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Atomic Structure, Theories
and Periodic Table
Structure Of An Atom
Parts
Found in Nucleus
Protons
Neutrons
Found in shells or orbits
Electrons
Protons
Found in the nucleus
Positively charged
Determines the identity
of the element.
Mass = 1 amu
Neutrons
Found in nucleus
Neutral charge
Mass = 1amu
Electrons
Found in the shells/orbits around the
nucleus
Negatively charged
Determines chemical
behavior
Mass = 1/1832 amu

(negligible mass)
Neutral Atoms
Number of protons = Number of
electrons
# of Positive charges = # of Negative
charges
 Example:
A neutral atom of Neon has 10 protons and 10 electrons.
A neutral atom of chlorine has 17 protons and _____ electrons.
Atomic Number
 Atomic number = number of protons
Found on the periodic table
Protons determine the identity of the
element
Example:
If boron has an atomic number of 3 then it has 3 protons.
a. If argon has an atomic number of 18, then it has ___ protons?
b. What element has an atomic number of 20?
Mass Number
 Mass number = number of protons + number of neutrons
 # Neutrons = Mass # - Atomic #
 No mass numbers found on the periodic table
Example:
If an atom has 3 protons and 4 neutrons it has a mass number of
7.
If an atom has a mass number of 11 and has 5 protons, it has ___
neutrons.
If an atom has a mass number of 15 and an atomic number of 7,
how many protons does it have? ____ How many neutrons does
it have? ____
Give the number of protons and neutrons
for the following atoms.
Symbol
Protons
Electrons
Neutrons
IONS
 Number of protons ≠ Number of electrons
Positive Ion
# of Positive charges > # of Negative charges
4 protons and 2 electrons
 Negative Ion
# of Positive charges < # of Negative charges
9 protons and 10 electrons
Development of Atomic Models
Philosophies and Discoverers of the Parts
(Subatomic Particles) of the Atom:
 Electron- Crookes, JJ Thomson, Millikan
 Proton- Rutherford
 Neutron- Chadwick
The Atom
The discovery of the atom is a progression
Many scientists contributed to the development of
present day atomic theory.
With each new discovery dealing with the nature
of the atom, a new atomic model was
constructed.
 The first person to believe in the existence of the
atom was Democritus in 400 B.C.
Democritus
400 BC
Democritus-suggested
the existence of
atoms without the
support of
experimental
evidence.
Philosopher 2: Aristotle
 very influential
 disagreed with Democritus
 helped disprove Democritus’ theory of atom
 this caused the denial of the existence of
atoms for the next 2000 years
John Dalton
1785-1844
English school
teacher that
performed
experiments to
correct his atomic
theory
Believed all atoms
are indivisible
spheres
Dalton’s Atomic Theory
 John Dalton- proposed the first Atomic Theory
a. All elements are composed of indivisible
particles called atoms.
b. Atoms of the same element are identical
c. Atoms of different elements can mix
physically or combine chemically in whole
number ratios to form compounds
d. Chemical reactions occur when atoms are
separated, combined or rearranged
however, atoms are never changed into
another atom because of a chemical
reaction.
Eugene Goldstein
In 1886 used a
modified
cathode ray to
discover canal
rays and
protons
John Thompson
Discovered the
electron in 1897
Passed electric
current through gases
at low pressure with
as cathode ray tube
Developed the Plum
Pudding Model of an
atom
Millikan
1909 performed Oil
Drop Experiment
Determined the
charge to mass ratio
of an electron and
then determined the
mass of an electron
Ernest Rutherford
1909 Performed the gold foil
experiment and concluded that an
atom is mainly empty space with a
tiny densely-packed positive nucleus
The Proton-- Rutherford
 Using a gold foil
experiment, he aimed a
beam of radiation through
a piece of gold foil, he
expected the beam to go
straight through. Instead,
some of the radiation
bounced off.
 Developed the nuclear
atom model
 Discovered the existence
of the nucleus
 Discovered the proton in
the nucleus.
Neils Bohr
1913 Developed
the Planetary
Model of an
atom with
electrons
orbiting the
nucleus in
specific orbits
Erwin Schrödinger (1926)
 Quantum mechanics
 electrons can only exist in specified energy
states
 Electron cloud model
 orbital: region around the nucleus where eare likely to be found
James Chadwick (1932)
 Discovered neutrons
 neutral particles in the nucleus of an atom
 Joliot-Curie Experiments
 based his theory on their experimental
evidence
The Periodic Table
 There are 116 elements on the periodic table.
