Principles of Chemistry
7 marks · EasyThis question is about sub-atomic particles and electron configurations.
State the relative mass and relative charge of an electron.
Describe where electrons are found within an atom.
An atom has atomic number 12. How many electrons does it have?
Write the electronic configuration of an atom with atomic number 12.
State the rule for filling electron shells up to the third shell.
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) Relative mass = 1/1836; relative charge = -1 · must show relative mass, relative charge
(b) Electrons are found in shells (energy levels) around the nucleus. · must show shells, around nucleus
(c) 12 · must show 12
(d) 2,8,2 · must show 2,8,2
(e) The first shell holds a maximum of 2 electrons, the second and third up to 8 electrons each; each shell is filled before the next one begins to fill. · must show first: 2, second: 8, third: 8, fill lowest first
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Marking points
B1 (a) · 1 mark · Gives the relative mass of an electron as 1/1836 (or approximately correct fraction).
B1 (a) · 1 mark · Gives the relative charge of an electron as -1.
B1 (b) · 1 mark · Describes electrons as being in shells/energy levels around the nucleus.
A1 (c) · 1 mark · States that an atom with atomic number 12 has 12 electrons.
B1 (d) · 2 marks · Gives electronic configuration of magnesium as 2,8,2 (1 mark for correct method, 1 mark for completely correct answer). (follow-through)
B1 (e) · 1 mark · States correct rules for filling shells up to the third shell.
Worked solution
a) Relative mass = 1/1836; relative charge = -1.
b) Electrons are found in shells around the nucleus.
c) Number of electrons = atomic number = 12.
d) Fill the shells as follows: 2 in the first shell, 8 in the second, 2 in the third. So, configuration is 2,8,2.
e) Electron shells fill from lowest energy to highest: first shell holds up to 2 electrons, second and third shells up to 8 each.