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Big Science from the Small World of Atom
- Understanding Atomic Structure with 3D
Visualization
By Dawen Li, Shoieb Shaik, Scott Wehby
Atomic Structure
 Quark: Smallest
particles existed in
proton and
neutrons. Further
discovery is still
ongoing.
 Electrons: Negatively charged electrons form
electron cloud surrounding nucleus
 Protons: One unit positively charge.
 Neutrons: No charge, along with protons forms
nucleus center.
Discovery of Atomic Structure
400 B.C
1800s
1904
1911
1913
Current
 Greek
John
Dalton,
philosopher
an English
Democritus
Niels Bohr,
a Danish
scientist, put
proposed
scientist,
brought
the
idea
back
that
the
atoms
 Another
English
physicist
electrons
into orbits.
 all
An
English
scientist
J.J.
make
atom Ernest
idea
up
and
substances
proved
atoms
Rutherford
concluded
Thomson
proposed
a so
He
hypothesized
that
electrons
exist
by
experiment.
fromcalled
experiments
that almost
“Plum Pudding”
 He named
the smallest
piece
travelled
in
a
definite
orbits
model:
negatively
charged
all the
mass
ofmeaning
an atom,
and
 The current electron  of
Allmatter
elements
“atomos,”
are
composed
of
around
the
nucleus
at
a
specific
electrons
were
scattered
allmodeled
its
positive
charges,
were
atoms,
to
be
cut.”
as
an
cloud model: a cloud of“not
energy
level,
much
like
throughout
a
positively
concentrated
in a centralplanets
sphere.
electrons surrounding indivisible
charged
sphere,
like raisins
circle
the
sun.
atomic
nucleus
surrounding
the dense nucleus.  Atoms of in
a same
pudding
the
element
by
electrons.

James
Chadwick,
a student of
are
However,
a
famous
Greek
exactly
alike.
Atoms
of
Rutherford,
concluded that the
philosopher,
Aristotle,
different
elements
are
nucleus
contained
positive
disputed
Democritus’s
theory.
different.
protons and neutral neutrons.
 Aristotle’s incorrect hypothesis
was accepted for about 2000
years.
Atomic Mass and Atomic Number
Atomic Mass:
Atomic Number:
 Mass of an atom is so small,
even gram is extremely big
measurement unit.
 Different elements have
different number of protons
called atomic number.
 Nucleus contains most of the
atom mass. Mass of proton =
mass of neutron, both are
much larger than electrons.
 Each element can be identified
by atomic number.
 Mass number is the sum of # of
protons and neutrons.
Atomic Mass Unit
= 1/12 of Carbon12 (six protons and six
neutrons)
= mass of each proton or neutron
 In an atom, # of protons = # of
electrons, so atom is charge
neutral.
# of neutrons
= mass number - # of protons
Isotopes and Average Atomic Mass
 Isotopes:
atoms of the
same element
that have
different # of
neutrons
 Average atomic mass of an
element: weighted-average
mass of the mixture of its
isotopes.
Valence and Core Electrons
 Valence electrons are those
occupying the outermost shell
(the highest energy level) of an
atom
Core electrons
 Core electrons are placed in the
inner (low) energy levels – close to
nucleus.
 Valence electrons involve chemical
bonding. Therefore, these
electrons are critical for chemical
properties.
 When an atom loses or gains an
electron, it becomes either
positively or negatively charged
ion.
Valence
electrons
Electron Arrangement in Shells
 Electrons first fill in the lower
shell (low energy level), than
only move to higher one.
 The maximum number of
electrons in a shell = 2n2,
where n stands for the
number of shell.
 Each shell can further divide
into sub-shells (orbitals), such
as s, p, d, f orbitals.
 Electrons fill orbitals in the
order of 1s2, 2s2, 2p6, 3s2, 3p6,
4s2, 3d10, 4p6, 5s2 4d10 and so
on.
Elements Organized by Periodic Table
 In late 1800s, Russian chemist,
Dmitri Mendeleev, organized all
elements known at that time
by atomic mass.
 He found repeating patterns of
chemical properties using his
periodic table.
 Based on periodic table, he
predicted properties of missing
elements, which are extremely
close to what scientists
discovered later on.
Atom and Current Periodic Table
 In 1913 a young English scientist, Henry
Moseley, re-arranged elements based on
atomic number instead of atomic mass, which
is the current periodic table.
 Elements in each group (vertical column)
have similar properties because they have the
same number of valence electrons for
chemical bonding.
 The number of valence electrons can be
found from I, II, … VIII, group number.
 Each periodic row ends with noble gas. The
outer energy level is fully filled with eight
valence electrons (two electrons at row 1),
leading to be “inert” for chemical reactions.
 Solid metals are located in the left side of
periodic table, while nonmetal gases are in
right-top region.
 Niels Bohr, aDanish scientist, put
electrons into orbits.
Information from Core Electrons
 Moseley’s law: the difference
in energy between shells
changes as the atomic
number varies.
 When an electron transits
from outer shell to the inner
shell, a X-ray photon is
emitted.
 Energies of X-ray photons are
characteristic. They are
specific to the elements in the
specimen, which is basis for
element identification.
Nanotechnology from Electron Beam
 Electron-beam
lithography to
make nanopieces.
 Electron
microscopies
to see
nanoscale
structures.
Nanopillar arrays from Prof. Dawen Li’s
lab at the University of Alabama
Discovery Using Scientific Method
Recognize the Problem
Form a Hypothesis
Test and Analyze Data
Draw Conclusions
Review Questions
 What are different parts of an atom and draw its structure? What are the
particles that make up protons and neutrons?
 Where is the majority of the mass of atom located?
 Discuss the charge from a proton, an electron, a neutron, and the charge in
an atom as a whole.
 Which element is an atom with 14 protons in the nucleus? What is the
number of valence and core electrons?
 Why do elements in the same group undergo similar chemical properties?
 How isotopes are defined? How to calculate number of neutrons?
 What are the properties (metal or nonmetal, solid or gas phase) of the
elements located on the left side of the periodic table?
 Why are the noble gases in group VIII chemically stable?
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