Electronic Structure

Section 1 of 7Rules for filling shells

1

Electrons occupy the shell closest to the nucleus first, because it has the lowest energy.

2

The first shell holds a maximum of 2 electrons.

3

The second shell holds a maximum of 8 electrons.

4

The third shell holds up to 8 electrons for the first 20 elements, after which the fourth shell starts to fill.

5

When a shell is full, extra electrons go into the next shell out. Every shell inside the outer one must be full.

Section 2 of 7Two ways to show electronic structure

Shell diagrams

• Shell diagrams The nucleus is drawn at the centre with concentric circles for shells.
  • Electrons are shown as dots or crosses on each circle.

Number notation

• Number notation Write the number of electrons in each shell from the innermost outwards, separated by commas.
  • Carbon has 6 electrons, so its structure is 2,4.

Ions

• Ions For an ion, count the electrons it actually has.
  • A sodium atom is 2,8,1;
    • the sodium ion has lost one electron so it is 2,8.

Outer shell

• Outer shell The outermost shell is called the valence shell. Atoms are most stable when this shell is full.

Section 3 of 7Links to the Periodic Table

Electronic structure and position

Period

• Period The number of numbers in the notation is the number of occupied shells, which is the period number.
• Chlorine is 2,8,7: three numbers, so Period 3.

Group

• Group The last number is the number of outer electrons, which is the group number.
• Chlorine has 7 outer electrons, so it is in Group 7.

Group 0

• Group 0 Noble gases have full outer shells, such as helium (2) and neon (2,8).
  • Helium is the exception: it has only 2 outer electrons.

Similar reactions

• Similar reactions Elements in the same group have the same number of outer electrons, so they react in similar ways.

Section 4 of 7Examples

Element	Electrons	Structure	Period	Group
Lithium	3	2,1	2	1
Carbon	6	2,4	2	4
Oxygen	8	2,6	2	6
Sodium	11	2,8,1	3	1
Chlorine	17	2,8,7	3	7
Calcium	20	2,8,8,2	4	2

Section 5 of 7Stability and reactions

1Outer shell not full

• Outer shell not full Most atoms have an incomplete outer shell, which makes them reactive.

2Electrons move or are shared

• Electrons move or are shared Metals lose outer electrons, non-metals gain them, and non-metals can also share them.

3Full outer shell

• Full outer shell Losing, gaining or sharing electrons gives each atom a full outer shell, which is a more stable arrangement.

Section 6 of 7Key terms

Term	Meaning
Electron shell	A region around the nucleus where electrons are found, also called an energy level.
Electronic structure	The arrangement of electrons in the shells of an atom, e.g. 2,8,1.
Valence shell	The outermost occupied shell of an atom.
Period	A horizontal row of the Periodic Table; its number equals the number of occupied shells.
Group	A vertical column of the Periodic Table; its number equals the number of outer electrons (apart from Group 0).
Noble gas	An unreactive Group 0 element that has a full outer shell.

Section 7 of 7Exam technique

1

Putting more than 2 electrons in the first shell or more than 8 in the second shell.

2

Starting a new shell before the previous one is full. All inner shells must be complete.

3

Mixing up period and groupthe number of shells gives the period, the outer electrons give the group.

4

Writing the structure of an ion as if it were the atom. Count the electrons after gaining or losing them.

5

Saying electron transfer involves inner electrons. Only outer-shell electrons are lost, gained or shared.