Electronic structures and the periodic table

The electronic structure of an element is linked to its position on the periodic table.

FeatureLink
Number of shellsPeriod number
Number of electrons in outermost shellGroup number
Numbers added togetherAtomic number

The electronic structure of sodium (2.8.1) shows that sodium, Na:

  • is in period 3
  • is in group 1
  • has an atomic number of (2 + 8 + 1) = 11

Note that there are some exceptions to the pattern shown with the example above.

ExceptionReason
HydrogenHas 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 groupThese elements have full outer shells - helium is 2, neon is 2.8, argon is 2.8.8, and their group number is group 0

Electronic structures and properties of elements

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:

Table showing electronic structures of group 1 elements, lithium, sodium and potassium. Group 1 elements have similar properties and reactions as they all have one electron in their outer shell.

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.

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