Counting the number of electrons in an atom
4 marks · NormalA student is studying atoms of three elements: Element X, Element Y, and Element Z.
| Element | Atomic number |
|---|---|
| X | 5 |
| Y | 12 |
| Z | 8 |
State the number of electrons in a neutral atom of element X.
Explain why the number of electrons in a neutral atom of element Y is 12.
Calculate the total number of electrons in 2 neutral atoms of element Z.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) 5
(b) Because a neutral atom has the same number of protons and electrons. The atomic number of Y is 12, so it has 12 protons and therefore 12 electrons.
(c) 16
Common mistakes: Multiplying atomic number by number of atoms incorrectly in part c
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Marking points
B1 (a) · 1 mark · States that element X has 5 electrons.
B1 (b) · 1 mark · Explains that a neutral atom has equal numbers of protons and electrons.
B1 (b) · 1 mark · Links the atomic number 12 to number of protons (and electrons) in element Y.
B1 (c) · 1 mark · Calculates that 2 atoms of Z have 2 × 8 = 16 electrons.
Worked solution
a) The atomic number of X is 5, so a neutral atom has 5 electrons.
b) A neutral atom has the same number of protons and electrons. The atomic number shows both the number of protons and the number of electrons in a neutral atom, so element Y with atomic number 12 has 12 electrons.
c) The atomic number of Z is 8, so 1 atom has 8 electrons. 2 atoms have 2 × 8 = 16 electrons.