In 1869, the Russian chemist Dmitri Mendeleev first
proposed that the chemical elements exhibited a "periodicity of properties."
Mendeleev had tried to organize the chemical elements according to their
atomic weights, assuming that the properties of
the elements would gradually change as atomic weight increased.
What he found, however, was that the chemical and physical properties of the
elements increased gradually and then suddenly changed at distinct
steps, or periods. To account for
these repeating trends, Mendeleev grouped the elements in a table that
had both rows and columns.
The modern periodic table of elements is based on Mendeleev's observations; however, instead of being organized by atomic weight, the modern table is
arranged by atomic number (z). As one moves from
left to right in a row of the periodic table, the properties of the elements
gradually change. At the end of each row, a drastic shift occurs in
chemical properties. The next element in order of atomic number is more
similar (chemically speaking) to the first element in the row above it; thus a new row
begins on the table.
For example, oxygen (O), fluorine (F), and neon (Ne) (z = 8, 9 and 10,
respectively) all are stable nonmetals that are gases at room
temperature. Sodium (Na, z = 11), however, is a silver metal that
is solid at room temperature, much like the element lithium (z =
3). Thus sodium begins a new row in the periodic table and is
placed directly beneath lithium, highlighting their chemical
Rows in the periodic table are called periods. As one
moves from left to right in a given period, the chemical properties of
the elements slowly change. Columns in the periodic table are
called groups. Elements in a given group in the periodic table
share many similar chemical and physical properties. The link
below will open a copy of the periodic table of elements in a new
Electron Configuration and the Table
The "periodic" nature of chemical properties that
Mendeleev had discovered is related to the electron configuration of
the atoms of the elements. In other words, the way in which an atom's electrons are
arranged around its nucleus affects the properties of the atom.
Bohr's theory of the atom
tells us that electrons are not
located randomly around an atom's nucleus, but they occur in specific electron
shells (see our Atomic Theory II module for more information). Each shell has a limited capacity for electrons.
As lower shells are filled, additional electrons reside in more-distant shells.
The capacity of the first electron shell is two electrons and for the
second shell the capacity is eight. Thus, in our example discussed
above, oxygen, with eight protons and eight electrons, carries two electrons in
its first shell and six in its second shell. Fluorine, with nine
electrons, carries two in its first shell and seven in the second. Neon,
with ten electrons, carries two in the first and eight in the second.
Because the number of electrons in the second shell increases, we can
begin to imagine why the chemical properties gradually change as we move
from oxygen to fluorine to neon.
Sodium has eleven electrons. Two fit in its first shell, but
remember that the second shell can only carry eight electrons.
Sodium's eleventh electron cannot fit into either its first or its second
shell. This electron takes up residence in yet another orbit, a third
electron shell in sodium. The reason that there is a dramatic
shift in chemical properties when moving from neon to sodium is because
there is a dramatic shift in electron configuration between the two
elements. But why is sodium similar to lithium? Let's look
at the electron configurations of these elements.
you can see in the illustration, while sodium has three electron shells and
lithium two, the characteristic they share in common is that they both have only one electron in their outermost
electron shell. These outer-shell electrons (called valence
electrons) are important in determining the chemical properties of the
An element's chemical properties are determined by the way in which its atoms
interact with other atoms. If we picture the outer (valence)
electron shell of an atom as a sphere encompassing everything inside, then it is
only the valence shell that can interact with other atoms - much the same way as
it is only the paint on the exterior of your house that "interacts" with, and
gets wet by, rain water.
The valence shell electrons in an atom determine the way it will interact with neighboring atoms, and therefore determine its chemical properties. Since both sodium and lithium have one valence electron, they share similar chemical properties.
Electron Configuration Shorthand
For elements in groups labeled A in the periodic
table (IA, IIA, etc.), the number of
valence electrons corresponds to the group number. Thus Li, Na, and
other elements in group IA have one valence electron. Be, Mg, and
other group-IIA elements have two valence electrons. B, Al and other
group-IIIA elements have three valence electrons, and so on. The
row, or period, number that an element resides in on the table is equal
to the number of total shells that contain electrons in the atom. H and He in the first period
normally have electrons in only the first shell; Li, Be, B, and other
period-two elements have two shells occupied, and so on. To write the
electron configuration of elements, scientists often use a shorthand in
which the element's symbol is followed by the element's electron shells,
written as a right-hand parentheses symbol ")". The number of electrons in
each shell is then written after the ) symbol. A few examples are
Li )2e- )1e-
F )2e- )7e-
Na )2e- )8e- )1e-
For further details, the table linked below shows the electron configurations of the first eleven elements.