2.9 - Electronic Structure

Electron shells and writing an electronic structure

• Electrons occupy shells (also called energy levels) around the nucleus.
• The innermost shell has the lowest energy and fills first.

Rules for filling shells

• Each shell holds a maximum number of electrons:
  • First shell holds up to 2
  • Second shell holds up to 8
  • Third shell holds up to 8 (for elements up to calcium)
• An atom's number of electrons equals its atomic number (number of protons).

Why the outer shell matters

• A full outer shell is a very stable arrangement - the elements of Group 0 already have full outer shells.
• Most atoms have an incomplete outer shell, so they react to achieve a full outer shell.

Writing the electronic structure

• As numbers - Listed from the innermost shell outwards, separated by commas. For example, sodium is 2,8,1.
• As a diagram - The nucleus in the middle, occupied shells drawn as circles, and electrons drawn as dots or crosses on each circle (the diagram of sodium below uses small circles).

Worked example - Electronic structure of calcium

Work out the electronic structure of calcium.

Electronic structure and the periodic table

• The group number gives the number of outer-shell electrons (Groups 1 to 7).
  • Group 0 is the exception: these atoms have a full outer shell of 8 electrons (except helium, which has 2).
• The period number gives the number of occupied shells. A Period 2 element has electrons in two shells, and a Period 4 element has electrons in four shells.

Putting the two together

• Potassium sits in Group 1 and Period 4: four shells are occupied and the outer shell holds 1 electron, giving 2,8,8,1.
• Chlorine has the electronic structure 2,8,7: three occupied shells place it in Period 3, and seven outer electrons place it in Group 7.

Why a group shares its chemistry

• When atoms react, only the outer-shell electrons are involved.
• Elements in the same group have the same number of outer electrons, so they react in similar ways and share chemical properties.