Science of Water : Concepts and Applications

102 The Science of Water: Concepts and Applications
their symbols from languages other than English. The following is a list of common elements with
their common names and the names from which the symbol is derived.
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chlorine Cl
copper Cu (Cuprum—Latin)
hydrogen H
iron Fe (Ferrum—Latin)
nitrogen N
oxygen O
phosphorus P
sodium Na (Natrium—Latin)
sulfur S
As shown above, a unique capital letter or a unique combination of capital letter and a small letter
designates each unique element. These are called chemical symbols. As is apparent from the list
above, most of the time the symbol is easily recognized as an abbreviation of the atom name, such
as O for oxygen.
Typically, we do not fi nd most of the elements as single atoms. They are more often found in
combinations of atoms called molecules. Basically, a molecule is the least common denominator of
making a substance what it is.
A system of formulae has been devised to show how atoms are combined into molecules.
When a chemist writes the symbol for an element, it stands for one atom of the element. A
subscript following the symbol indicates the number of atoms in the molecule. O 2 is the chemical
formula for an oxygen molecule. It shows that oxygen occurs in molecules consisting of two oxygen
atoms. As you know, a molecule of water contains two hydrogen atoms and one oxygen atom, so
the formula is H 2 O.
√ Important Point: The chemical formula of the water molecule, H 2 O, was defi ned in 1860 by
the Italian scientist Stanislao Cannizzaro.
Some elements have similar chemical properties. For example, a chemical such as bromine (atomic
number 35) has chemical properties that are similar to the chemical properties of the element
chlorine (atomic number 17, which most water operators are familiar with) and iodine (atomic
number 53).
In 1865, English chemist John Newlands arranged some of the known elements in an increasing
order of atomic weights. Newlands’ arrangement placed the lightest element he knew about at the
top of his list and the heaviest element at the bottom. Newlands was surprised when he observed that
starting from a given element, every eighth element repeated the properties of the given element.
Later, in 1869, Dmitri Mendeleev, a Russian chemist, published a table of the 63 known elements.
In his table, Mendeleev, like Newlands, arranged the elements in an increasing order of
atomic weights. He also grouped them in eight vertical columns so that the elements with similar
chemical properties would be found in one column. It is interesting to note that Mendeleev left
blanks in his table. He correctly hypothesized that undiscovered elements existed that would fi ll in
the blanks when they were discovered. Because he knew the chemical properties of the elements
above and below the blanks in his table, he was able to predict quite accurately the properties of
some of the undiscovered elements.
Our modern form of the periodic table is based on work done by the English scientist Henry
Moseley, who was killed during World War I. Following the work of the New Zealand physicist
Ernest Rutherford and the Danish physicist Niels Bohr, Moseley used x-ray methods to determine
the number of protons in the nucleus of an atom.
The atomic number, or the number of protons, of an atom is related to its atomic structure.
In turn, atomic structure governs chemical properties. The atomic number of an element is more