Atomic symbol - A complete description of an element can be simplified by means of using the following notation:
where E is the atom symbol as shown in the periodic table. Z is the number of protons and A is the atomic mass number. Since the mass number is the
sum of protons and neutrons, the number of neutrons, N, can be worked out as: A - Z. The example shown above is the atomic symbol for the most common
isotope of uranium with the atomic number 92 and the mass number 238. There are 238 - 92 = 146 neutrons in the atomic nucleus.
Since Z is unique for each atom, it is common to write as 238U or 'uranium-238' in the text. The complete notation shown above are more commonly used
in radiochemistry, the study of radiation, nuclear fission and stuff like that. Otherwise, it is more commonly written as simply 'U' unless there is a need to refer to a
Molecular formula - This is the symbolic representation of a molecule. It indicates the actual numbers of atoms of different elements in one molecule of a compound. An example
is the good old dihydrogen oxide, or commonly known as water, which has a molecular formula H2O. This means there are 2 hydrogen atoms and one oxygen atoms in a water molecule.
Other examples such as calcium hydroxide, Ca(OH)2. There are one calcium atom, 2 oxygen atoms and 2 hydrogen atoms.
Quite often certain elements come together as a subgroup in a molecule. In this case, the bracket indicates the OH subgroup (the hydroxyl) and the subscript 2 indicates there are two of such group. However,
brackets are omitted if there is only one subgroup. For example, sodium hydroxide, NaOH.
Empirical formula - This is the simplest, non-reducible ratio of the numbers of atoms of different elements in a compound. For example, hydrogen peroxide with molecular formula H2O2 has an empirical
formula of H1O1 or HO. (Note: this is not a water! Hydrogen peroxide in concentrated form is explosive, while dilute form is useful as a mild antiseptic for mouth wash!).