STRUCTURE OF AN ATOM
A brief review of the structure of atoms
HISTORY OF ATOMIC THEORY


See the Powerpoint on Atomic Theory Scientists for more
information and pictures/models.
Review/Summary:
Democritus, 460 BC – proposes particles so small they can not
be destroyed or divided, and describes them as “atomos”
(uncuttable)
 John Dalton, early 1800’s - first to use “atom”; proposes that
atoms of the same element are identical, and atoms of
different elements have different weights/masses; Solid
Sphere Model
 JJ Thomson, ~1897 – discovers the first subatomic particle,
electrons; Plum Pudding Model
 Ernest Rutherford, ~1912 – discovers the proton, and nucleus;
Nuclear Model
 Niels Bohr, ~1915 – discovers energy levels for the electrons;
Planetary Model
 Erwin Schrodinger, ~1926 – developed the Electron Cloud
Model
 James Chadwick, ~1932 – discovers the neutron in the
nucleus


Most Modern Model – The electron cloud model with
all three subatomic particles
DEVELOPMENT OF ATOMIC THEORY

A more extensive review of Atomic Theory
Development:

http://www.abcte.org/files/previews/chemistry/s1_p1.
html
Use the “next” button at the bottom of the webpage
above to cycle through the different scientists and their
theories/models.
WHAT IS AN ATOM?

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Smallest particle of an element (pure
substance) that still has the properties of that
element.
Most of the atom is made of EMPTY
SPACE

Example: an atom of gold is the smallest particle
of gold (an element or pure substance) that still
has the properties of gold.

Video: http://www.youtube.com/watch?v=o3I1JGW-Ck

Brainpop (username: sclee


password: cougars):
Atom:
http://www.brainpop.com/science/matterandchemistry/at
oms/
Atomic Model:
http://www.brainpop.com/science/matterandchemistry/at
omicmodel/
TWO MAIN REGIONS OF THE ATOM

Nucleus
Positive charge
 Almost all of the mass of an atom
 Tiny, dense center of the atom
 Can be broken apart in chain reactions to produce
HUGE amounts of energy for use in electricity,
medicine, weapons, etc (nuclear energy)


Electron cloud
Negative charge
 Almost 0 mass
 Energy levels/shells

Max # electrons – 2,8,8*
 Valence level – outermost energy level
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THE NUCLEUS OF AN ATOM
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Tiny and dense

Center of the atom
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Contains protons and neutrons

Positive charge (positive protons plus neutral neutrons)

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Very little of the volume (space) in an atom, but almost all of the
mass
 Mass = protons + neutrons
 Units for mass are amu (atomic mass unit)
Held together by a strong nuclear force
Can be split apart in controlled chain reactions to release nuclear
energy:
 http://www.youtube.com/watch?v=ufEAg9h9wU8
 http://www.youtube.com/watch?v=mBdVK4cqiFs
 Fermi is the first scientist to conduct controlled chain
reactions
 Nuclear energy in weapons:
http://www.brainpop.com/science/energy/radioactivity/
Video tutorial: http://www.youtube.com/watch?v=FSyAehMdpyI
ELECTRON CLOUD
Most of the volume in the atom, but almost 0
mass
 Contains only electrons
 Negative charge
 Energy levels

Closest to the nucleus is the lowest level
 Outermost level is the valence level

SUBATOMIC PARTICLES
Protons
 Neutrons
 Electrons


Video tutorial:
http://www.youtube.com/watch?v=FSyAehMdpyI
ISOTOPES
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Atoms of the same element can have different
numbers of neutrons.

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Naming isotopes: use the name of the element,
followed by the mass number of that atom
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
This means their mass is also different (mass =
protons + neutrons)
Ex: Carbon-12, Carbon-13
Video Brainpop:
Isotopes:
http://www.brainpop.com/science/matterandchemistr
y/isotopes/
 Radioactivity:
http://www.brainpop.com/science/matterandchemistr
y/periodictableofelements/

IONS
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In a neutral atom, the number of protons equals the
number of electrons.
Neutral atoms are often unstable, because they do not have
a full valence level.
 Unstable atoms will gain or lose electrons to become
stable.
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Atoms which have gained electrons (gained negative
particles) are negative ions. They will have more
electrons than protons.
Atoms which have lost electrons (lost negative
particles) are positive ions. They will have fewer
electrons than protons.
To see if it is an ion, compare protons and electrons.
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Video Tutorial Brainpop:



Ions:
http://www.brainpop.com/science/matterandchemistry/ions/
PERIODIC TABLE ARRANGEMENT

Dimitri Mendeleev developed the first (near)
modern Periodic Table of Elements
Page 71 in your book
 Based on physical properties, especially atomic mass
(later rearranged by atomic number)

Periods are rows, and tell how many energy
levels the neutral atom has; elements increase
left to right across the periods and are very
different in properties/reactivity;
 Groups or families are columns; atoms in the
same group have the same number of valence
electrons, and similar chemical reactivity.

CHEMICAL REACTIVITY ON THE PERIODIC TABLE
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Metals on the left (2/3 of the table), metalloids on the break,
metals on the right (except hydrogen).
Most reactive metals – group 1, with 1 valence electron;
 Most reactive nonmetals – group 17, with 7 valence electrons;
 Nonreactive (inert) elements – group 18, with full valence level

METALS, NONMETALS, METALLOIDS ON THE
PERIODIC TABLE
Property
Metals
Metalloids
Nonmetals
Luster (shine)
Yes
No
No
Malleable
Yes
No (brittle)
No (brittle)
Ductile (wires)
Yes
No
No
Conductivity
Yes
Semi-
No
State at rom
temp.
Solid (except
mercury)
Solid
Solid, liquid, or
gas
PERIODIC TABLE TRENDS
Atomic Mass Increases
Atomic number increases
Atomic number increases
Atomic Mass Increases
Metallic Properties Increase