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How Big is an Atom?
Imagine that you could increase the size of an atom
to make it as big as an orange.

How Big is an Atom?
At this new scale, the orange would be as big as the
Earth.

What is an atom?


Smallest unit
into which
matter can be
divided, while
still maintaining
its properties.
An Element is
composed of
ONE type of
atom.
Aim: The scientific study of the atom began
with John Dalton in the early 1800’s and has
been revised through the years.
The atomic Model of Matter
John Dalton (1803-1807)
J.J. Thomson (1897)
Ernest Rutherford (1910)
Niels Bohr (1913)
Wave-Mechanical Model
The Dalton Model 1803
1) Each element is composed of indivisible atoms.
2) In an element, all the atoms are identical; atoms of
different elements have different properties, including
mass.
3) In a chemical reaction, atoms are not created,
destroyed, or changed into other types of atoms.
4) Compounds are formed when atoms of more than
one element combine.
5) Dalton’s model = solid, indivisible, sphere.
DOES NOT mention subatomic particles
Solid Sphere No Charged Particles
J.J. Thomson 1897
1.
2.
Examined cathode rays (electric charge that flows
from the negative electrode, (cathode) to a positively
charged electrode, (anode).
These cathode rays were the paths of negatively
charged particles he called electrons.
Cathode Ray Tube - YouTube
PLUM PUDDING model:
Positively charged sphere with electrons embedded in it
like raisins in plum pudding. He gave the atom some
structure.
Plum Pudding Model
What happens when you bite into
a peach?
The Rutherford Model 1911
Bombarded thin gold foil with the radioactive nuclei of
Helium atoms (+ charged alpha particles) and
observed how these particles were scattered by the
foils.
1 in 20,000 particles bounced back or were deflected, the
rest past through the gold sheets.
Conclusion
1) Most of the atom is empty space with a dense (+)
charged nucleus.
2) The atom has a dense (+) nucleus.
3) The electrons are present and orbit the space
surrounding the nucleus.
Rutherford's Model of the Atom - YouTube
Rutherford Experiment
Neils Bohr 1913
The Bohr Model – electrons travel in fixed circular
pathways or ORBITALS around the nucleus,
held in place by the proton(s) in the nucleus.
Why we see Light
1) Electrons absorb energy and jump to higher
orbitals. The electron gives off the excess
energy as light and falls back down to lower
energy levels.
2) Light given off by the atoms corresponds to
certain frequencies or energies.
Wave Mechanical Model
Shows the area with the greatest probability
of finding an electron(s)
Bohr Model vs. Wave Mechanical Model
Bohr Model
Bohr's model gives the
electron orbit an exact
travel path.
Wave Mechanical
1.
The electron(s) make
an orbital cloud of the
most probable location
around the nucleus.
Bright Line Spectra
Structure of an Atom
-
+
+
+
+

-
-
Energy Levels or Orbitals
Made up of the subatomic
particles:
+
 Protons
(positive)
 Neutrons
(neutral)
 Electrons
(negative)
The Atom’s “CENTER”
Protons and neutrons are grouped together to
form the “center” or nucleus of an atom.
-
+
-
+ +
-
Notice that the electrons are not apart of the nucleus. They are found in the electron cloud.
Subatomic Particle Weight Comparison
Expressed in Atomic Mass Units (AMU)
- - - - - - - - - - - - - -
- - - - - - - - - - - - - -
1839 electrons = 1 neutron
+
1836 electrons = 1 proton
+
How do you think the mass of a neutron
compares to that of a proton?
1 neutron ≈ 1 proton
Atoms are Electrically Neutral
+
+
+
+
+
+
8 protons
+
+
+
-
- - -
-
=
No Charge
8 electrons
Number of Protons = Number of Electrons
Sub-atomic
Particle
Symbol Location
Electric
Mass
Charge Atomic mass unit
(AMU)
Electron
e-
Outside
the
nucleus
1-
1/1840
Proton
p
nucleus
1+
1 amu
Neutron
n
nucleus
0
1 amu
Atomic Number
Neutrons
1) Neutrons add mass to an atom, but they do not change
the atom’s identity as an element.
2) Neutrons are located inside the nucleus with the
protons.
# of neutrons + # of protons = MASS NUMBER
Mass number – Atomic number = # of neutrons
3) All atoms on the periodic table are electrically neutral.
Give the symbol, atomic number, # of protons, neutrons,
and electrons, and atomic mass for Argon.
Isotopic Symbols
Isotope Notation – Concept Check
1.
2.
3.
4.
5.
Atomic Number =
Atomic Mass =
Protons =
Electrons =
Neutrons =
Mass Number
 The
total number of protons and neutrons in the nucleus
What is the mass number
of this atom?
-
-