 How many different types of atoms are there?
 116
 How do all of these atoms/elements differ from one another?
 They differ in the # p+, # n°, # e
–
 The periodic table is organized left to right, top to bottom by increasing
atomic number.
 Horizontal rows– periods
 Columns– families/groups
 The atoms of each element contain a unique positive
charge
 Atoms are neutral (0 charge) because they are
balanced by a + and – charge.
# e-- = # p+ = atomic number
How many p+ and e-- are in each of the following?
a. B
b. Ra
c. Pt
d. Mg
e. An atom contains 66 e-- . What is it?
f. An element contains 14 p+ . What is it?
Atomic Mass Number and Isotopes
Atomic Mass number
Is the average of all masses of isotopes combined
Is equal to #p+ + #n° in the nucleus
To find the number of n° :
= atomic mass # - atomic #
= atomic mass # - #p+
= atomic mass # - #e-
Isotopes
 are atoms with the same number of p+, but different #n°. The number in the
isotope name gives the atomic mass of the isotope. Potassium-39 and
potassium-40 are isotopes.
Isotopes all have basically the same chemical behavior.
 Why?
 Chemical behavior depends upon e-. Isotopes all have the same # of e-.
Ex. potassium- 39
 Has an atomic mass of 39.
 What is the #p+ and #n° in the nucleus?
 #p+ = 19, #n°= 20
Ex. potassium-40
 Has an atomic mass of 40.
 What is the #p+ and #n° in the nucleus?
 #p+ = 19, #n°= 21
Isotopes (find p+, e-, n0)
Element
Neon
Atomic
Number
10
Atomic Mass
Number
22
Calcium
20
46
Oxygen
8
17
Iron
26
57
Zinc
30
64
Mercury
80
204
Ex. 1
Using % Abundance to Find Atomic Mass
Contribution
ISOTOPE
 Isotope 6X
 Isotope 7X
MASS
%Abund.
6.015amu
7.016amu
7.5%
92.5%
 6X: mass contribution= (mass) (% abundance)
= (6.015 amu)(.075)
6 X=
.451 amu
 7X: mass contribution= (mass) (% abundance)
= (7.016 amu)(.925)
= 6.490 amu
 Now add the numbers together:
= .451amu + 6.490 amu
= 6.941 amu
 Element is Lithium.
 Boron has 2 naturally occurring isotopes: boron-10
(abundance=19.8%, mass=10.013 amu), boron11(abund.= 80.2%,mass=11.009amu). Calculate the
atomic mass of boron.
 Calculate the atomic mass of Mg. The 3 Mg isotopes
have atomic masses and abund. of 23.985amu (78.99%),
24.986amu (10.00%), and 25.982amu (11.01%).
Radioactive Decay
 Why don’t atoms of one element change into atoms of another element
during chemical reactions?
 Because chemical reactions involve an atoms e-. The nucleus remains
unchanged.
 What determines the identity of an atom?
 The number of protons in an atom.
Nuclear Reactions
 There is another type of reaction that involves the nucleus
undergoing change; this reaction involves one radioactive
element changing into another element- nuclear reactions.
 Radioactive atoms undergo significant changes that can alter
their identities.
There are three types of radioactive decay:
 Alpha radiation (α)- made up of 2p+
and 2n0; is equivalent to a He
nucleus
4 He
2
 Beta radiation (β)- made up of fast
moving electrons called beta
particles.
0 β
-1
 Gamma radiation (γ)- made up of
high energy radiation that possess
no mass or electrical charge.
0 γ
0
C. Isotopes
 Atoms of the same element with different numbers of neutrons.
 Isotope symbol:
Mass #
Atomic #
“Carbon-12”
12
6
C
C. Isotopes
© Addison-Wesley Publishing Company, Inc.
C. Isotopes
 Average Atomic Mass
 reported on Periodic Table
 weighted average of all isotopes
Avg.
(mass # )(# of atoms)  (mass # )(# of atoms)
Atomic 
total # of atoms
Mass
C. Isotopes
 EX: About 8 out of 10 chlorine atoms are chlorine-35. Two out of 10 are chlorine37.
