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Atomic History and Structure
Chapter 4
Early Theories of The Atom
DemocritusDemocritus (b. c. 460 BC; d. c. 370 BC) postulated the
existence of invisible atoms, characterized only by quantitative properties:
size, shape, and motion. Imagine these atoms as indivisible spheres, the
smallest pieces of an element that still behave like the entire chunk of matter.
Dalton
Thomson
“The PLUM PUDDING MODEL”
Rutherford
Rutherford’s Experiment
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Visit this website and click on the tutorial for
“Section 3.2 Rutherford Experiment” Play and
watch through all 5 parts
Visit this website and see what Rutherford’s
Experiment looked like.
What is an ATOM?
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Atom- the smallest particle of an element
that retains the properties of that element
Subatomic particles
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Protons – Positive charge, found in the nucleus
and have a mass of 1 amu. ( Identify)
Neutrons- No charge, found in the nucleus, and
have a mass of 1 amu ( Isotopes)
Electrons- Negative charge, found in the energy
levels outside of the nucleus, have relatively no
mass ( Ions)
By the Numbers…
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Atomic number –
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This determines the elements position in the
periodic table ( Identifies the atom)
In atoms not ions
Atomic # = # protons= # of electron
Mass number
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Mass number = # protons + # neutrons
Why are electrons not included in the mass of an
atom?
You will not look to the periodic table to
determine the mass number, the number on
the periodic table is an average.
Isotopes
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An Isotope is the same element (at.#) with
a different number of neutrons.
When naming an isotope you write name of
the of the element, dash, then write the
mass number
Symbol of element
Example: Carbon-14
Atomic number
from periodic
table
14
6
C
More about the mass
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Knowing the mass number you can
determine the number of neutrons in a
specific atom
# of neutrons = mass number - # of protons
atomic mass unit (amu)- the mass of the
carbon atom.
Isotopic Notation
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Isotope notation
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C
C
Determine the number of protons, electrons
and neutrons in an isotope.
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Example #1
Carbon-14
Carbon-12
Examples
H
H
What number is different?
H
Check your work!
Calculating Atomic Mass
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Atomic mass- weighted average mass of the
isotopes of that element.
This is the decimal number on the periodic table.
To determine the atomic mass you must know what
percent of each isotope of the element is found in
nature and then it can be calculated.
Example: For Chlorine
25% is chlorine-37
75% is chlorine – 35
What is the average atomic mass of chlorine?
To Calculate:
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25% is chlorine-37
75% is chlorine-35
1. Take mass 37 x .25 = ans A
2. Take mass 35 x .75 = ans B
3. Add ansA + ansB = Average Atomic mass
for Chlorine
Ions-a charged particle
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An atom that has either gained or lost
an electron. Electrons are lost and
gained to make ions
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When they are gained (-Neg) ions (anion)
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When they are lost (+ Pos) ions (cations)
Ion Examples
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Example:
What is the charge of an ion that has 11
protons and 10 electrons. Write the isotope
notation for this atom.
Tell the number of P, E and N in the following
ions.
24
12
Mg
2
p=12
n=12
e=10
15
7
N
-3
p=7
n=8
e=10
History of Periodic Table
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Dmitri Mendeleev – listed
the elements in several
vertical columns in order of
increasing atomic MASS.
He left blanks in the table
for elements that were not
discovered yet.
Henry Mosley – Arranged
elements on the periodic
table in order of increasing
atomic NUMBER.
http://www.chemheritage.org/EducationalServices/chemach/ppt/lm04.html
Arrangement of Periodic Table
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Periodic law – the properties of elements are
periodic functions of their atomic number
Periods – horizontal rows; 7; correspond to
energy levels
Groups/Families – vertical columns; “group
A” Roman numerals correspond to the
number of valence electrons
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Group I – Alkali metals, Group II – Alkaline earth
metals, Group VII – Halogens, Group VIII –
Noble gases
Metals, Nonmetals, and Metalloids
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Metals – everything to the left of the stairstep;
including aluminum; does not include hydrogen
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Properties: Have luster (shiny), good conductors of heat
and electricity, malleable ( able to be pounded into
sheets), ductile (able to be pulled into a wire), tend to
lose electrons in chemical reactions, most are solids
Transition metals – middle block over to stairstep
Inner transition metals – bottom 2 rows; sometimes
called “lanthanide series” and “actinide series”
Metals, Nonmetals, and Metalloids
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Nonmetals – everything to the right of the
stairstep; includes hydrogen
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Properties – Dull, poor conductors, brittle, tend to
gain or share electrons in chemical reactions,
most are gases
Metalloids – either side of the stairstep; does
not include aluminum
Periodic Table colored to show metals,
nonmetals, and metalloids
http://hyperphysics.phy-astr.gsu.edu/hbase/pertab/pertab.html
Answers to practice problems
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14
6
Example
12
6
C
The mass
number
differs due to
the
difference in
neutrons!
Isotopes of Hydrogen
1
1
H
2
1
H
3
1
C
H
Back to isotopic notation