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Periodic Table
A Tutorial
The periodic table arranges elements, both naturally occurring and man made, in an
orderly arrangement by increasing atomic number. Remember, the atomic number is, by
definition, the number of protons. The number of protons determine the properties of the
element as well as the reactivity of the element. Because the number of protons does not
change it makes sense that the elements would be arranged by atomic number.
In an unbonded atom, the number of protons will equal the number of electrons. The
periodic table deals with unbonded atoms of all the elements. Because of this, the periodic
table can tell us a lot about an element.
The periodic table is made of columns and rows. The columns are called groups, they
are read top to bottom, and are labelled with roman numerals I through VIII. The rows are
called periods, they are read left to right, and are labelled with Arabic numbers 1-7. These rows
and columns, these periods and groups assign each element with two specific numbers, much
like the coordinates on a point on graph paper.
I
1
2
3
4
5
6
7
H
Li
Na
K
Rb
Cs
Fr
II
Be
Mg
Ca
Sr
Ba
Ra
III
B
Al
Ga
In
Ti
IV
C
Si
Ge
Sn
Pb
V
N
P
As
Sb
Bi
VI
O
S
Se
Te
Po
VII
F
Cl
Br
I
At
VIII
He
Ne
Ar
Kr
Xe
Rn
By definition: Groups tell us the number of valence electrons
Periods tell us the number of energy shells found on an atom
Look at Be. It is found in Group II and Period 2. This information tells us the following:
Because it is in Group II, Be has 2 valence electrons.
Because it is in Period 2, Be has 2 energy shells, K - L
If we put those two pieces of information together, we can write the electron
configuration as:
K - L
2 - 2
Now that we know the electron configuration, we can add those two numbers together and get
the total number of electrons. 2+2 = 4 electrons.
Remember: the number of protons is equal to the number of electrons in an unbonded atom.
The periodic table deals with unbonded atoms.
With that said, if Be has 4 electrons then Be has 4 protons. If Be has 4 protons that will
give us the atomic number for Be. The atomic number, the number of protons, of Be is 4.
The Group tells us the number of valence electrons. If we know the number of valence
electrons then we can write the electron dot. Remember the electron dot shows only the
valence electrons found on an atom. Be has 2 valence electrons.
Be
Metals have less than 4 valence electrons. Be has 2 valence electrons, making Be a metal.
Let’s list what we have found out about Be:
It is a metal,
It has 2 valence electrons ( Group II).
It is a metal (less than 4 valence electrons)
It has 2 energy shells
(Period 2).
It has an electron configuration of 2-2
The total number of electrons is 4
The number of protons is 4
The atomic number is 4.
The electron dot is
Be
Let’s take a look at another element. Cl
Chlorine, Cl, is found in Group VII Period 3
What do we know?
Cl has 7 valence electrons (Group VII)
Cl has 3 energy shells (Period 3) K-L-M
The electron configuration is 2 – 8 – 7
The total number of electrons is 17
The total number of protons is 17
The atomic number is 17
It is a nonmetal, more than 4 valence electrons
Its electron dot is
Cl