The way electrons are arranged in an atom is called the 'electronic structure'. As you have seen on the previous page, there is a link between an element's electronic structure and its place in the periodic table. You can work out an element's electronic structure from its place in the periodic table.
The diagram below shows a section of the periodic table, with the elements arranged as usual in order of their atomic number from 2 to 20. The red numbers above each chemical symbol show the electrons in each shell.
Moving across each period, you can see that the number of shells is the same as the period number.
As you go across each period from left to right the outer shell gradually becomes filled with electrons. The outer shell contains just one electron on the left hand side of the table, but is filled by the time you get to the right hand side.
Moving down each group, you can see that the number of electrons in the outermost shell is the same as the group number.
Each element in a group therefore has the same number of electrons in its outer shell.
Group 0 is a partial exception to this rule, since although it comes after group 7 it is not called 'group 8'; and it contains helium, which has only two electrons in its outer shell.
Here's how to use the periodic table to work out an electronic structure:
Find the element in the periodic table. Work out which period it's in, and draw that number of circles around the nucleus.
Work out which group the element is in and draw that number of electrons in the outer circle - with eight for Group 0 elements (except helium).
Fill the other circles with electrons (remember: two in the first, eight in the second and third, 18 in the fourth).
The atomic number of an atom is the number of protons it has. This is the same as the number of electrons. If we know the atomic number we can work out the arrangement of the electrons. Fill the shells starting from the smallest and going outward.
For example silicon has atomic number 16. So we have to fill the shells with 16 electrons. That makes 2 in the first (to fill it), 8 in the second shell (to fill that) and 6 left to go into the third shell. So silicon has electronic structure 2.8.6
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