Download All atoms of the same element are the same This is not actually

Mr. Shields
Regents Chemistry
U01 L04
1
Dalton’s Atomic Theory
Recall that one of Dalton’s postulates said:
All atoms of the same element are the same
This is not actually correct. Let’s see why.
2
Nucleons
All nuclei of the same element MUST have the
same number of Protons. Why?
The proton is the nucleon that determines atomic
number and thus the identity of the element
The electrons balance the positive charge of the
nucleus and it’s the electron that determines the
elements chemical properties
-
+
-
+
=0
3
Mass Number
Only protons determine an elements chemical properties
Because it alone causes a change in the # of electrons.
Therefore – Changing the number of neutrons WILL NOT
change the chemical properties of an atom.
- It WILL CHANGE however the atoms Mass Number
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Mass Number
•
So What’s not correct about Dalton postulate that
All atoms of the same element are the same)?
• Atoms of the same element can have different
nos. of neutrons. SO THEY ARE NOT THE
SAME!
– Example:
12C 13C 14C
or
234U 235U 238U
– No. of Neutrons = 6, 7, 8 and 142, 143, 146
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Isotopes
•
Atoms of the same element that have different nos. of
Neutrons are said to be ISOTOPES of that element
•
The Chemical properties of all isotopes of an element
are the same i.e. they react exactly the same
•
However, Physical properties of isotopes may not be
the same. For example mass, and
Rate of diffusion.
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Isotopes
•
All elements found in nature are MIXTURES of
isotopes
•
Isotopes of an element are designated by mass
number in either of two ways
– 1: 23592U
– 2: U-235 or Uranium-235
•
Using the above how would we designate the
isotope of carbon that has 8 neutrons?
14 C
6
or C-14 (Carbon-14)
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Isotopes
•
Only HYDROGEN has special names for its
isotopes
•
•
•
•
1
1H
1 proton
0 neutrons
Hydrogen
2H
1
3H
1
1 proton
1 proton
1 neutron 2 neutrons
Deuterium Tritium
This isotope is Radioactive
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