Knowledge Organiser: Electronic Configuration
Key Terms
- Shell: Energy level where electrons are found
- Configuration: Arrangement written as e.g. 2,8,1
- Outer shell electrons: Determine chemical properties
- Group: = number of outer electrons
- Period: = number of occupied shells
Must-Know Facts
- Shell capacities: 1st = 2, 2nd = 8, 3rd = 8
- Electrons fill lowest energy shell first
- Same group = same outer electrons = similar properties
- Full outer shell = stable = unreactive (Group 0)
- Group 1: 1 outer electron, very reactive
- Group 7: 7 outer electrons, very reactive
Key Equations
- Group number = number of outer shell electrons
- Period number = number of occupied electron shells
- Total electrons = atomic number (for neutral atoms)
Common Mistakes
- Writing 2,8,3 for aluminium (Al, Z=13): Correct — but students often put all 13 in one shell; remember 2 + 8 + 3
- Mixing up group and period: Group = column (outer electrons); Period = row (number of shells)
- Saying 3rd shell holds 18 electrons: At GCSE the 3rd shell holds maximum 8 electrons
- Configuration doesn't determine reactivity: It does — number of outer electrons directly determines how reactive the element is
Memory Aid
- Shell capacity: "2 then 8, 8, 8..." — The first shell is special with only 2. All subsequent shells (at GCSE) hold up to 8.
- Periodic table link: "Group = Outer, Period = Shells" — Group number tells you how many electrons are on the outside; period number tells you how many shells are occupied.
- Reactivity reminder: "1 outer = very reactive (Group 1), 7 outer = very reactive (Group 7), 8 outer = stable (Group 0)." The extreme groups are the most reactive; the noble gases are the most stable.