Atomic Structure
5 marks · EasyAn element X has two naturally occurring isotopes: X-64 and X-66. In a sample of X, 75% of the atoms are X-64 and 25% are X-66. The atomic number of X is 30.
Calculate the relative atomic mass of element X. Show all your working.
How many electrons are present in one atom of X?
State the number of electrons in an ion of X with a 2+ charge.
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.
Show answer
(a) Relative atomic mass = 64.5
(b) 30
(c) 28
Common mistakes: Mixing up isotope mass and atomic number; Using wrong percentage as fraction; Forgetting to remove electrons for the 2+ ion
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Multiplies each isotope mass by percentage abundance (or fraction) and adds together.
M2 (a) · 1 mark · Divides total mass by 100 (or uses correct fractions for each isotope) to obtain average/relative atomic mass.
A1 (a) · 1 mark · Gives correct answer: 64.5 or a value within tolerance.
A2 (b) · 1 mark · Gives correct number of electrons for neutral X atom (30).
A3 (c) · 1 mark · Gives correct number of electrons in X2+ ion (28).
Worked solution
a) To calculate the relative atomic mass of X:
b) Number of electrons in a neutral atom of X equals its atomic number = 30.
c) For X with a 2+ charge, it has lost 2 electrons: