Atomic
Number The
number of protons in an atom
defines what element it is.
For example, hydrogen atoms
have one proton, carbon atoms have
six protons and oxygen atoms have
eight. Each element has
a unique number of protons. No two elements have the
same number of protons, therefore each element has a
unique atomic number. The number of protons
in an atom is referred to as
the atomic number of that element.
The number of protons in an
atom also determines the chemical
behavior of the element.
Atomic
Symbol The
atomic symbol is one or two
letters chosen to represent
an element ("H" for
"hydrogen", etc.). These symbols are used internationally. Typically,
a symbol is the truncated name
of the element or the truncated
Latin name of the element. The
colors of the atomic symbols
were chosen by us for
purposes of clarity and not
part of the format of the Periodic
Table. Atomic symbol colors
are further explained below in "State of the Element".
Atomic
Weight The
atomic weight is the ratio
of the average mass of a chemical element's atoms to
a standard. Since 1961 the standard unit of atomic mass
has been one-twelfth the mass of an atom of the isotope
carbon-12. An isotope is one of two or more species of
atoms of the same chemical element that have differenct
atomic mass numbers (protons + neutrons). The atomic
weight of carbon is 12.0107, which is the average that
reflects the typical ratio of natrual abundances of
its isotopes.
The concept of atomic weight is fundamental to chemistry, because most chemical
reactions take place in accordance with simple numerical relationships among
atoms. Since it is almost always impossible to count the atoms involved
directly, chemists measure reactants and products by weighing them and reach
their conclusions through calculations involving atomic weights.
Periods
and Groups Periods
are arranged horizontally across
the Periodic Table. Each row
is referred to as a Period.
There are 7 Periods. The elements
in a Period have the same number
of valence shells. All elements
in the same row tend to have
their valence electrons in the
same energy level. For example
in Period 4, potassium (K),
vanadium (V), germanium (Ge)
and bromine (Br) all have their
valence electrons in the fourth
energy level (4th shell).
Each
column is referred to as a Group.
There are 18 Groups. In each
Group the elements of that group
tend to have the same number
electrons in their outer most
shell, (valence shell). Elements
in Group 17, such as
fluorine (F), chlorine (Cl)
and bromine (Br), have seven
valence electrons, elements
in Group 13 have three valence
electrons and so on.
State
of the Element The
colors of the letters of the
atomic symbol indicates the
physical state of the element:
black for solids; blue for liquids;
red for gases; fuchsia for
synthetics.
The
colors of the atomic symbols
were chosen by us for
purposes of clarity and not
part of the format of the Periodic
Table.
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