If opposite charges
attract, why are the
negative electrons
not attracted to the
positive nucleus?
Electrons are
always moving.
Electrons have
inertia in motion
= momentum.
If electrons are always
moving and they weigh
very, very little, why
don’t they get thrown
away out of the atom?
Because of the
electromagnetic
force!
Forces inside the atom
This force of attraction binds the electrons to
the atom. The electromagnetic force becomes
stronger as the distance between charges
becomes smaller.
If opposite charges
attract, how do the
protons get attracted
with the neutrons?
Because of the
strong nuclear
force!
Forces inside the atom
Protons and neutrons in the nuclei of atoms are
held together by the strong force.
If the atomic mass/weight is
just the sum of the protons
and neutrons, why are there
decimal values for the
atomic weight written on
the periodic table?
Atomic Weight is an Average!
It is the weighted average of the individual atomic
weights, or mass numbers, of the isotopes.
Example
• Chlorine = 35.48 (atomic weight)
Isotopes of Chlorine
Chlorine-35 = 76% abundance
Chlorine-37 = 24% abundance
Σ(mass number x %abundance)
Example
• The mass of Isotope 1 is 6.015 amu and
its abundance is 7.5%
• The mass of Isotope 2 is 7.016 amu with a
92.5% abundance.
• What is the average atomic mass?
• What is the element?
The
Periodic
Table
Triads!
Johann
Dobereiner
(1829)
Calcium, Strontium,
Barium
Chlorine, Bromine,
Iodine
The elements in a triad had similar chemical
properties and orderly physical properties.
Octaves!
John
Newlands
(1863)
After arranging the elements in order of
increasing atomic mass, certain properties
repeated every 8th element.
Increasing
atomic mass!
Lothar
Meyer
(1867)
Increasing
atomic
mass!
Dmitri
Mendeleev
(1869)
“Father of the
Periodic
Table”
Both scientists left vacant spaces where
unknown elements should fit.
Periodic Table,
I am your
Father.
Dmitri
Mendeleev
(1869)
The weight
must be wrong.
Reclassify!
Dmitri
Mendeleev
(1869)
Increasing
atomic
NUMBER!
Henry
Moseley
(1913)
Through his work with X-rays, he determined the actual
nuclear charge (atomic number) of the elements. He
rearranged the elements in order of increasing atomic
number.
Actinide and
Lanthanide
Series!
Glenn
Seaborg
(1944)
He reconfigured the periodic table by
placing the actinide series below the
lanthanide series
Periodic Table
Geography
The Modern Periodic Table
Columns of elements are called groups.
George M. McKelvy, Ph.D.
33
The Modern Periodic Table
Rows of elements are called periods.
George M. McKelvy, Ph.D.
34
The Alkali Metals
• 1 valence electron
• Highly reactive with water
• Form ionic compounds
• Do not occur in nature as pure
elements (always in
compounds)
Alkali-Earth Metals
• Have 2 valence electrons
• Reactive, but less reactive
than alkali metals
• Are ductile, malleable and
have a silvery luster
Transition metals…
and inner transition metals
• Are less reactive than groups 1
and 2.
• Tend not to react in water.
• Are malleable and ductile, but
still harder than group 1 & 2.
• Tend to be solids at room
temperature.
• Have variable chemical
properties
 Inner transition metals
• Are good conductors of
tend to be radioactive
electricity and heat.
Nonmetals
• Poor conductors of heat and electricity
• Often are found as gases or liquids,
sometimes solids.
Halogens
• Are nonmetals
• highly reactive with metalsmost reactive is flourine,
lease reactive is astatine
• Mostly exist as gases or
liquids (except At -solid)
• Have 7 valence electrons
Noble gases
• At room temperature,
exist as gases.
• Are completely unreactive
• Have full s and p orbitals
• Are odorless, colorless,
nonflammable
Metalloids
• Tend to be solids
• Have properties similar to both metals and nonmetals
• Tend to be semiconductors (which means they are useful
for technological uses)
Periodic Law
When elements are arranged in
order of increasing atomic number,
there is a periodic pattern in their
physical and chemical properties.
Metals, metalloids (semi-metals) and nonmetals
Properties of Metals
• Good conductors of heat
and electricity: lots of
mobile electrons to move
energy and charge around
• Have luster (shiny)
• Are ductile (stretchable)
and malleable (bendable)
• Solids at room temp.
except for Hg
Properties of Metalloids
• Have properties of both
metals and non-metals
• Some of them are
semiconductors (like silicon)
which behave sometimes as
conductors and sometimes
as insulators
• Solids at room temperature
Properties of Non-Metals
• Poor conductors of heat
and electricity
• Can be solids, liquids or
gases (contrary to
popular belief not all
non-metals are gases!)
• Are not ductile,
malleable
Periodic Trends - Reactivity
–how likely or vigorously an atom is to
react with other substances
–Usually determined by how easily
electrons can be removed and how
badly they want to take other atom’s
electrons since it is the
transfer/interaction of electrons that is
the basis of chemical reactions
Periodic Trends - Reactivity
• Metals
–Period: DECREASES from left to right
–Group: INCREASES down a group
WHY? The farther left and down the
periodic table, the easier it is for
elections to be given or taken away,
resulting in a higher reactivity
Periodic Trends - Reactivity
• Non-metals
–Period: INCREASES from left to right
–Group: DECREASES down a group
WHY? The farther right and up the
periodic table, the higher the
electronegativity, resulting in a more
vigorous exchange of electrons