Chapter 3- Section 1
Atomic Structure
Objectives:
-Democritus’s theory
-Dalton’s theory
-Bohr’s Model vs. Modern Model of the Atom
Democritus
• Ancient Greek philosopher – 4th century B.C
• Suggested that the universe was made of
invisible units called atoms
• His theory explained some observations but
could not provide enough evidence to
convince people that atoms really existed
John Dalton
• 19th century -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
• Every element is made of tiny, unique
particles called atoms
• Atoms of the same element are identical/
exactly alike
• Atoms of different elements combine in
constant ratios to form compounds.
• Dalton’s theory was widely accepted because there
was much evidence to support it
1808 John Dalton
• Dalton’s Model of the Atom:
– Uniform Shape
– Uniform Density
– Indivisible
• So what did Dalton’s model
look like?
• A simple design that
promoted future research
into atomic theory…
The Atom
• Small, dense center called the nucleus
• Nucleus is positively charged and made up
of protons and neutrons
• Nucleus is surrounded by tiny negatively
charged particles called electrons
What’s in an Atom?
Atoms are made up of 3 subatomic particles:
• Protons- positively charged particle, found in the
nucleus
• Neutrons – neutral particle (no charge), found in
the nucleus
• Electrons- tiny negatively charged particle,
moving outside the nucleus
Subatomic Particles
Particle
Charge
Mass (kg)
Location in
the atom
Proton
+1
1.67 x 10-27
In the nucleus
Neutron
0
1.67 x 10-27
In the nucleus
Electron
-1
9.11 x 10-31
Moving
around
outside the
nucleus
Atoms have NO overall charge
• Atoms are not charged even though they are
made up of charged particles
• Atoms have an equal number of protons (positive
charge) and electrons (neg. charge) whose
charges exactly cancel.
• Ex: Helium (He)
– 2 protons, 2 neutrons, 2 electrons
+2
0
-2
= 0 total charge
Models of the Atom
• All previous models contributed to modern
model of the atom
• New discoveries required consideration of
new models
• Like most scientific models and theories, the
model of the atom has been revised many
times
Bohr’s Model
• In 1913, Bohr suggested that electrons in an
atom move in set paths around the nucleus
much like planets orbit the sun
Modern Theory
• Electrons behave more like waves on a
vibrating string
• Electrons exist in orbitals (areas of probability)
• Demo– just like turning blades of a fan, the
exact position, speed, direction and location
of electrons in an atom cannot be determined
Modern Model of the Atom
• Small, dense nucleus
consisting of protons
and neutrons
• Electrons exist in
orbitals within each
energy level
• Space between energy
levels decreases at
higher energy (higher
energy levels are closer
together)
Valence Electrons
• Valence electrons- electrons in the outermost
shell of an atom and determine the atom’s
chemical properties
• Valence electrons are the electrons in an atom
that participate in chemical bonding.
• Every atom has between one and eight
valence electrons.
How many valence electrons??
•
•
•
•
•
•
•
Br
Al
K
Mg
Cl
S
N
Orbitals
• Orbitals- a region in an atom where there is a
high probability of finding an electron
• Electrons are not in neat fixed orbits like Bohr
proposed but in orbitals.
– 3-D region around the nucleus that indicates the
probable location (90%) of an electron.
Orbitals
• An orbital is the actual region of space
occupied by an electron in a particular energy
level.
• There are many different types of orbitals and
each have their own fundamental shape. They
are designated by letters s, p, d,and f.
So how many orbitals are there
and what do they look like??
s-subshell
How many
electrons can
occupy an ssubshell?
•
•
•
•
1 orbital x 2 electrons = 2
Consists of one s orbital
Has a spherical shape
It can hold 2 electrons
Every energy level has an s-orbital
p-subshell
How many electrons may
occupy the p-subshell?
3 orbitals x 2 electrons each= 6
•
•
•
•
Consists of 3 p orbitals
Shaped like a dumbbell
Each orientation can hold two electrons
Energy levels 2 and higher have 3 p orbitals
d- subshell
How many electrons may occupy a
d- subshell?
5 orbitals x 2 electrons per orbital= 10
• Consists of 5 d orbitals
• Each orientation can hold two electrons
• Energy levels 3 and higher have 5 d
orbitals
f- subshell
How many electrons
may occupy an
f-subshell?
7 orbitals x 2 electrons per orbital= 14
• Consists of 7 f orbitals
• Each of these orientations can hold up to 2
electrons
• Energy levels 4 and higher have 7 f orbitals
Check it out!
https://www.youtube.com/watch?v=KjNgq16jEY#t=86
Sublevel Summary
Number of Orbitals it
contains
Totals number of
electrons
s
1
2
p
3
6
d
5
10
f
7
14
Sublevel
(2 x number of orbitals)
Electron Configuration!!
What is it?
c
ab
Electron configuration of an atom is
a notation that describes the
a)energy levels
b)orbitals (s, p, d or f)
c)total number of the electrons in
each orbital.
When you are giving directions to your
house, you need to include several
pieces of information….
What town, name of street, number of
house, etc.
The same is true for giving the
“address” of an electron in an atom of
an element.
What information do we need in order
to write the address (electron
configuration) of electrons in an
element?
• 1) Which energy level(s) does it occupy?
• 2) Shapes of the orbitals – considering
different orientations
• 3) How many total electrons??
An electron’s “address” will be
different (specific) for each
element….. WHY?
How do we use this info?
• The periodic table is organized based on
electron configuration.
Diagonal rule: the order of filling
HOW TO: write electron configurations
Step 1) Look up the atomic number of the element to
determine the number of electrons
Step 2) Consult the periodic table (or arrow diagram) to
determine the order in which the sublevels should be
filled
Step 3) Write the configuration, filling in up to two
electrons in each “s” subshell, up to 6 in “p”, up to 10 in
“d”, and up to 14 in “f”
Step 4) When you think you are finished, add up the
exponents (superscripts) to see if you have the correct
number of electrons