Electron shell
A level around the nucleus that can hold a fixed number of electrons. The first shell holds 2, the second holds 8 and the third holds 8 (for the first 20 elements).
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Analogy
Shells are like rows of seats around a stage. The front row fills first.
Example
Carbon (6): 2, 4Oxygen (8): 2, 6Sodium (11): 2, 8, 1Chlorine (17): 2, 8, 7
In practice
Sodium has 11 electrons, so its shells fill as 2, 8, 1.
Practice
Flashcards
- How many electrons fit in the first and second shells?2 in the first shell and 8 in the second.
- What is the electron arrangement of chlorine (atomic number 17)?2, 8, 7.
- What does the period number tell you?The number of electron shells the atom has.
Questions people ask
Why does the second shell hold only 8 electrons?
Each shell has a fixed number of spaces, set by how electrons are arranged in energy levels. The first has room for 2 and the second for 8. At school level you simply learn the rule: fill the first shell, then the second, then the third, working outwards.
How do you write the electron arrangement of an atom?
Find the atomic number, which is the number of electrons in a neutral atom. Put 2 in the first shell, up to 8 in the second, and so on. For phosphorus, number 15, you write 2, 8, 5. The last number shows the group.