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2.1.3The Particulate Nature of Matter

Electronic Configuration — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 4 of 13 Unit quiz

📚 Combined Science 0653 - Chemistry Hub / Topical Revision
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Section 1 of 7Electron shells

Shells and energy
  • Shells and energy Electrons move around the nucleus in shells, also called energy levels.
  • The further a shell is from the nucleus, the more energy it has.
Filling order
  • Filling order Electrons fill the shell closest to the nucleus first.
    • When it is full, extra electrons go into the next shell.
Shell capacity
  • Shell capacity At this level the first shell holds up to 2 electrons, the second up to 8 and the third up to 8, before the fourth begins to fill.
Valence shell
  • Valence shell The outermost shell is the valence shell.
    • An atom is most stable when this shell is completely full.

Section 2 of 7Writing the configuration

1
Find the atomic number. For a neutral atom this is also the number of electrons.
2
Put the first 2 electrons in shell one.
3
Put the next electrons, up to 8, in shell two.
4
Put the following electrons, up to 8, in shell three, then any left over in shell four.
5
Write the number in each shell separated by commas. Magnesium has 12 electrons, so its configuration is 2,8,2.

Section 3 of 7First twenty elements

ElementAtomic numberConfiguration
Hydrogen / Helium1 / 21 / 2
Lithium to Neon3 to 102,1 up to 2,8
Sodium112,8,1
Magnesium122,8,2
Chlorine172,8,7
Argon182,8,8
Potassium192,8,8,1
Calcium202,8,8,2

Section 4 of 7Configurations of ions

1Start with the atom
  • Start with the atom A sodium atom has 11 electrons, so its configuration is 2,8,1.
2Lose or gain electrons
  • Lose or gain electrons Sodium loses its single outer electron, leaving 10 electrons.
3Rewrite the shells
  • Rewrite the shells The sodium ion is 2,8, which is the same as neon.
    • Non-metals gain electrons instead, for example chlorine becomes 2,8,8 as Cl-.

Section 5 of 7Link to the Periodic Table

Configuration and position
Number of numbers
  • Number of numbers The count of numbers in the configuration shows how many shells are occupied, which is the period number.
Last number
  • Last number The final number is the number of outer electrons, which equals the group number for Groups I to VII.
Group VIII
  • Group VIII Noble gases have full outer shells (2 for helium, 8 for the others), which is why they are unreactive.

Section 6 of 7Key terms

TermMeaning
Electronic configurationThe arrangement of electrons in the shells of an atom, written as numbers separated by commas.
ShellAn energy level around the nucleus that can hold a limited number of electrons.
Valence shellThe outermost occupied shell of an atom.
PeriodA horizontal row of the Periodic Table; its number equals the number of occupied shells.
GroupA vertical column of the Periodic Table; elements in it have the same number of outer electrons.

Section 7 of 7Exam technique

1
Putting 8 electrons in the first shell. The first shell holds only 2.
2
Using atomic mass instead of atomic number to count electrons.
3
Forgetting to change the number of electrons when writing the configuration of an ion.
4
Putting more than 8 electrons in the third shell for the first twenty elements; potassium and calcium use the fourth shell.

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