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Atomic Structure
• The smallest part of an element is an
ATOM
• All atoms consist of subatomic particles:
– Protons
– Neutrons
– Electrons
Nucleus
• The “core” of the atom
• Contains 99.9% of the mass of the atom
• Protons and neutrons are found in the
nucleus of an atom
Mass Number = mass of the nucleus or
mass of protons + neutrons
Protons
• Positively charged particles
• The number of protons identifies the
element
• 1 proton = 1 atomic mass unit (amu)
# protons = Atomic Number (whole number)
Neutron
• Neutral Particles
• Found in the nucleus
• 1 neutron = 1 amu
# neutrons = Mass number – Atomic Number
Electrons
• Negatively charged particles
• Found in the electron cloud
• Electrons have a very small mass, so we
don’t even count it!
• Electrons give atoms their chemical
properties
• Atoms are neutral so;
# electrons = # protons
Mass Number
• mass # = protons + neutrons
• always a whole number
• NOT on the
Periodic Table!
+
Electrons
eNucleus
e-
ee-
ee-
+
+
+ + +
Nucleus
Carbon-12
Neutrons 6
Protons
6
Electrons 6
Neutron
Proton
Shorthand Notation
Mass number
SYMBOL OF ELEMENT
Atomic number
Isotopes
• Atoms of the same element with different
mass numbers.
• Different number of neutrons & different
mass
• Nuclear symbol:
Mass #
12
Atomic #
6
• Hyphen notation: carbon-12;
carbon-13
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
C
Isotopes
+
Electrons
Nucleus
+
+
Neutron
+ + +
Proton
Proton
Nucleus
+
Carbon-12
Neutrons 6
Protons
6
Electrons 6
+
Nucleus
+
+
Neutron
+
Electrons
+
Carbon-14
Neutrons 8
Protons
6
Electrons 6
Nucleus
6Li
7Li
3 p+
3 n0
3 p+
4 n0
2e– 1e–
2e– 1e–
Neutron
Electrons +
Nucleus
+
+
Proton
Neutron
Electrons +
Nucleus
+
+
Nucleus
Nucleus
Lithium-6
Neutrons 3
Protons
3
Electrons 3
Lithium-7
Neutrons 4
Protons
3
Electrons 3
Proton
17
Cl
Isotopes
37
• Chlorine-37
– atomic #:
17
– mass #:
37
– # of protons:
17
– # of electrons: 17
– # of neutrons: 20
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
37
17
Cl
Average Atomic Mass
• weighted average of all isotopes
on the Periodic Table
• round to 2 decimal places
Avg.
(mass)(%) + (mass)(%)
Atomic =
100
Mass
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Calculating Average Atomic Mass
You will be given the mass number and the
Percent Abundance (how common the
isotope is in nature) of the isotope
Atomic mass = (% abundance in decimal form
isotope1)(mass isotope1) +
(%abundance in decimal form isotope2)(mass isotope2)
Average Atomic Mass
• EX: Calculate the avg. atomic mass of oxygen if its
abundance in nature is 99.76% 16O, 0.04% 17O, and
0.20% 18O.
Avg.
(16)(99.76) + (17)(0.04) + (18)(0.20)
16.00
Atomic =
=
amu
100
Mass
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Example
• Carbon has 6 protons and 6 neutrons,
what is Carbon’s mass number?
C-12
• An isotope of Carbon has 6 protons and 8
neutrons, what is carbon’s mass number?
C-14
Remember the number of protons NEVER
changes for an element!
Isotope Review
• Same element different number of
NEUTRONS
• Different Mass number
• Same Atomic number, therefore the
same number of protons and electrons
Isotope Applications
• Complete the read around the room
station activity
Atomic Structure
• ATOMS
– Differ by number of protons
• IONS
– Differ by number of electrons
• ISOTOPES
– Differ by number of neutrons
Electrons
• Electrons give elements their chemical
properties
• The arrangement of the electrons will
determine how it reacts with other
substances
• Gain electron the atom becomes negative
(anion)
• Lose an electron the atom becomes
positive (cation)
Humor
Two atoms are walking down the street.
One atom says to the other, “Hey! I think I lost an electron!”
The other says, “Are you sure??”
“Yes, I’m positive!”
A neutron walks into a restaurant and orders a couple
of sodas. As she is about to leave, she asks the waiter
how much she owes. The waiter replies, “For you,
No Charge!!!”
Electrons are found in the
electron cloud
The cloud has regions of space called
energy levels
• The first energy level holds 2 electrons
• The second energy level holds 8
electrons.
• The third energy level holds 18 electrons
Valence Electrons
• Are found furthest from the nucleus
• Dictate the physical and chemical
properties of an element
• Use the periodic table to determine the
number of valence electrons.
• All atoms want 8 valence electrons
Lewis Dot Diagram
• A way to illustrate the number of valence
electrons
– Use one dot for each valence electron
– Place the dot around each side of the symbol
before pairing the electrons
– The symbol represents the nucleus plus all
the inner electrons for the element.
Examples
H
O
N
F
Ne
Electrons and Light
• Electrons are normally in the ground state
• When the atom is given energy the
electrons move to the excited state.
• When the electrons lose this energy they
fall back to the ground state and emit light.
• Each element has a unique emission
spectrum
Electromagnetic Spectrum
• Electromagnetic Radiation –
– A broad range of energetic emissions
– made up of photons
• Photons – bundles of energy
– Travel like waves
– Move at the speed of light = 3.0 x 108 m/s
– Electromagnetic waves do not require a
medium to move
Parts of the wave
• Amplitude – the height of the wave
• Wavelength – the distance between the
two successive waves
• Frequency – the number of waves that
pass a given reference point per second
• Wavelength = lamda unit is nanometer
• Frequency = nu in units of 1/s or s-1
What is the difference between
these waves?