Electronic structure
Electrons fill the lowest available energy level (the innermost shell)
first
The first shell holds up to 2 electrons, the second up to 8 ,
the third up to 8 for the first 20 elements
Write the structure as numbers: sodium (11 electrons) is 2,8,1 ;
chlorine (17) is 2,8,7; calcium (20) is 2,8,8,2
Or draw it: a nucleus with the electrons as dots or crosses on rings
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Electronic structure — electrons fill the shell nearest the nucleus first Na 2,8,1 sodium Cl 2,8,7 chlorine Ca 2,8,8,2 calcium Shell 1 holds up to 2 electrons, shell 2 holds up to 8, shell 3 holds up to 8. The number of electrons in the outer shell is the group number. Electronic structure diagrams for sodium (2,8,1), chlorine (2,8,7) and calcium (2,8,8,2), with shells drawn as concentric circles
The number of electrons in the outer shell is the group number . The
number of shells is the period number
Sodium 2,8,1: Group 1, Period 3. Potassium 2,8,8,1: Group 1, Period 4
The outer shell is the single outermost shell. An atom has one outer
shell, however many shells it has in total
If a question gives you a periodic table and asks for an electronic
structure, use it: the atomic number is the number of electrons. Don't try to
work it out from the mass number
"Potassium has more electrons than sodium" is not a comparison of electronic
structure. Say potassium has four shells and sodium has three, and
both have one electron in the outer shell
An electronic structure of 2,8,8 is argon, not sulfur: count the electrons
(18) and find atomic number 18. Don't subtract
The rest of this topic is in the full Exam Guide The complete AQA GCSE Chemistry guide — every topic, exam checklist and common mistake — is coming soon. Until then, the first topic in every unit is free to read.
In the full topic
• Electronic structure• The periodic table• Development of the periodic tableRead the free chemistry topics →