Atomic Structure
5 marks · EasyA sample of an element X consists of two isotopes. 30% of the atoms are isotope X-24 (with mass number 24) and 70% are isotope X-25 (with mass number 25). The atomic number of element X is 12.
Calculate the relative atomic mass of element X from its isotopic abundances.
How many electrons are there in an atom of X? State your answer and explain your reasoning.
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) 24.7 · must show Correct calculation of total mass from isotope abundances, Divide total mass by 100
(b) 12; because number of electrons equals atomic number in a neutral atom · must show States 12 electrons, Explanation that number of electrons equals atomic number for a neutral atom
Common mistakes: Mixing up percentages of each isotope; Using the wrong atomic number; Assuming different number of electrons than protons
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Multiplies the percentage of each isotope by its mass number
M2 (a) · 1 mark · Sums the total mass of all atoms and divides by 100
A1 (a) · 1 mark · Correct final value for relative atomic mass (24.7) (follow-through)
B1 (b) · 1 mark · Correct number of electrons given (12)
B2 (b) · 1 mark · Explanation that number of electrons in a neutral atom equals its atomic number
Worked solution
a) Calculate total mass for 100 atoms:
Total mass from X-24 = 720
Total mass from X-25 = 1750
Relative atomic mass
b) Atomic number of X is 12, so number of electrons in a neutral atom is 12 (same as the number of protons).
Explanation: Atoms are electrically neutral, so the number of electrons equals the number of protons (atomic number).