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Chapter 4
The Structure of the Atom
Chapter 4
Democritus: Greek philosopher coined the word
“atom”. Greek word comes from “atomos”
meaning indivisible.
Antoine Lavoisier: Law of Conservation of Matter
Joseph Proust: Law of Constant Composition
(aka Law of Definite Proportion)
John Dalton’s Atomic Theory
All matter is made up of atoms, which are
indivisible.
All atoms of the same element are
identical, but differ from those of other
elements.
Atoms are neither created nor destroyed in
any chemical reaction.
A given compound always has the same
relative numbers and kinds of atoms.
Dalton Model of the Atom
Atoms are indivisible
No subatomic
particles have been
discovered!
Atoms of each
element are identical
Isotopes unknown at
this time!
1803!
Electricity
Ben Franklin: studied
static electricity;
objects could have
negative or positive
charge.
The word electricity
comes from elektron
(Greek for amber)
Static electricity is
charge in motion.
Discovering Atomic Structure
Opposite charges
attract
Like charges repel
Crookes: studied
behavior of gases in a
vacuum tube.
Cathode Ray Tube
Thomson: Discovered Electrons
He used a magnet in the CRT to deflect the rays
away from the negative pole of the magnet (rays
were pulled toward the positive pole).
No deflection without the magnet, so the rays
must be negative.
Using other gases in the tube gave the same
effect, so it is characteristic to all elements.
Thomson discovered electrons.
He thought the positive charges were like
pudding and that electrons were like raisins!
Plum Pudding Model
Millikan’s Oil Drop Experiment
He is known for measuring the charge on
an electron (from which he calculated the
mass of an electron).
He concluded the charge on an electron
was 1.60 x 10-19 coulombs
He used a charge/mass ratio to calculate
the mass of an electron as 9.109 x 10-28 g.
Millikan’s Experiment
Rutherford’s Gold-Foil Experiment
He discovered the positively charged
nucleus by shooting positive alpha
particles through gold foil. Most went
through, but some were deflected.
Since some of the particles deflected
backwards, it had to get close to
something positive; the nucleus is positive;
protons were discovered.
Rutherford
He concluded the
most dense part of
the atom was in the
center, which he
called the nucleus.
Most of the atom was
empty space (shells).
Rutherford’s Lighter Side
Rutherford Model
The Neutron
Discovered in 1932
by James Chadwick
(Coworker of
Rutherford)
Same mass as proton
No electrical charge
Also found in the
nucleus
Subatomic Particles
Particle
Location
Mass
Charge
Proton
Nucleus
1 amu
+1
Neutron
Nucleus
1 amu
0
Electron
Outside
Nucleus
1/1836
amu
-1
Radioactivity
Henri Becqerel
accidently discovered
uranium exhibits
radioactivity.
Marie and Pierre
Curie isolated radium
(Ra) and polonium
(Po) which are both
radioactive.
Types of Radiation
Modern Atomic Theory
Atoms consist of subatomic particles:
protons, neutrons, and electrons.
Atoms are not permanently changed
during chemical reactions, merely
rearranged.
Atoms can be changed to other elements,
but not by ordinary chemical reactions
(nuclear).
Cont.
Atoms of the same element are not all
alike (masses may differ-ISOTOPES).
Atomic Number (Z)
An atom’s identity comes from the number
of protons in its nucleus.
Atomic number is defined as the number
of protons (which is also the number of
electrons) in an atom’s nucleus.
Atoms are electrically neutral.
Atomic # = # protons = # electrons
(abbrev. Z)
Mass Number (A)
The total number of protons and neutrons
of an isotope.
# neutrons = mass # - atomic #
or A - Z
Isotopes
Atoms of the same element with the same
number of protons, but different number of
neutrons.
Ex. 3 isotopes of hydrogen
Hydrogen 1 (protium)
1 p+, 0 no, 1 eHydrogen 2 (deuterium) 1 p+, 1 no, 1 eHydrogen 3 (tritium)
1 p+, 2 n0, 1 eradioactive!
It gets heavier!
Designating Isotopes
Hyphen notation- the mass # is added with
a hyphen to the name of the element. Ex.
Carbon – 12
**LOOK UP THE ATOMIC # on the periodic
chart to find protons, (which is = to the
electrons). Carbon -12 has 6 protons, 6
electrons, and 6 neutrons (12-6 = 6).
Isotopes, cont.
Nuclear symbol- the atomic # is added to
the symbol for the element as a subscript
to the LEFT and the mass # is added as a
superscript on the left.
Ex. 1737Cl How many protons, neutrons
and electrons does this isotope of chlorine
have?
Ex.
How many protons, neutrons, and
electrons does uranium -238 have?
Ions
An atom that has gained or lost one or
more electrons.
An ion may have a positive or negative
charge.
Cations are positively charged because
they lose electrons.
Anions are negatively charged because
they gain electrons.
Example
Calculate the charges ION Protons Electrons
on the following ions
and indicate whether
35
36
the ions are cations or X
anions.
Y
3
2
Z
20
18