|Number of shells||Period number|
|Number of electrons in outermost shell||Group number|
|Numbers added together||Atomic number|
The electronic structure of sodium (2.8.1) shows that sodium, Na:
Note that there are some exceptions to the pattern shown with the example above.
|Hydrogen||Has only 1 electron but not placed in group 1 as it does not share properties with the rest of the group - it is placed by itself on the periodic table|
|Helium, neon and other elements in this group||These elements have full outer shells - helium is 2, neon is 2.8, argon is 2.8.8, and their group number is group 0|
The electronic structures of atoms help explain the properties of elements and the structure of the periodic table. When atoms collide and react, it is the outer electrons that meet and interact. So, elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell.
The electronic structures of the elements in group 1:
The atoms of all group 1 elements have similar chemical properties and reactions because they all have one electron in their outer shell. Similarly, the atoms of all group 7 elements have similar chemical properties and reactions to each other because all of them have seven electrons in their outer shell.