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The arrangement of the elements
The periodic table is simply a list of the chemical elements arranged in order of increasing atomic number. The elements are also arranged in order of increasing relative mass, although in a couple of places this isn't followed.
Reading like a book from left to right and top to bottom, the atomic number increases in single units from 1 to above 110.
Although this all sounds simple enough, it was the result of investigations and thought by many notable scientists, most famously Mendeleev. The arrangement is such that similar elements are found next to one another in repeating cycles or periods. From this pattern comes the term 'The Periodic Table'.
However, the periodic table is more than just a list of elements. Its clever structure makes it one of the most useful datatables in all of science. Hence its ubiquitous place of honour on laboratory walls.
Groups
The vertical columns of elements are groups, numbered from 1 to 18.
There are four blocks, or regions named according to the types of orbitals being filled in the outer energy shell. The transition metals,or 'd' block are not nowadays given group numbers. Neither are the 'f' block elements otherwise known as lanthanoides and actinoids (not shown above).
The heavier elements above number 94 are artificial and radioactive. Their chemistry is of little importance as regards pre-university studies.
Some of the groups are also referred to by group names (in times past, they all had names).
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Group number
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Group name
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1
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alkali metals
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2
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alkaline earth metals
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17
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halogens
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18
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noble gases
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Hydrogen is the only element of elements 1-20 that does not easily fit into any of the groups. For this reason it is usually to be found suspended in no-man's land, in the middle of the periodic table.
Groups share similar physical and chemical properties owing to the similarity of electronic configuration in terms of the outer (valence) electrons. For example, group 1 - the alkali metals all have one electron in the outer shell. They are all soft metals, which react with water making metal hydroxides.
Periods
The horizontal rows, or periods, give the periodic table its name. Each period represents the elements in which the same energy level is being filled by electrons.
Thus, the second period has elements with an incomplete second energy level (except for neon, in which the second level is now full)
Electronic configuration
Putting together the group number and the period number it is possible to work out the electronic configuration for all of the main group elements. Main group excludes the 'd' block transition metals and the 'f' block (lanthanoides and actinoides).
- Group number = number of electrons in the outer shell (valence shell electrons)
- Period number = total number of shells including the valence shell.
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Example: What is the electronic configuration of the element from period 3 in group 16? Period 3 has a total of 3 occupied energy shells, i.e. two filled and one outer (valence shell) Group 16 elements have an outer energy shell with 6 electrons Therefore electronic configuration = 2, 8, 6 |
Blocks
Blocks are not generally used to categorise the periodic table, nevertheless they are useful when considering the electronic configurations of the sub-shells.
The groups 1 and 2 are referred to as the 's' block, as the elements of these two groups are filling up an outer 's' orbital.
- Potassium [Ar] 4s1
- Calcium [Ar] 4s2
The transition metals (and scandium and zinc) are in the 'd' block as they all have 4d electrons as the outer shell.
The right hand side of the periodic table has the main groups elements, also known as the 'p' block. Here the 'p' orbitals are being filled up.
- Carbon - 1s2 2s2 2p2
- Silicon - 1s2 2s2 2p6 3s2 3p2
- Chlorine - 1s2 2s2 2p6 3s2 3p5
