Download The Atom

The Structure
of the Atom
Chapter 4
4.1 - Early Theories of the Atom
4.2 - Subatomic Particles
4.3 - How Atoms Differ
4.4 - Unstable Nuclei & Radioactivity
– CHEMISTRY 112
History of the atom
• Not the history of atom, but the idea of the
atom
• Original idea Ancient Greece (400 B.C..)
• Democritus and Leucippus Greek
philosophers
• Atomos - not to be cut
Another Greek
•
•
•
•
•
•
•
Aristotle - Famous philosopher
All substances are made of 4 elements
Fire - Hot
Air - light
Earth - cool, heavy
Water - wet
Blend these in different proportions to get all
substances
•
•
•
•
•
•
•
•
Who Was Right?
Greek society was slave based
Beneath Famous to work with hands
did not experiment
Greeks settled disagreements by
argument
Aristotle was more famous
He won
His ideas carried through middle ages.
Alchemists change lead to gold
4.1 Early Theories of Matter
• Dalton’s Atomic Theory
1. All matter is composed of extremely
small particles called atoms.
2. All atoms of a given element are
identical. Atoms of different elements
are different from one another.
3. Atoms cannot be created or divided into
smaller particles or destroyed.
4. Different atoms combine in simple
whole number ratios to form
compounds.
5. In a chemical reaction, atoms are
separated, combined, or rearranged.
Dalton’s Atomic Model
Atom - the smallest
particle of an element
that retains the
properties of the
element.
4.1 Early Theories of Matter
• E. Goldstein discovered the proton in 1886.
• J.J. Thomson discovered the electron in
1897 during cathode ray tube experiments in
the late 1890’s.
Thomson’s Experiment
Voltage source
-
+
Vacuum tube
Metal Disks
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source

+
Passing an electric current makes a
beam appear to move from the negative
to the positive end
Thomson’s Experiment
Voltage source

+
Passing an electric current makes a
beam appear to move from the negative
to the positive end
Thomson’s Experiment
Voltage source

+
Passing an electric current makes a
beam appear to move from the negative
to the positive end
Thomson’s Experiment
Voltage source

