Counting the number of electrons in an atom
2 marks · EasyExplain why a neutral atom of aluminium (atomic number 13) also contains 13 electrons.
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Must show: Aluminium atom has atomic number 13 (13 protons), Neutral atom: number of protons = number of electrons
Common mistakes: Assuming number of electrons is always 13, regardless of charge; Confusing proton number with neutron number
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Marking points
B1 · 1 mark · States that atomic number is 13, so atom has 13 protons.
B2 · 1 mark · Explains that in a neutral atom, the number of electrons equals the number of protons.
Worked solution
Aluminium has atomic number 13, so it has 13 protons in its nucleus.
In a neutral atom, there is no overall charge because the number of electrons (negative charge) is equal to the number of protons (positive charge).
So, a neutral aluminium atom contains 13 electrons.