
The periodic table as we have it today has not
always existed; it developed much in the
same way as atomic theory did. In the early
1800’s scientists began looking for ways to
classify the elements that had been
discovered.
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**Two other
triads he found
included Cl, Br,
and I as well as
S, Se, and Te.
A German chemist, Johann
Dobereiner, found, that the
properties of three metals
(Ca, Ba, Sr) were very
similar. He also
recognized that the atomic
mass of Sr was about
midway between that of Ca
and Ba. He grouped these
three elements together
and called them a triad.

John Newlands saw
that if he put the
elements in rows of
seven that there
would be a
repeating pattern
every eighth
element. This is
often referred to as
the Law of Octaves.

In 1860, Stanislao Cannizaro
described a method for
accurately determining the
atomic weights of elements.
This gave scientists a set of
standardized measurements to
use in all countries. Based on
Cannizaro’s work other
chemists began working on
other ways to organize the
elements.

Dmitri Mendeleev, a Russian
chemist in the mid 1800’s, was
looking for a way to organize all
known elements. By organizing
the elements by density, reactivity,
and melting point, he came up
with a periodic table that put the
elements in order of increasing
atomic mass.
There were gaps (undiscovered elements) in
Mendeleev’s table. For each of these gaps, Mendeleev
predicted the properties of the missing elements.

In the early 1900’s, English
scientist, Henry Moseley
found a new pattern in
Mendeleev’s table. This
pattern showed that the
nuclei of each element in the
table increased by one
positive charge (proton).
This observation plus the
discoveries of new elements
to fill in Mendeleev’s gaps
led to the modern periodic
table being organized by
increasing atomic number.
 From
both Mendeleev and
Moseley’s work, the periodic law
was developed. The periodic law
states that the chemical and
physical properties of the
elements are periodic functions
of their atomic number.

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


Group
Group
Group
Group
Group
1 or IA= Alkali Metals (does not include hydrogen)
2 or IIA = Alkaline Earth Metals.
13 or IIIA = Boron family
14 or IVA = Carbon family
15 or VA = Nitrogen family.
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

Group 16 or VIA = Calcogens.
Group 17 or VIIA = Halogens.
Group 18 or VIIIA = Noble Gases - once
known as the Inert Gases.


The middle section of the periodic table
contains the Transition Metals (d-block)
The two rows below the main part of the
table are the Inner Transition Metals (f-block)


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The metals are found on the left side of the
periodic table.
The non-metals are found on the right side
of the table.
The metalloids are found above and below
the stair-step line started between boron and
aluminum.


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The most solid elements are located to
the left and below the metalloid line.
Two elements are liquid at room
temperature: mercury and bromine.
Several elements are gases at room
temperature: hydrogen, helium,
nitrogen, oxygen, fluorine, neon,
chlorine, argon, krypton, xenon, and
radon.
•Shiny, metallic
luster
•Mostly solid, yet
easily deformed –
malleable &
ductile
•Good conductors
of heat &
electricity
•Loosely held
electrons
Metal Activity Increases
•Solids lack luster
•Many gases at
room temperature
•Poor conductor of
electricity
•Lower melting
points
•Tightly held
valence electrons
Nonmetal Activity Increases

Energy levels – Increases down a group,
does not change across a period
In other words, there are more layers of
electrons as move down the table (think
about an onion)
Remember – energy levels are regions of
space in which electrons can move around
the atom’s nucleus

What is atomic radius?
◦ It is one half the distance between 2 adjacent atoms
of the same element.


It increases as you go down a group.
It decreases as you go from left to right in a
period.

Cation- a positively charged ion because it
has LOST electrons. The radius always
decreases with a lose of electrons
◦ Metals tend to form cations!

Anion – a negatively charged ion because it
has GAINED electrons. The radius always
increases with a gain of electrons.
◦ Nonmetals tend to form aniona!

What is electronegativity?
◦ The measure of the ability of an atom in a chemical
compound to attract electrons
◦ Does NOT include the Noble Gases!!

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Decreases as you move down a group.
Increases from left to right across a period.

What is Ionization Energy?
◦ The amount of energy necessary to remove an
electron from a gaseous atom.


Decreases as you move down a group.
Increases as you move from left to right
across a period.