Download The Structure of an Atom

The Structure of an Atom
Ch.4 Notes
Section 1: Early Theories of Matter
•Atom- the smallest particle of an element that retains the
properties of the element.
•Size? The world’s population is 6,840,000,000
•The number of atoms in a penny is
29,000,000,000,000,000,000,000
Early Theories of Matter
• Ancient Greek philosopher Democritus speculated matter
was made up of atomos
• The theory was rejected for 2000 years due to being
challenged by Aristotle
Discovery of the Electron
• John Dalton- proposed his atomic theory in
1803 based on his research that was not
completely accurate
• Atoms can be subdivided
• Atoms of the same element can have slightly
different masses
Discovering the Electron
•Discovering the electron
•Sir William Crookes - used a cathode ray tube to do experiments
on the atom based on the relationship between mass and charge.
Electrons are negatively charged
Discovering the Atom
•JJ Thomson (1856-1940)- in the 1890’s found a
cathode ray could be deflected by magnetic and
electric fields. He concluded the ray was a stream of
negatively charged particles dislodged from an atom
with a mass much less than a hydrogen atom.
http://www.yout
ube.com/watch?
v=GzMh4q2HjM
Discovering the Atom
•Robert Millikan (1868-1953) – in 1909 he accurately
determined the mass of an electron through the Oil
Drop Experiment, it had a charge of -1
•The actual mass of an electron is 9.1 x 10-28 g
Nucleus
• Plum Pudding Theory-( aka the chocolate chip cookie
model)
• Matter is neutral, so Thomson proposed: spherical
atoms have uniform (+) charge with (-) electrons
embedded throughout.
Nucleus
http://www.youtube.com/wa
tch?v=wzALbzTdnc8
• Ernest Rutherford, in 1911
• To test the theory, he aimed a thin beam of (+) charged
Alpha particles at a thin sheet of gold.
• He expected the alpha particles to pass through with little
deflection
• Instead, most passed through, but few bounced back and
deflected at big angles.
• He concluded atom is mostly empty space with large (+)
charged nucleus in center
Atomic Summary
*Nucleus
-protons + neutrons
-contains all the mass
-positive charge
*Electrons
-located in the cloud
-fast moving
-very small
Properties of Subatomic Particles
Particle Symbol Location
Relative
Charge
Mass
Electron
e
Electron
Cloud
-1
Proton
+
p
Nucleus
+1
1
Neutron
0
n
Nucleus
none
1
1/1840
Actual
Mass
9.11 x 10-28
1.673 x 10-24
1.675 x 10-24
How atoms differ:
•Atomic number is how an atom is identified, it is equal to the
number of protons in an atom, & determines the location of the
atom in the periodic table.
•The periodic table is arranged left to right & top to bottom by
increasing atomic number.
atomic #
Chemical Symbol
Average atomic mass
1
H
1.008
Atomic Number
Number of protons in nucleus of an element
This number identifies the element
Periodic table is arranged in sequence of
increasing atomic numbers
Atoms are neutral: (+) charges must equal (-) charges
We know: atomic # = #protons = # electrons
•How many protons and electrons are there in:
•Be: p= 4
e= 4
•F: p= 9 e= 9
Ions
• Charged particles: Number of electrons has
changed- proton # stays the same
• Be2+ has lost 2 electrons:
p= 4 e= 2
• F1- has gained one electron:
p=9
e=10
Isotopes
•To identify an isotope a number is added to the end of the
name, it is called a mass number.
•Mass Number = # of protons + # of neutrons
•Ex: Carbon-14, Neon-22, Potassium-41
• Isotopes
• All atoms of same element have
some number of protons
• Most have different numbers of
neutrons
Mass Number
• An isotope is identified by its mass number
• Mass number= number of protons + number of neutrons
• Number of neutrons = mass number – atomic number
Atomic Mass
• Atomic mass units (amu) = ½ the mass of a Carbon-12 atom
• Atomic mass given in Periodic Table is not given in whole
numbers
• Atomic mass is the weighted average of the atomic masses of
all naturally occurring isotopes. This weighted average takes
into account the mass and abundance of each isotope
Atomic Mass
• Average atomic mass= mass * abundance for each
isotope: then add
Copper-63
Copper-65
Number of protons
29
29
Number of neutrons
34
36
Atomic mass
62.930 amu
64.928 amu
Abundance
69.17%
30.83%
• Mass Contribution = mass x abundance
• Cu-63= (62.930)(.6917)= 43.53
• Cu-65= ( 64.928)(.3083) =20.02
• 43.53+20.02= 63.55