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The periodic Table developed as Chemists began to get a basic understanding of the principles underlying the Chemistry they understood at the time. Mendeleev, in 1869, arranged the elements into vertical groups of elements of similar chemical properties and his work was subsequently built on by Moseley, who introduced the concept of the atomic number (number of protons). The modern Periodc Table contains the elements arranged in order of increasing atomic number, each vertical column is a GROUP, containing elements with similar chemical properties and crucially with the same number of outer electrons. Each row is a PERIOD and as we move along a period, the number of outer electrons in the elements increases by one.
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Key Groups of the Periodic Table
Group 1 also known as the alkali metals have one electron in their outermost shell. The alkali metals are soft, highly reactive metals, and their reactivity increases down the group. Hydrogen also has a single electron in its outermost shell, however hydrogen behaves very differently from elements in the lower periods and so scientists disagree over whether it should belong to group 1 or 7.
The Group 2 elements are also called the ‘alkaline earth metals’. They are all reactive metals with distinctive flame colours. In general they are harder, denser and have higher melting points than each alkali metal in the same period. Group 2 elements have two electrons in their outermost shell.
Group 7 is called the halogens. This is a group of highly reactive non-metals. This is the only group that contains elements in all three states of matter at room temperature and pressure. Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid. Fluorine and chlorine have seven electrons in their outer electron shell.
The Group 8 elements are commonly known as the noble gases. They are typically unreactive. At one time they were known as the inert gases, but some compounds (particularly of Xe) are now known. Reactivity increases down the group with radon being the most reactive. The noble gases each have a full outer electron shell.
