Counting the number of electrons in an atom
4 marks · NormalThe table below lists the names and atomic numbers of four different elements.
| Element | Atomic number |
|---|---|
| Carbon | 6 |
| Oxygen | 8 |
| Sodium | 11 |
| Chlorine | 17 |
Write down the number of electrons in a neutral atom of oxygen.
Write down the number of electrons in a neutral atom of sodium.
Write down the number of electrons in a neutral atom of chlorine.
Explain why the number of electrons is the same as the atomic number in a neutral atom.
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(a) 8
(b) 11
(c) 17
(d) Number of protons (atomic number) must equal number of electrons for a neutral atom so that charges balance.
Common mistakes: Atomic mass used instead of atomic number
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Marking points
A1 (a) · 1 mark · Gives 8 as number of electrons for oxygen.
A2 (b) · 1 mark · Gives 11 as number of electrons for sodium.
A3 (c) · 1 mark · Gives 17 as number of electrons for chlorine.
E1 (d) · 1 mark · Correct explanation relating neutral charge to protons and electrons.
Worked solution
a) Oxygen: atomic number = 8, so 8 electrons.
b) Sodium: atomic number = 11, so 11 electrons.
c) Chlorine: atomic number = 17, so 17 electrons.
d) In a neutral atom, the number of electrons equals the atomic number because protons (positive) and electrons (negative) must be equal for no overall charge.