This topic accounts for approximately 9% of your exam marks.
stable
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Stable9%
Subatomic particles and electronic configuration appear in every exam series.
Electrons occupy shells around the nucleus, filling from the innermost shell outwards
For the first 20 elements at IGCSE, the shell capacities are:
1st shell: up to 2 electrons
2nd shell: up to 8 electrons
3rd shell: up to 8 electrons (at IGCSE level)
4th shell starts to fill once the 3rd has reached 8
An electronic configuration is written as the number of electrons in each shell, separated by commas, starting from the innermost shell:
sodium (Z = 11): 2,8,1
chlorine (Z = 17): 2,8,7
calcium (Z = 20): 2,8,8,2
Reading an electronic configuration
The total of all the numbers equals the atomic number (= number of electrons)
The number of electrons in the outermost shell equals the element's group number for groups 1–7 (this is why same-group elements behave alike chemically)
The number of shells used equals the element's period number on the periodic table