Avg.
(35 u)(8 atoms)  (37 u)(2 atoms)
 35.4 u
Atomic 
10 atoms
Mass
Periodic Table
atomic number # of protons
mass number
-atomic number
# of neutrons
8
O
15.999
round to 16 - mass
number ( # protons
& neutrons)
unrounded –mass
number (average
mass of the
isotopes)
Masses of Atoms
 A scale designed for atoms gives their small
atomic masses in atomic mass units (amu)
 An atom of
12.00 amu
12C
was assigned an exact mass of
 Relative masses of all other atoms was
determined by comparing each to the mass of
12C
 An atom twice as heavy has a mass of 24.00 amu.
An atom half as heavy is 6.00 amu.
Atomic Mass
 Listed on the periodic table
 Gives the mass of “average” atom of each element
compared to 12C
 Average atom based on all the
isotopes and their abundance %.
 Atomic mass is not a whole number
due to isotopes.
Na
22.99
Isotopes
 Isotopes – atoms of the same element with different
numbers of neutrons.
Oxygen-16 Oxygen-17
16
Oxygen-18
17
8
p+
‗‗‗‗
e-
‗‗‗‗
nº
‗‗‗‗
18
8
8
‗‗‗‗
‗‗‗‗
‗‗‗‗
‗‗‗‗
‗‗‗‗
‗‗‗‗
Calculating Average Atomic Mass
 Percent(%) abundance of isotopes
 Mass of each isotope of that element
 Weighted average =
mass isotope1(%) + mass isotope2(%) + …
100
100
Atomic Mass of Magnesium
Isotopes Mass of Isotope
24Mg
= 24.0 amu
25Mg
26Mg
= 25.0 amu
=
26.0 amu
Abundance
78.70%
10.13%
11.17%
Atomic mass (average mass) Mg = 24.3 amu
Mg
24.3
 Finding An Isotopic Mass
Naturally occurring boron is 80.20% boron11 (atomic mass 11.0 amu) and 19.80% of a
different isotope of boron. What must the
mass of this isotope be if the average atomic
mass of boron is 10.81 amu?
Radioactivity
 Mosely’s X-ray analysis of atoms was an attempt to
explain radioactivity.
 1896 – Henri Becquerel – Uranium spontaneously emits
energy.
 1898 – Marie & Pierre Curie – first isolated a radioactive
element - Radium
The Periodic Table
A. Dmitri Mendeleev
 Dmitri Mendeleev (1869, Russian)
• Organized elements by
increasing atomic mass.
• Predicted the existence of
undiscovered elements.
B. Henry Mosely
 Henry Mosely (1913, British)
• Organized elements by
increasing atomic number.
• Fixed problems in Mendeleev’s
arrangement.
A. Metallic Character
1
2
3
4
5
6
7
Metals
Nonmetals
Metalloids
B. Table Sections
1
2
3
4
5
6
7
Representative Elements
Transition Metals
Inner Transition Metals
B. Table Sections
1
2
3
4
5
6
7
Overall Configuration
Lanthanides - part of period 6
Actinides - part of period 7
C. Columns & Rows
1
2
3
4
5
6
7
Group (Family)
Period
A. Terms
 Periodic Law
• Properties of elements repeat
periodically when the elements
are arranged by increasing
atomic number.
A. Terms
 Valence Electrons
 e- in the outermost energy level
 Atomic Radius
 First Ionization Energy
• energy required to remove an efrom a neutral atom
B. Periodic Trends
 Atomic Radius
• Increases to the LEFT and DOWN.
1
2
3
4
5
6
7
B. Periodic Trends
 First Ionization Energy
• Increases to the RIGHT and UP.
1
2
3
4
5
6
7
B. Periodic Trends
 Which atom has the larger radius?
•Be or Ba
Ba
•Ca or Br
Ca
B. Periodic Trends
 Which atom has the higher 1st I.E.?
•N or Bi
N
•Ba or Ne
Ne
B. Periodic Trends

Group # = # of valence e- (except He)


Families have similar reactivity.
Period # = # of energy levels
1A
1
2
3
4
5
6
7
8A
2A
3A 4A 5A 6A 7A
C. Dot Diagrams
 Dots represent the valence e-.
 EX: Sodium
 EX: Chlorine