Atomic number and mass number
4 marks · NormalUse the periodic table and your knowledge to answer the following questions about atomic numbers and mass numbers.
Define mass number.
An atom of element X has atomic number 9 and mass number 19.
State how many protons, neutrons and electrons are present in a neutral atom of X.
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) Mass number is the total number of protons and neutrons in the nucleus of an atom.
(b) Protons: 9, Electrons: 9, Neutrons: 10
Common mistakes: Mass number confused with atomic number; Neutron calculation error
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Defines mass number as the total number of protons and neutrons in the nucleus.
B2 (b) · 1 mark · