Chemistry
Ms. Pollock
2013 - 2014
4.3 ATOMIC STRUCTURE
Introduction
 Dalton’s atomic theory
very good but not
entirely correct
 Atoms able to be
broken into smaller
particles
Electrons, Protons, and
Neutrons
 Electrons
 Negative charge
 Repulsed by negative
objects
 Attracted to positive
objects
 Able to “stake out”
territory and “defend”
that territory from other
electrons
Electrons, Protons, and
Neutrons
 Protons
 Positive charge
 Attracted to positive
objects
 Repulsed by negative
objects
 Forced to group
together into one big
clump – strong nuclear
forces
 Form dense, positively
charged center
Electrons, Protons, and
Neutrons
 Neutrons
 Proposed by Rutherford
 Neutral
 Neither attracted nor
repelled
 Bound in nucleus by
strong nuclear forces
Electrons, Protons, and
Neutrons
 No real interaction of
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neutrons
Protons and electrons
interacting
Expected attraction
between protons and
electrons
Electrons much smaller
than neutrons and
protons
Most atom mass in
nucleus
Electrons, Protons, and
Neutrons
Particle
Relative
Mass
(amu)
Mass in
Grams (g)
Electric
Charge
Location
Electron
1/1840
9.11 X 10-28
-1
Outside
nucleus
Proton
1
1.67 X 10 -24
+1
Nucleus
Neutron
1
1.67 X 10 -24
0
nucleus
•One proton for every
electron in neutral atom
•Two protons = two
electrons
•Ten protons = ten
electrons
 Atomic mass and
charge for
subatomic particles
in table
 Atomic mass unit –
1/12 mass of carbon
atom; mass of
proton and neutron
one unit; also called
a dalton
Atomic Number and Mass Number
 Different elements
distinguished by
number of protons
 Atomic number –
number of protons in
nuclei of any atom of
an element; symbol Z
 Also number of
electrons for neutral
atoms
Atomic Number and Mass Number
 Mass number – total
number of protons and
neutrons in nucleus;
symbol A
 No electrons in mass
number due to tiny
mass
 Mass number A =
(number of protons) –
(number of neutrons)
Atomic Mass Calculation
Example
 What is the mass number of an atom that
contains 3 protons and 4 neutrons?
 (number of protons) = 3
 (number of neutrons) = 4
 mass number A = (number of protons) + (number
of neutrons)
 mass number A = (3) + (4) = 7
Atomic Mass Calculation
Example
 What is the mass number of an atom of
helium that contains 2 neutrons?
 (number of protons) = 2 (Remember that an atom
of helium always has 2 protons.)
 (number of neutrons) = 2
 mass number A = (number of protons) + (number
of neutrons)
 mass number A = (2) + (2) = 4
Isotopes and Atomic Mass
 Number of neutrons possibly different
 Isotopes – atoms of same element with
different numbers of neutrons
 Protons not varied – same atomic number
 Neutrons varied – different mass number
Isotope Example
 What is the atomic number
 What is the atomic number
(Z) and the mass number
(A) of an isotope of lithium
containing 3 neutrons? A
lithium
(Z) and the mass number
(A) of an isotope of lithium
containing 4 neutrons? A
lithium
 atom contains 3 protons in its
 atom contains 3 protons in its
nucleus.
atomic number Z = number
of protons = 3
number of neutrons = 3
mass number A = (number of
protons) + (number of
neutrons)
mass number A = (3) + (3) = 6
nucleus.
atomic number Z = number
of protons = 3
number of neutrons = 4
mass number A = (number of
protons) + (number of
neutrons)
mass number A = (3) + (4) = 7
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Isotopes and Atomic Mass
 Isotopes part of Dalton’s theory that has been
disproved – not all atoms of an element
identical
 Natural samples of elements combinations of
different isotopes
 Isotope abundance important to calculation
of atomic mass
 Periodic table mass weighted average mass
of atoms in naturally occurring sample
Average Atomic Mass Example
 Boron has two naturally occurring isotopes. In
a sample of boron, 20% of the atoms are B10, which is an isotope of boron with 5
neutrons and a mass of 10 amu. The other
80% of the atoms are B-11, which is an
isotope of boron with 6 neutrons and a mass
of 11 amu. What is the atomic mass of boron?
 0.2(10) + 0.80(11) = 10.8 amu
Average Atomic Mass Example
 Neon has three naturally occurring isotopes. In a
sample of neon, 90.48% of the atoms are Ne-20,
which is an isotope of neon with 10 neutrons and
a mass of 19.99 amu. Another 0.27% of the
atoms are Ne-21, which is an isotope of neon
with 11 neutrons and a mass of 20.99 amu. The
final 9.25% of the atoms are Ne-22, which is an
isotope of neon with 12 neutrons and a mass of
21.99 amu. What is the atomic mass of neon?
 0.9048(19.99) + 0.0027(20.99) + 0.0925(21.99) =
20.20 amu
Isotopes and Atomic Mass