+
Passing an electric current makes a
beam appear to move from the negative
to the positive end
Thomson’s Experiment
Voltage source
• By adding an electric field
Thomson’s Experiment
Voltage source
+
 By adding an electric field
Thomson’s Experiment
Voltage source
+
 By adding an electric field
Thomson’s Experiment
Voltage source
+
 By adding an electric field
Thomson’s Experiment
Voltage source
+
 By adding an electric field
Thomson’s Experiment
Voltage source
+
 By adding an electric field
Thomson’s Experiment
Voltage source
+
 By adding an electric field he found that
the moving pieces were negative
Thomson’s Model
• Found the electron
• Couldn’t find
positive (for a
while)
• Said the atom was
like plum pudding
• A bunch of positive
stuff, with the
electrons able to
be removed
4.1 Early Theories of Matter
• Robert A. Millikan determined the mass and
charge of the electron in 1916.
– one unit of negative charge
– mass is 1/1840 of a hydrogen atom
4.1 Early Theories of Matter
• In 1911 Ernest Rutherford discovered the
nucleus during his gold foil experiment.
4.1 Early Theories of Matter
Ernest Rutherford’s gold foil experiment.
Lead
block
Florescent
Screen
Uranium
Gold Foil
He Expected
• The alpha particles to pass through without
changing direction very much
• Because
• The positive charges were spread out evenly.
Alone they were not enough to stop the alpha
particles
What he expected
Because
Because, he thought the mass
was evenly distributed in the atom
Because, he thought
the mass was evenly
distributed in the
atom
What he got
+
How he explained it
• Atom is mostly empty
• Small dense,
piece
• Alpha particles
deflected by
close
+
enough
positive
at center
are
it if they get
Density and the Atom
• Since most of the particles went through, it was
mostly empty.
• Because the pieces turned so much, the
positive pieces were heavy.
• Small volume, big mass, big density
• This small dense positive area is the nucleus
4.1 Early Theories of Matter
• Neils Bohr developed
the planetary model of
the atom
– Electrons are in a particular
path have a fixed energy
– Energy level-region around a
nucleus where the electron is
likely to be moving
4.1 Early Theories of Matter
• Erwin Schrodinger
developed the Quantum
Mechanical Model
– Describes the electronic
structure of the atom as the
probability of finding
electrons within certain
regions of space
4.1 Early Theories of Matter
• James Chadwick discovered
the neutron in 1932.
• In 1913 Henry Mosley used
X-rays to count the number of
protons in the atomic nuclei of
different atoms.
4.2 Subatomic Particles & the
Nuclear Atom
• Located within the Nucleus
– Proton (p+)
• Positively charged particle (each carries a charge of +1)
• Relative mass = 1 amu
• Actual mass = 1.673 X 10-27 kg
– Neutron (n0)
•
•
•
•
Neutrally charged particle
Relative mass = 1 amu
Actual mass = 1.675 X 10-27 kg
Serves as the glue that holds the nucleus together as well as a buffer
between the charges of protons and electrons
Subatomic Particles
• Located outside the nucleus in the electron
cloud
– Electron
•
•
•
•
Negatively charged particle (each carries a charge of -1)
Relative mass = 1/1840 amu
Actual mass = 9.11 X 10-31 kg
The electron is the part of the atom that will function in
bonding and reactions
4.3 How Atoms Differ
• Atomic Number
– the number of protons in the nucleus of an atom
– indicated at the top of the element’s block on the
periodic table
8
Oxygen has an atomic number of 8
O
There are 8 protons in an atom of Oxygen
15.999
12
Magnesium has an atomic number of 12
Mg
24.305
There are 12 protons in an atom of Magnesium
Isotopes
• Atoms of the same element with the same number of
protons, but different numbers of neutrons
• Since the atoms have different numbers of neutrons,
they also have different mass numbers
– Mass number = # of protons + # of neutrons
Abbreviating Isotopes
• Hyphen Notation
– Simply write the Name of the atom, put a
hyphen, and then write the mass number
• Carbon-12 vs. Carbon-14
– Carbon 12 has 6 protons and 6 neutrons
– Carbon 14 has 6 protons and 8 neutrons
• Nuclear Designation
– Element symbol is written in the center
– Mass number goes in the upper left corner
– Atomic number goes in the lower left corner
12
C
6
Different Isotopes
• Identify the number of protons, neutrons, and
electrons each of the following have.
35
Br
p+: ________ no: ________ e-: ________
79.904
Boron-10
Boron 11
35
17
p+: ________ no: ________ e-: ________
p+: ________ no: ________ e-: ________
p+: ________
Cl
no:
________
e-: ________
p+: ________
66Zn
30
no: ________
e-: ________
Calculating Atomic Mass
• Mass Number
– the number of protons + neutrons in a given isotope
• Atomic Mass
– The weighted average mass of all of the isotopes of
that element
[(Mass of isotope A)(percent abundance )] + [(Mass of isotope B)(percent abundance)]
Practice Calculating
Atomic Mass
Calculate the atomic mass of helium given the
following information:
There are two naturally occurring isotopes of helium:
Isotope
% Abundance Mass
helium-3
0.0001
3.0160
helium-4
99.9999
4.0026
Practice Calculating
Atomic Mass
There are two naturally occurring isotopes of helium:
Isotope
% Abundance
Mass
helium-3
0.0001
3.0160
helium-4
99.9999
4.0026
(3.0160 x 0.000 001) + (4.0026 x 0.999999)
0.000 003 0160 + 4.002595997
0.000 003 0160 + 4.0026
= 4.002603016
= 4.0026
Practice Calculating
Atomic Mass
There are three naturally existing isotopes of silicon:
silicon-28, silicon-29, and silicon-30. Their percents of
natural abundance is listed respectfully: 92.21 %,
4.70 %, and 3.09 %.
Calculate the average atomic mass of silicon and
express your answer in 4 significant digits.
4.4 Unstable Nuclei &
Radioactive Decay
• Nuclear Reactions
– reactions that involve a change in the nucleus of an atom.
• Radioactivity
– the spontaneous release of radiation.
• Radiation
– rays and particles emitted by radioactive materials
• Radioactive atoms emit radiation because their nuclei
are unstable.
• There are three main types of radiation
– Alpha decay
– Beta decay
– Gamma decay
Alpha (α) radiation
• two protons and 2 neutrons
• Positive charge
• Symbols:


4
4
2
2
He
• reduces the atomic number by 2
• reduces the mass by 4
Beta (β) radiation
• Fast moving electron
• Negative charge
0
• Symbols:

-1
• increases the atomic number by 1
• does not change the mass
Gamma (γ) radiation
• high energy radiation
• released with alpha and beta radiation
• symbol:

0

0
• does not change the mass or atomic number
Half lives
• The time it takes for 1/2 of the mass of the isotope to
be decayed.
• If I have a 60g sample and the half life is 2 years, how
long will it take for there to be 7.5g left of the sample?
60g  30g  15g  7.5g
2 years
4 years
6 years
So, it takes 6 years for the 60g sample to decay into 7.5g.