Transcript Document 7617536

Atomic
Structure
Early Theories of Matter
• Science as we know it did not exist
several thousand years ago
• Most philosophers thought that matter
was made up of air, water, earth, and
fire
Democritus (460-370 BC)
• The first to propose that matter was not
infinitely indivisible
• Believed that matter was made up of atomos
(or atoms)
• He also said that atoms could not be created
or destroyed
Democritus’ Theory
1. Matter is composed of empty space through
which atoms move
2. Atoms are solid, homogeneous, indestructible,
and indivisible
3. Different atoms have different sizes and shapes
4. The differing properties of matter are due to the
size, shape, and movement of atoms
5. Changes in matter result from changes in the
groupings of atoms and not the atoms
themselves
Democritus
• Democritus was on the right track, but had no
scientific data to back up any of his claims
• The foremost thinker of the day, Aristotle,
rejected Democritus’ ideas because he
believed that nothingness could not exist
• No other theories of the atom came about
until nearly 2000 years later
John Dalton
• John Dalton was the next scientist to
propose a theory about the atom in the
19th century
Dalton’s Atomic Theory
1. All matter is composed of extremely small particles
called atoms
2. All atoms of a given element are identical, having
the same size, mass, and chemical properties.
Atoms of a specific element are different from other
elements
3. Atoms cannot be created, destroyed, or broken into
smaller particles
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 Theory
• This was a HUGE step toward the
atomic model, but was not totally correct
• Atoms ARE divisible into smaller
subatomic particles
• Atoms of a given element can have
different masses
Basic Definitions
• Atom – smallest unit of an element that
retains the properties of that element
• Atoms are made up of several
subatomic particles called protons,
neutrons, and electrons
Protons, Neutrons, & Electrons
• Protons – have a +1 charge and are
found in the nucleus of the atom
• Neutrons – have no charge and are
also found in the nucleus of an atom
• Electrons – have a -1 charge and are
found on the outside of the nucleus
• Nucleus – made up of protons and
neutrons, has an overall + charge
Atomic Structure
JJ Thomson
• JJ Thomson used the cathode ray
experiment to determine the charge to
mass ratio of an electron.
• He identified the first subatomic particle,
the electron
• He proposed the plum pudding model of
the atom
• Credited for discovering the electron
Robert Millikan
• Millikan is noted for his famous
Millikan’s Oil Drop Experiment
• This experiment determined the charge
and the mass of an electron
Earnest Rutherford
• Rutherford’s Gold Foil Experiment helped to
determine the existence of the nucleus
• Rutherford proposed that the nucleus was
small, dense and positively charged
• Proposed the nuclear atomic model which
stated that there was a nucleus with a
positive charge and electrons around the
outside
James Chadwick
• Chadwick showed that the nucleus also
contained neutrons
• He is credited for the discovery of the
neutron
Atomic Numbers
• The atomic number of an element is
the number of protons in the nucleus of
an atom of that element.
• It is the number of protons that
determines the identity of an element,
as well as many of its chemical and
physical properties.
• The number of protons for an element
CANNOT be changed.
Atomic Numbers
• Because atoms have no overall electrical
charge, the number of protons must equal
the number of electrons.
• Therefore, the atomic number of an
element also tells the number of electrons
in a neutral atom of that element.
• The number of electrons can be changed
when determining the charge of an ion.
Masses
• The mass of a neutron is almost the same as
the mass of a proton.
• The sum of the protons and neutrons in the
nucleus is the mass number of that particular
atom.
• Isotopes of an element have different mass
numbers because they have different
numbers of neutrons, but they all have the
same atomic number (number of protons)
Isotopes
• When writing isotopes, the atomic number (or
number of protons) will appear at the bottom left of
the formula
• The mass number (number of protons plus neutrons
will appear at the top left of the formula.
• The element symbol will appear to the right of the
numbers
• The different number of neutrons has NO bearing on
chemical reactivity
Writing the Names of Isotopes
• When writing the name of an isotope,
you will write the name of the element –
the mass number
• For example 126 C would be named:
• Carbon - 12
Try the following
Name
Symbol
# Protons
# Neutrons
1
2
# Electrons
Mass #
25
55
Carbon – 11
197
Au
79
Oxygen - 15
Try the following
Name
Symbol
# Protons
# Neutrons
# Electrons
Mass #
Carbon – 11
11
6
5
6
11
79
118
79
197
1
2
1
3
C
6
Gold - 197
Hydrogen –
3
197
Au
79
3
H
1
Manganese 55
55
Mn
25
25
30
25
55
Oxygen - 15
15
8
7
8
15
O
8
Atomic Mass
• Atomic mass –the
weighted average
mass of all the
naturally occurring
isotopes of that
element.
• The number is
usually located at the
bottom of the periodic
table and has
decimal places
Calculating Atomic Mass
Calculating Atomic Mass
• Copper exists as a mixture of two
isotopes.
• The lighter isotope (Cu-63), with 29
protons and 34 neutrons, makes up
69.17% of copper atoms.
• The heavier isotope (Cu-65), with 29
protons and 36 neutrons, constitutes the
remaining 30.83% of copper atoms.
Calculating Atomic Mass
• First, calculate the contribution of each
isotope to the average atomic mass,
being sure to convert each percent to a
fractional abundance.
Calculating Atomic Mass
• The average atomic mass of the
element is the sum of the mass
contributions of each isotope.
Try this one…
Calculate the atomic mass of germanium.
72.59 amu
You can tell many things from an
isotope formula
• Hydrogen has three naturally occurring
isotopes in nature: Hydrogen – 1,
Hydrogen – 2, and Hydrogen – 3.
– Which is the most abundant in nature?
• Hydrogen – 1
– Which is the heaviest?
• Hydrogen - 3