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Atomic Theory
Atomic Theory
• Democritus-
440 B.C.
• He proposed that if you kept cutting something
in half you would eventually end up with an
“uncutable” particle. Which he called an atom
• Greek: Atomos - indivisible
• Aristotle-
338-322 B.C
• He disagreed with Democritus. He believed
you would never end up with an indivisible
particle.
Atomic Theory
• Dalton-
1808
• In search of understanding why elements
combine in specific ratios he discovered
through his experiments that this happens
because they are made of individual atoms.
• His theory states 3 things
1- Atoms are small and cannot be
created, divided or destroyed.
2- Atoms of the same element are all alike
3-Atoms join with other atoms to make
new substances
Atoms
Atomic Structure
The Atom
• Atoms are extremely small.
• Atoms contain a nucleus.
• Atoms are made of subatomic particles
• Protons
• Neutrons
• Electrons
How small is small?
• If you could enlarge a penny until it
was as wide as the continental United
States(about 3000miles), each of its
atoms would be only about 3 cm in
diameter- about the size of a ping pong
ball.
Protons
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Positively charged particles (+).
Found in the nucleus.
All protons are identical
Each proton= 1 amu (atomic mass unit)
Every atom of an element has the same
number of protons.
Neutrons
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Particles that have no charge
Found in nucleus
All neutrons are identical
Protons & Neutrons are the most
massive particles in the atom but
located in a very small area (nucleus)
Electrons
• Negatively charged particles (e-)
• Located outside of the nucleus in rings or
levels called atomic clouds
• Their mass is so small that it is usually
considered zero.
• It takes more than 1,800 electrons to equal the mass of
one proton.
• However electrons occupy most of an atoms volume.
Some Things to Remember!
• In an atom, the number of protons equals the
number of electrons. As a result, the positive
charge from protons equals the number of
electrons which makes the atom neutral.
• All atoms of an element have the same
atomic number.
Periodic Table Key
Atomic Number
• Atomic # = the number of protons in
an atom.
Atomic Mass
• Atoms are too small to be measures in
everyday units of mass (grams).
• Scientists use units knows as atomic
mass units (amu).
• A proton or a neutron has a mass equal to
about 1 amu.
• Atomic Mass is equal to the sum of
protons and neutrons in an atom.
Isotopes
• The number of neutrons in an atom can
vary.
• Atoms with the same number of
protons and a different number of
neutrons are called isotopes.
• We identify an isotopes by the
atoms mass.
Isotopes
Arrangement of Electrons
• The first ring can only hold 2 electrons
Arrangement of Electrons
• The 2nd and 3rd rings can both hold 8
electrons