Download Atomic Notation

Atomic Notation
Feb.21, 2012
-Chemical symbols were proposed in 1817 and were adopted internationally
-The atom was recognized as being the smallest part of an element at that time
-Atoms are composed of protons (positively charged), electrons (negatively charged),
and neutrons (no charge)
-Protons and neutrons are found in the nucleus and they make up most of the mass of
the atom
-Electrons are found in electron shells around the nucleus
-The electrons do not contribute greatly to the mass of the elements.
-We describe each element using a unique Standard or Atomic notation
Where:
A – the mass number
X – the element name
Z – the atomic number
-The mass number is the number of protons and neutrons in that element. The number
is rounded to the nearest whole number (no decimals in standard notation)
-The atomic number is the number of protons
-All atoms of the same element have the same number of protons (or atomic number)
-The number of electrons in an atom is equal to the number of protons.
-The charge on an atom will be zero when the number of electrons and protons are
the same.
-The number of neutrons can be determined by subtracting the atomic number from
the mass number (atomic number which is rounded)
ie. Lithium (refer to reference Periodic Table)
Mass Number (A) :7
Atomic Number (Z): 3
Element Name (X): Li
Number of Protons: 3
Number of Neutrons: 7 – 3 = 4
Number of Electrons: 3 (so that there will be a neutral charge)
But why is the mass number rounded?
-because there are different forms of an element in nature (especially those
which have different atomic masses due to different numbers of neutrons). The
average mass number of those different forms of the same element
(characterized by all having the same number of protons or atomic number) is
used in the periodic table.
Isotopes
-isotopes are variants of a chemical element
-On the periodic table, the mass number is actually an average value based on the
relative concentration of the different isotopes of an element found in its natural state.