AAM = f1m1 + f2m2 + …
m1 = mass of isotope 1
f1 = fractional abundance of isotope 1
m2 = mass of isotope 2
f2 = fractional abundance of isotope 2
.
.
.
Average Atomic Mass
Rubidium has 2 naturally occurring isotopes:
Rb-85 which has a mass of 84.9118 amu and
a natural abundance of 72.17%, and Rb-87 which
has a mass of 86.9092 amu and a natural
abundance of 27.83%. Calculate the AAM of
Rubidium.
85.47 amu
Average Atomic Mass
Silver (Ag) has 2 naturally occurring isotopes,
Ag-109 and Ag-107 which has a natural abundance
of 51.84% and a mass of 106.905 amu. What is
the isotopic mass of Ag-109?
108.9 amu