I Used to Know That_ Stuff You Forgot From School - Caroline Taggart [27]
molecule: the smallest particle of a compound that can exist independently and retain its properties. So in the previous example, the smallest imaginable quantity of hydrogen and oxygen joined together in the right conditions and right proportions will still produce a molecule of water. Only when the hydrogen and oxygen are chemically separated again do they lose the properties that make them water and return to being atoms of hydrogen and oxygen.
☞ THE PERIODIC TABLE OF THE ELEMENTS
The periodic table was first devised in 1889 by the Russian chemist Dmitri Mendeleev. When putting the table together, Mendeleev realized there were gaps between some of the elements. Based on this, he predicted that some elements had yet to be discovered.
The table arranges the elements in ascending order of atomic number (the number of protons that each possesses) in such a way that the vertical columns contain groups or families with similar chemical properties. The horizontal rows represent periods, with the most electropositive (an alkali metal) on the left and the so-called inert gases on the right, and the whole thing proves that “the chemical properties of the elements are periodic functions of their atomic weights”—or, in other words, that similar properties in an element recur at regular intervals.
PERIODIC TABLE OF THE ELEMENTS
The elements are traditionally designated by a one-, two-, or three-letter abbreviation, as you can see in the table, and there are 118 of them. The table above lists 103; listed below them are elements 104 through 118. From 93 upward the elements don’t occur naturally but have been synthesized in particle accelerators. The last few are recent achievements, and they have temporary names based on their atomic numbers. Element 117, which will be called Ununseptium, hasn’t been synthesized yet, but scientists are working on it. The lanthanides and actinides are usually separated from the rest of the table, as shown above, because—unlike the other rows—they have similar properties as you read across.
104 Rutherfordium Rf
105 Dubnium Db
106 Seaborgium Sg
107 Bohrium Bh
108 Hassium Hs
109 Meitnerium Mt
110 Darmstadtium Ds
111 Roentgenium Rg
112 Ununbium Uun
113 Ununtrium Uut
114 Ununquadium Uuq
115 Ununpentium Uup
116 Ununhexium Uuh
117 Ununseptium Uus
118 Ununoctium Uuo
☞ ACIDS, BASES, AND SALTS
An acid is a substance (often sour and corrosive) that contains hydrogen atoms that, when dissolved in water, dissociate into ions and may be replaced by metals to form a salt.
A base is a compound that combines with an acid to form a salt plus water. Bases that are soluble in water are called alkalis. Many bases are oxides (so their formula ends in O, possibly with a little number after it) or hydroxides (OH).
A salt is a (usually crystalline) solid compound formed from the combination of an acid and a base by the replacement of hydrogen ions in the acid by positive ions in the base.
For example, combine sulphuric acid with the base cupric oxide in the right conditions and you have copper sulphate (that lovely bright blue stuff) and water:
H2SO4 + CuO → CuSO4 + H2O.
In a school lab you test whether a substance is an acid or a base with litmus paper. Acids turn litmus red; bases turn it blue. Serious scientists use the pH—potential of hydrogen—which is measured by sensors and electrodes and such. Pure water has a pH of 7, with anything less considered acidic and anything higher alkaline. Gardeners use this as a way of testing soil; you also sometimes see the pH listed on shampoo bottles.
Another term you might remember—and one worth mentioning here—is valency, which means the number of atoms of hydrogen that an atom or group displaces when forming a compound. Hydrogen has a valency of 1 and oxygen a valency of 2, which is why the formula for water is H2O and not just HO—because you need two atoms of hydrogen to “match” one of oxygen. Copper can have either of two valencies, which is why the one mentioned a moment ago is called cupric oxide, not just copper