Download Chapter 4.1

Chapter 4.1
Atomic Theory
Democritus Positives
World made of two things
1. Empty space
2. Small particles
2. Atoms were the smallest
possible particles
3. Different atom for each
substance
1.
Democritus Negatives
1.
2.
Changes in matter were the
result of changes in the
groupings of atoms and not
changes in the atoms
themselves
Very general, not supported
experimentally
Aristotle
 Contradicted
Democritus on
the theory that empty space
could not exist
 Since Democritus could not
experimentally support his
theory, Aristotle's reputation
killed the idea of the atom for
1600 years
Isaac Newton & Robert
Boyle
 Renewed
support of
Democritus
 Still no experiments or
predictions
Dalton’s Atomic Theory

Positives
1. All matter composed of
atoms
2. All elements have different
atoms
Dalton’s Atomic Theory
3.
4.
Atoms combine in simple
whole number ratios to form
compounds
In a reaction atoms are
separated, combined or
rearranged
Dalton’s Atomic Theory

Negatives
1. Atoms are the smallest unit
of matter
2. All atoms within an element
are the same
Nanotechnology
 Manipulating
individual atoms
in order to create new
products
Chapter 4.2
Subatomic Particle
History
Electron
 Thomson
 Discovered
the electron by
using a cathode ray tube to
compare the ratio of charge
to mass
 Determined mass was less
than Hydrogen
Electron

–
Importance of Thomson’s
Experiment
1. There must be particles smaller
than the atom
2. Dalton’s theory was wrong
Not believed at first, but further
experiments proved him right
Electron

Mendeleev’s Response
 No internal structure but
Thomson had discovered
a new element
Thomson’s New Element
1.
2.
3.
Atomic mass 1/1,000,000 of H
Traveled at 1398 miles/second
Capable of moving freely through
space
Electron
 Millikan
 Determined
the electron had
a negative charge
Electron
 Millikan
 Determined
the mass of the
electron
 Mass of electron is 1/1840
of H
-28 g
  mass equals 9.1x10
Atomic Models
 Thomson’s
Plum
Pudding
Model
Rutherford’s Model
 Beginning
of
the modern
model of the
atom
 Atom is mostly
empty space
Rutherford’s Model
 Nucleus
(Gold foil experiments)
Centrally located in the atom
 Positively charged
 Extremely dense – virtually all the
mass of the atom
 Electrons are held to the nucleus
by the electromagnetic force

Questions still to be
resolved
1.
2.
3.
How are the electrons arranged
around the nucleus?
How do atoms of different
elements differ in structure?
What accounts for the mass of
the elements since the mass of
an electron is so small?
Proton

Discovered by Henry Moseley
 Shot x-rays at samples of
elements and found that
wavelength decreased with
increasing atomic mass
 Decreased wavelength was
caused by an increase in number
of electrons
Proton
 Carries
the opposite but equal
charge to the electron
Neutron
 Bothe
& Chadwick
 Energy particles in the atom
with no charge
 Mass identical to the proton
Modern Atomic Theory
1.
2.
Atoms are composed of a
nucleus with electron(s)
circling and held in place by
protons
Protons and electrons are
identical in number in a
neutral atom
Modern Atomic Theory
3. Nucleus
1. Positively charged
2. Composed of protons and
neutrons
3. 99.97% of the mass of the
atom
Chapter 4.3
Periodic Table
Information
Atomic Number
 Number
of protons in the
nucleus
 Moseley determined atomic
number for each element
Re-ordered Mendeleev’s table
 Experimental methods were used
to support/refute claims of new
elements

Isotopes



Atoms of an element with the
same number of protons but
different numbers of neutrons
(p.100)
Most elements exist as a mixture
of isotopes
All react the same chemically and
physically
Mass Number
 Sum
of the protons and
neutrons in an element
 Recorded in the upper left
quadrant
i.e. K-39 = 39K
 # N0 = Mass # – Atomic #
i.e. K-39 = 39-19 = 20 N0
Atomic Mass
 Weighted
average of the mass
of all the isotopes of an
element
 Majority of the mass is in the
nucleus
Atomic Mass
 Measured
in atomic mass
units(amu’s)
 1 amu = 1/12 mass of a C-12
atom
 1 amu = approximate mass
of a proton or neutron
Calculating Atomic Mass
1.
2.
Mass of each isotope multiplied by
the percentage of the time it
occurs
Add the products derived for each
isotope
Cl-35 = 76%
26.6
Cl-37 = 24%
8.9
Cl = 35.5 amu
Chapter 4.4
Radioactive Decay
Nuclear Reaction
 Any
reaction that involves a
change in the nucleus of an
atom
 Changes the identity of the
element
Radioactivity
1.
2.
3.
Spontaneous emission of
radiation
Atoms gain stability by losing
energy through radioactive
decay
Radioactive decay continues
until a non-radioactive stable
element is formed
Types of Radiation

Alpha 
• Equivalent to a He nucleus
+
0
 2 P and 2 N emitted
 Overall 2+ charge
Types of Radiation
 Alpha

 Mass number decreases by 4

Atomic number decreases by 2
Types of Radiation

Beta 
 E- emitted from the nucleus


Generated by the decay
of a N0
N0 decay every 930 s
Types of Radiation
 Beta

 Mass number does not change

Atomic number increases by 1
Types of Radiation
 Gamma

 High energy x-rays
 Usually accompanies the
other two types of radiation
 Accounts for the bulk of the
energy emitted
Nuclear Equation
1.
2.
3.
Representation of radioactive
decay
1. 226Ra  222Rn + 
2. 14C  14N +  (-1 )
3. 238U  234Th +  + 2
Conservation of mass and atomic
numbers
Stability determined by ratio of N0/ P+
Strong Force
 Force
that holds N0 & P+ in the
nucleus
 Effective over only very short
distances ( 10-3 pm)
 100,000 times stronger than
gravity