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
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
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.