PERIODIC TABLE

  As elements were discovered and their properties became known, Dmitri Mendeleev

arranged them into a chart, organizing them by increasing atomic mass.  Elements with

similar properties were placed in the same column or group (also called a family). 

There were many open spaces where Mendeleev predicted new elements would be

discovered whose mass would fall between the known elements and whose properties

would align with the other elements in that particular group. 

  When the additional elements were discovered and placed into the chart in order of

increasing atomic mass, the properties of some elements did not match the other elements

in the group.  For example, potassium, K, has a mass of 39.1 and is a soft, shiny gray metal

that is highly reactive with water.  Argon, Ar, has a mass of 39.9 and is a unreactive gas. 

In order for their properties to coincide with members in their group, the two elements had

to be switched.  Argon needed to be listed before potassium.  The same was true for

Tellurium and Iodine.  This meant that the elements were no longer in order by increasing

atomic mass.

  Later, when the atomic number of each element became known, Henry Moseley

designed the modern periodic table placing the elements in order of increasing atomic

number.  The Periodic Law states that there is a reoccurring pattern in the properties

of the elements when the elements are listed in order of increasing atomic number.

  Groups are columns running up and down.  They are also called Families.  Elements

within a group have similar properties.  Groups can be numbered 1 - 18 across the top

of the chart (IUPAC--international method) or the tall columns (known as the group A

elements) can be numbered I - VIII using Roman Numerals.  The short groups in the

middle are known as group B elements.  Rows on the periodic table are called Periods.

  There are 7 periods whose numbers go down the left side of the chart.

  There is a stairstep that begins under the element boron and zigzags its way down the

chart separating the metals on the left from the non-metals on the right.  Elements

having properties of both metals and non-metals are called Metalloids and they touch

the stairstep on either side.  These include boron, silicon, germanium, arsenic, antimony

and tellurium. 

   Elements in group 1A are called alkali metals.  Those in group 2A are alkaline earth

metals.  The group B elements (the short groups in the middle of the chart) are the

transition metals, while the two periods pulled out from this section and lying

across the bottom of the chart are the inner transition metals or rare earths. 

Group 7A are the halogens (fluorine, chlorine, bromine, iodine and astitine). 

Group 8A are the noble gases or inert gases. 

IONS AND REACTIVITY AND THE PERIODIC TABLE

    The electrons found in an atom's outermost shell are the ones used in chemical reactions.

  These outermost electrons are called VALENCE electrons.  The number of valence

electrons determine how many bonds the atom will form with other atoms.  When using the

group number on the periodic table for the group A elements (the tall columns), the

group number equals the number of valence electrons.  There are 8 groups across the periodic

table that belong to group "A".  Hydrogen is in group 1 and everything in its group has 1 valence

electron.  Beryllium is in group 2 and everything in its group has 2 valence electrons, Boron's

group 3 has 3 valence electrons, Carbon's group 4 has 4, Nitrogen's group 5 has 5, Oxygen's

group 6 has 6, Fluorine's group 7 has 7 and Neon's group 8 has 8 valence electrons (Helium is

an exception to group 8 because it only has an atomic number of 2 and therefore only has 2

valence electrons).  However, group 8A is known as the Noble Gases and are unreactive. 

Elements having 8 valence electrons are stable and unreactive.  Helium is also stable and

unreactive even though it has only 2 valence electrons.  This is because Helium's electrons fill the

first shell.  The elements in group 2A have 2 valence electrons, but they are NOT in the first shell,

and so the element's are not chemically stable, but instead are chemically reactive. 

   Since having 8 valence electrons makes an element stable, then every element on the chart

(except the Noble Gases) will react in order to achieve 8 valence electrons.  This is known as

the OCTET RULE.  This means that group 7, which is only one electron away, will do

whatever it can to gain one more electron.  Elements in group 7A are very reactive.  Upon

gaining one electron, a negative one ion (1-) will form.  Group 6 will do the same, but they

will have to gain 2 electrons to achieve stability.  This makes group 6A form ions with a

2- charge.  Likewise group 5A will gain 3 more electrons forming a 3- ion. A change

occurs with group 4A since it has exactly half of what it needs to become stable. 

Usually, the elements in group 4A, which is Carbon's group, will share their 4 valence

electrons.  This is the basis for the chemistry of life (biochemistry).  Proteins, lipids and

carbohydrates are made of long carbon chains created when carbon atoms share valence

electrons with other carbon atoms.  So in terms of ionic charge, this group will be

skipped over.  (Note:  This does not mean that carbon will never form an ion). 

    Going to the far left on the periodic table, is group 1, with 1 valence electron. 

Since it is so far away from the 8 needed, it will lose the 1 outermost electron and the

shell it is in, making the next innermost shell be the new, complete and stable, outermost shell. 

By losing the 1 valence electron, the atom achieves the electron arrangement and stability

of the Noble Gas just preceeding it on the chart.  For example, when Na loses its 1 valence

electron, it now has only 10 electrons, like Neon, Ne.  These 10 electrons fill the first shell

with 2 and the second with 8.  Only needing to lose 1 electron makes this group a very

chemically reactive group.  Especially if an atom is placed near an atom of group 7 which

wants one more electron.  The elements of group 1 will all form 1+ ions as they lose this

electron.  Likewise, group 2 elements will lose their 2 outermost electrions and form

2+ ions and group 3A will lose their 3 valence electrons to form 3+ ions. 

   So looking at the pattern from left to right across the periodic table, the ionic

charges are as follows:   1+, 2+, 3+, skip, 3-, 2-, 1- and 0.