Electronic configuration and the Periodic Table
Electronic configuration
- Electrons fill shells from the inside out.
- The first shell holds up to 2; the next shells hold up to 8 each (first 20 elements).
- The electronic configuration lists the electrons per shell, e.g. sodium (11) is 2,8,1; calcium (20) is 2,8,8,2.
Practice
An atom with 13 electrons has the configuration:
Fill inside-out: 2 in the first shell, 8 in the second, 3 left over → 2,8,3.
Link to the Periodic Table
- Group number = electrons in the outer shell (so 2,8,1 → Group I).
- Period number = number of shells used (so 2,8,1 → Period 3).
- Noble gases (Group VIII/0) have a full outer shell, so they are very unreactive.
Practice
An element with configuration 2,8,1 is in:
One outer electron → Group I; three shells used → Period 3.
Practice
Noble gases are unreactive because they have a full outer shell.
A full outer shell is very stable, so noble gases hardly react.
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Key idea
- electrons fill shells 2, then 8, 8…; configuration written inside-out (e.g. 2,8,1)
- Group = outer-shell electrons; Period = number of shells
- a full outer shell (noble gases) means very unreactive