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Structure of the Atom
The idea of the atom goes
back to Ancient Greece.
We continue by looking at
more “recent” ideas of the
atom.
Dalton’s Atomic Theory (1808)
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All matter is made of atoms (particles to small
to see).
Each element has its own kind of atom.
Atoms of the same element have the same
mass, atoms of different elements have
different masses.
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Compounds are formed when different
atoms link to form “compound atoms”
(now called molecules). In a compound
all “compound atoms” are alike.
Atoms cannot be created or destroyed.
Modern Atomic Theory
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Even though Dalton’s work was 200
years ago, the basis of it is still valid
with only a few modifications.
Dalton thought atoms were solid
spheres like billiard balls, but we know
atoms are made up of smaller parts
Sub Atomic Particles
(smaller than atoms)
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Atoms are made up of three main type
of particles:
Protons
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Found in nucleus (center part) of atom
Positive charge (+1)
Mass 1 atomic mass unit (a.m.u.)
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Neutrons
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Found in nucleus of atom
Neutral ( 0 charge)
Mass 1 a.m.u.
Electrons
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Found moving outside the nucleus
Negative charge (-1)
Mass approx 1/1800 a.m.u.
Draw this in on front page of your notes
Atomic Number
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Number of protons in the atom
Each atomic number is unique and
defines each atom
E.g. If there are 8 protons, the element
is…
oxygen!
If the element is oxygen, there must be
8 protons
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The atomic number is found at the top
of each square on the periodic table.
Atomic number
See Periodic Table Here
If you have a periodic table you can
instantly find the number of protons.
Number of Electrons
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In any neutral (uncharged) atom the
number of protons is always equal to
the number of electrons.
The number of electrons, therefore, is
also equal to the atomic number.
If there are 8 protons, there will be 8
electrons.
Mass Number
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Total number of all protons and
neutrons in an element
always a whole number
Note - electrons are too small to
effect total mass
An element with 3 protons and 4
neutrons will have a mass number of 7
Mass Number
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To find mass number from a periodic
table, look under the symbol (or name)
Round off to a whole number (decimals
will be explained soon!)
See periodic table
Mass number = 12
(rounded off)
Number of Neutrons
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Can be determined from the periodic
table.
If mass number = protons + neutrons,
then:
Neutrons = mass number - protons
or
Neutrons = mass number - atomic
number
Calculating Protons, Neutrons
and Electrons
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Find number of protons, neutrons and
electrons for each of the following.
Carbon
Na
Element #5
Assignment
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Do PNE Worksheet
Isotopes
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Isotopes are atoms of the same
element that have different masses.
Why are these masses different??
Since number of protons doesn’t
change, the change in mass must be
due to different number of neutrons.
16 amu
17 amu
8 protons 8 neutrons
8 protons 9 neutrons
The Isotopes mass is written
next to it’s name.
16 amu
Oxygen - 16
17 amu
Oxygen - 17
The Isotopes are also
expressed as follows.
17 amu
16 amu
Oxygen - 16
16
8
O
Oxygen - 17
17
8
O
Example
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Hydrogen exists in 3 different isotopes
Most hydrogen atoms have one proton, (one electron)
and no neutrons
Mass is 1 a.m.u.
This is also called Hydrogen-1 or Protium
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Some Hydrogen atoms have one neutron
as well as the one proton and electron.
This is a different isotope - it is still
hydrogen, but it has a mass of 2 a.m.u.
This isotope is called Hydrogen-2
Also called Deuterium
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A third form of hydrogen has 2
neutrons with 1 proton and 1 electron.
This gives a mass of 3 a.m.u.
This is called Hydrogen-3
Also called Tritium
Atomic Mass
(Decimal Number)
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The mass reported on the periodic table
is actually a weighted average of the
masses of all the isotopes of an
element.
This is why the periodic table show
decimal number.
For most atoms, one isotope is much
more common than the other(s).
Assignment
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Do Isotope Worksheet