Counting the number of electrons in an atom
4 marks · NormalThe table shows the atomic numbers of four elements.
| Element | Atomic number |
|---|---|
| Fluorine | 9 |
| Neon | 10 |
| Sodium | 11 |
| Aluminium | 13 |
Which element in the table has 13 electrons in its atom? Give its name.
Which of these elements has 9 electrons in its atom? Give its name.
Explain why the number of electrons in a neutral atom is the same as its atomic number.
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(a) Aluminium
(b) Fluorine
(c) In a neutral atom, the number of protons (given by atomic number) is equal to the number of electrons because their charges balance each other.
Common mistakes: Atomic mass used instead of atomic number
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Marking points
A1 (a) · 1 mark · Gives correct name: aluminium.
A2 (b) · 1 mark · Gives correct name: fluorine.
E1 (c) · 1 mark · Mentions atoms are neutral or charges are balanced.
E2 (c) · 1 mark · Links the atomic number to number of protons and the need for this to equal number of electrons in neutral atom.
Worked solution
a) Aluminium (atomic number 13) has 13 electrons.
b) Fluorine (atomic number 9) has 9 electrons.
c) The atomic number tells us the number of protons in the atom. In a neutral atom, the number of electrons must be the same to balance the positive and negative charges, so that the overall charge is zero.