Download Atomic Structure - Tumwater School District

Atomic Structure
Chemistry
Chapter 4
The Atom
• The smallest particle of an
element that still behaves
like that element
– Ex) The smallest bit of gold
that still has the intensive
properties of gold.
• Started with the philosophy
of Democritus
– Atoms are indivisible and
indestructible!
Dalton buys the Democritus hype.
• John Dalton uses experiments to come up
with 4 ideas about the atom
1. Elements are composed of indivisible atoms.
2. All atoms of the same elements are identical.
Atoms of different elements must be different.
3. Atoms of different elements can be combined
together in ratios to form compounds.
4. Chemical reactions occur when atoms are
separated, attached, or rearranged. But atoms
aren’t changed, only rearranged.
What are atoms made of?
What are atoms made of?
• Pens!
The Parts of the Atom
• Three particles have been
discovered to be part of
the atom
– Protons
– Electrons
– Neutrons
• PENs!
– Okay, not really, but
hopefully it will help you
remember.
• http://www.youtube.com/watch?v=IdTxGJjA4J
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Thomson’s Discovery
• Thomson used electricity to
create a stream of particles
(cathode ray tube)
– He could deflect their flow
with positively charged plates
• The stream was pulled towards
positively charged objects
– Must be negatively charged!
– Thompson had discovered that
atoms MUST have a negative
particle.
• Electrons
• http://www.youtube.com/watch?v=XU8nMKk
zbT8&feature=related
More about the electron…
• The mass must be very small (1/1840th of the
mass of hydrogen)
• The charge must always be the same
– Regardless of the element
• Must be part of all elements
• http://www.youtube.com/watch?v=wzALbzTd
nc8&feature=channel&list=UL
The electrons get friends!
• Rutherford shot positively-charged
alpha particles at gold
• Most went straight through
• But some bounced straight back, or sideways
– There must be a solid positively charged
“kernel” at the center
• “The nucleus” made of protons
• Chadwick discovers that there is also
uncharged particles that make it heavier
– “Neutrons!”
• Have basically the same mass as protons
The atom gets a “shape”
• Basically all empty space
• Nucleus at the center
–
–
–
–
Very, very, very small
Positively charged protons
Neutrally charged neutrons
99.995% of the mass
• Electrons revolving around the
outside
– Zooming near the speed of light,
with very little mass
– Require more space than the
nucelus
If time permits
• http://www.youtube.com/watch?v=xqNSQ3O
QMGI
Atoms!
• Dalton was right that
different elements would
have different atoms.
– But some different atoms
are actually the same
elements!
• What determines the
element? THE NUMBER OF
PROTONS!
– Oxygen is oxygen because it
has 8 protons.
– Nitrogen is nitrogen because
it has 7 protons.
Atomic Number
• The number of protons is
called the “atomic number”
– Elements are separated and
arranged specifically by their
atomic number
• No two atoms with different
atomic numbers can be the
same element!
– EVER!
• EVVVVVVVVVVVVAAAAR!!!!!!
• For neutral atoms, the protons
and electrons are equal
Mass Number
• Mass number is how “heavy” the
nucleus of an atom is
– The number of protons plus neutrons
• The two sub-atomic particles that have a
large mass
• Protons + Neutrons = Mass
– Neutrons = Mass – Protons
• The mass number doesn’t affect
what element it is
– EX) Hydrogen can have a mass number of 1,
2 or 3, but it’s atomic number is ALWAYS 1!
Mass number
Okay, lets try some!
• I have 14 protons.
– What element am I?
• I also have 15 neutrons.
– What is my mass?
Okay, lets try some!
• I have 14 protons.
– What element am I? Silicon!
• I also have 15 neutrons.
– What is my mass? 29
• Therefore I am Silicon-29
How about another one?
• My mass is 14, my atomic number is 6.
– What element am I, and how many neutrons do I
have?
How about another one?
• My mass is 14, my atomic number is 6.
– What element am I, and how many neutrons do I
have?
• I am CARBON-14, and I have 8 neutrons
–14-6 = 8!
Isotopes
– Atoms of the same element MUST
have the same number of protons
• The number of neutrons can be
different in an element.
– Ex) Carbon-12
• 6 protons, 6 neutrons
– Ex) Carbon-14
• 6 protons, 8 neutrons
• Isotopes of elements have
different numbers of neutrons,
and therefore different
numbers of neutrons
– But Isotopes are still the same
element, just different masses
Consider the following…
• Mass number is just a COUNT of the number
of subatomic particles that have mass.
– Ex) 9 protons + 10 neutrons is 19 masses
• It gives you a very close, but general idea of
the mass of an atom.
– But it is not exact.
Atomic Masses Units (amu)
• The mass of protons and
neutrons is not always exactly 1
» (but really close)
• The only one that they are is
Carbon-12, and everything else is
based off that.
Mass = 14.003241
• Neutrons are (usually) a little
heavier than 1, protons less
• So we calculate based on
“atomic mass units”, which is
based off of Carbon-12=12.000
• Then we calculate the mass of
all the other atoms (sorry)
So, atomic mass is affected by…
•
•
•
•
The element
The number of protons compared to neutrons
The number of electrons (barely)
But as a general rule it is the protons plus the
electrons-ish
(Average)
Atomic Mass
• We can’t fit ALL the
isotopes on a periodic table
– and we don’t need to
because the isotopes are
usually all mixed together
with each other.
• So, what we do is take the
AVERAGE amu, and use
that as the mass of an
element.
Calculating average AMU
• You could find every single atom in the universe and get
the mass, then divide by the number of atoms.
• Or you could determine what fraction of the atoms
are which isotope, and determine the average.
– “Weighted average”
• Average AMU (with isotopes A, B, and C) =
– (% of A)(mass of A) + (% of B)(mass of B) + (% of C)(mass of C)
• % MUST BE WRITTEN AS A DECIMAL
– EX) 65% = 0.65
– This is where the significant digits of the AMU come from.
Calculating average AMU practice
• Carbon has 3 major isotopes.
– Actually, it has 16 known, but 13 are very, very, very rare
– Carbon 12
• AMU: 12.00 (repeating)
• 98.928%
– Carbon 13
• AMU: 13.00335483781
• 1.071%
– Carbon 14
• AMU: 14.0032419894
• ~0.000 000 000 001%
• Don’t forget to turn % into decimal before doing math!