Counting the number of electrons in an atom
5 marks · HardThe table below gives information about three elements.
| Element | Atomic number |
|---|---|
| A | 7 |
| B | 12 |
| C | 15 |
State the number of electrons in a neutral atom of element B.
Element C is found in a rock sample containing 5 atoms. Calculate the total number of electrons in all these atoms.
Explain why a neutral atom of element A contains the same number of electrons as its atomic number.
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) 12
(b) 75
(c) Because number of protons equals number of electrons in a neutral atom, so atomic number equals number of electrons. · must show States atomic number equals number of protons, and in a neutral atom protons=electrons
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Marking points
B1 (a) · 1 mark · States correct number of electrons in element B: 12.
M1 (b) · 1 mark · Attempts to multiply atomic number (15) by number of atoms (5). (follow-through)
A1 (b) · 1 mark · Correct answer: 75 electrons. (follow-through)
B1 (c) · 1 mark · States that the number of electrons equals the number of protons or atomic number in a neutral atom.
B2 (c) · 1 mark · Explains this is because the atom is neutral and the charges balance.
Worked solution
a) Number of electrons in B = atomic number = 12.
b) Number of electrons in one atom of C = 15. For 5 atoms: 5 × 15 = 75 electrons.
c) In a neutral atom, number of electrons equals atomic number, because the positive charge from protons balances the negative charge from electrons so the atom is neutral.