Download Atomic Structure

Atomic Structure
Atom
Definition – smallest particle of an element
that retains the chemical identity of that
element
Term proposed by Democritus in 450 BC
Democritus proposed that all matter is
composed of tiny, indivisible particles
Rejected by Aristotle because Democritus
could not explain how particles “stuck
together”
Dalton’s Atomic Theory of Matter- 1803
 Based on observations made by scientists
during the 1700’s, John Dalton proposed a
theory that could explain the properties of
matter
 Four postulates:
1.
2.
3.
4.
Each element is made of small particles called atoms
All atoms of a given element are identical, but different
from those of any other element
Atoms are neither created nor destroyed in a chemical
reaction
A given compound always has the same relative
numbers and kinds of atoms
Are the four postulates all true?
What do we know about atoms that means
we might need to re-state one of the
postulates?
Postulate Number 2 – not all atoms of the
same element are the same
You can have different Isotopes of the
same element
Example: Iodine-125 and Iodine-127
are both iodine, but are different from each
other. Iodine-125 is radioactive and Iodine127 is not.
History of Discovering about Atomic
Structure
Dalton thought atoms were hard and
round like tiny marbles
Early 1800’s –
Franklin- two kinds of electric charges
(positive and negative)
Electrons were First Particles Found
An English scientist discovered electrons
and named them. An Amercian scientist
found their mass.
Mass of an electron to be 9.11 x 10-28 grams
Protons were next particles found
Atoms are electrically neutral so the
negative electrons must be balanced by
an equal number of positive particles
An English scientist found these particles
and called them protons.
He also discovered these positively
charged particles were all concentrated in
the center of the atom – called it the
nucleus
Modern Atomic Theory
Atoms are composed of protons, neutrons,
and electrons
At first, scientists visualized electrons
orbiting the nucleus much like planets
orbiting the sun
Last particle found was the neutron
Neutrons have a mass approximately
equal to that of a proton
Neutrons have no charge
Neutrons help “glue” the nucleus together
Fundamental Subatomic Particles
Particle Location
Proton
Charge
Mass (amu)
Nucleus
+1
1
Neutron Nucleus
0
1
Electron Outside
Nucleus
-1
0
Moseley’s Discovery
Found that atoms of each element contain
a unique positive charge in their nucleus
Conclusion: atom’s identity comes from
the number of protons in the nucleus
Called “Atomic Number”
 Neutral atoms – positive charges must
equal negative charges
Conclusion – for neutral atoms, no. of
electrons equals no. of protons
Information in Periodic Table
7
N
Atomic Number (sometime not in center)
Chemical Symbol
Nitrogen
14.0067
Atomic Mass
Mass Number
 Hydrogen -1
The 1 after the hydrogen is the mass number. Used
to help keep track of what isotope you have.
 Definition
Sum of the neutrons and protons
Always a whole number
 Another way to represent
Mass Number
1
2
H
1
Atomic Number
3
H
1
H
1
Mass of an Atom
Atomic Mass Unit – approximately the
weight of one neutron or proton
Used because the weight in grams is very
small
1 a.m.u. = 1.66 x 10-24 g.
Element’s atomic mass
 Weighted average of all of the isotopes of
an element
7
N
Nitrogen
14.0067
Atomic Mass
Nitrogen contains mostly Nitrogen-14 with a small amount
of Nitrogen-15
Practice
Find sodium (Na) on the periodic table I
gave you
Find the atomic number. What does that
number mean?
Find the atomic mass. What does that
number mean?
How many electrons does one sodium
atom have?
I have one atom of sodium represented by
46
V
23
How many protons does it have?
How many neutrons does it have?