+34
+
++
5 protons + 6 neutrons =
a mass number of 11 amu
-
+
-
What is the overall charge of this atom?
Bohr Model vs. Wave Mechanical
Model
Niels Bohr s Atomic Model - YouTube
1.
2.
Niels Bohr based his atomic model on his
observations using Hydrogen…
Hydrogen has only 1 electron.
Bohr
Diagrams
1)
2)
3)
4)
Find your element on the periodic table.
Determine the number of protons and neutrons
in the atom. How can we do this?
Atomic Number = Proton number.
Mass Number – Atomic Number = Neutron #
Bohr Diagrams
1)
2)
C
3)
Draw a nucleus with the
number of protons and
neutrons.
Carbon has two energy
levels, or shells. (electron
configuration)
Draw the shells around the
nucleus.
Bohr Diagrams
1)
2)
C
3)
4)
Add the electrons.
Carbon has 6 electrons.
The first shell can only
hold 2 electrons.
The second shell will
contain the other 4
electrons.
Bohr Diagrams
Build a Bohr Diagram
1)
2)
C
3)
Check your work.
You should have 6
total electrons for
Carbon.
How many total
electrons can fit in the
third shell? 8
Absorption and Release of Energy by an
electron
1) When an electron absorbs a specific amount of energy
(known as quanta or quantum of energy), the electron
becomes excited and moves or “jumps” to a higher energy
orbital.
2) When the electron “jumps” to a higher energy level or
orbital it is said to be in the excited state.
3) When the electron releases this excess energy, it releases
the energy as a photon of light and falls to the ground state.
4) The color light that is emitted or released is determined by
how many orbitals and which orbitals the electron “falls”
back.
Orbitals
Atomic Emission Animation - YouTube
n=3 -----------------------------------------------------
n=2 ----------------------------------------------------
n=1 ----------------------------------------------------
Neils Bohr 1913
Bohr - YouTube
Bohr Model vs. Wave Mechanical
Model
Quantum Mechanics: The Structure Of Atoms - YouTube
Bohr Model
Wave Mechanical
When a Hydrogen e– was
excited, the light emitted
was found to be
composed of regularly
spaced lines. Each
element has a
Visible-line spectrum.
An atomic orbital is the
region of space around
the nucleus where the
probability of locating an
e– with a given energy is
greatest.
The bright-line spectra for three elements and a mixture of elements are
shown below.
1)
2)
3)
Identify all the elements in the mixture.
Explain, in terms of both electrons and energy, how the bright-line spectrum
of an element is produced.
State the total number of valence electrons in a cadmium atom in the ground
state.
Ground State vs. Excited State
1) Ground State – all electrons are in
the lowest possible energy levels
(normal) ex. 2 – 7
2) Excited State – if given additional
energy, electrons will “jump up” to
higher energy levels, temporarily.
Excited State ex. 2 – 5 – 2
Ground State ex. 2 – 7
Bright Line Emission Spectra
How does this happen?
1. “Excited electrons” at higher energy
levels will eventually release the extra
energy and “fall back down” to
ground state conditions.
2. During the “fall back”, energy is
released as Visible Light Energy.
Bright Line – Emission
Spectra
What evidence indicates that
electrons move around the
nucleus in definite pathways?
How would you compare
different element’s spectral
line patterns to an
individual’s DNA?
Unknown DNA Sample
Match
1) Each element has a specific
electron configuration and a
corresponding emission spectrum.
2) Emission (bright line) spectrum
can be used to identify
(“fingerprint”) each element.
Use your Sun block to block UVA and UVB rays!
Lewis Structures of Atoms
1) The chemical symbol for the atom is surrounded
by a number of dots corresponding to the number
of valence electrons.
2) Valence electron(s) – number of electrons in the
atom’s outermost orbital. These are the electrons
involved in chemical bonding between atoms.
3)

Kernel – Nucleus (protons and neutrons) and all
non-valence electrons.
Lewis and Bohr
Lewis Structures
In your notes, try these
elements on your own:
a) H
b) P
c) Ca
d) Ar
e) Cl
f) Al
Lewis Structures of atoms and
ions
Yellow flame
with Na salts
like NaCl,
NaBr
Potassium (K) –
Violet flame
Isotopes of an element account for the
average atomic mass
Isotopes- Atoms of the same element can have the
same number of protons, BUT different numbers
of neutrons.
Atoms with a few too many neutrons, or not quite
enough, can sometimes exist for a while, but
they're unstable.
How can we calculate the Average
Atomic Mass for an atom?
Atomic mass or atomic weight - the average mass
of atoms of an element, calculated using the
relative abundance of naturally-occurring
isotopes of an element.
Remember
The mass on the periodic table is the average of all
the naturally occurring isotopes of that element.
C = 12.011
238U
with an abundance
of 99.28%
235U
has an abundance
of 0.72%
Carbon’s Naturally Occurring Isotopes
Carbon
Naturally Occurring Isotopes
98.89% of C atoms are 12C 1.108% of C atoms are 13C
Σ (mass of isotope × relative abundance)
Average mass of C with 98.89% C-12 and
1.108% C-13
***Remember to divide the percentages by 100
98.89%/100% = .9889
1.108%/100% = .01108
Mavg= [(.9889)(12) + (.01108)(13)]
Mavg = [(11.8668) + (0.14404)]
Mavg = 12.011 amu
Chemistry: Average Atomic Mass
Ions

Ions: neutral atoms that have either gained or
lost electron(s). Number of protons and neutrons
remains the same.

Cation: atom loses 1 or more electrons to become
a Positively charged ion.

Anion: atom gains 1 or more electrons to become
a Negatively charged ion.
Which has a larger atomic radius,
the neutral atom or the ion? Explain.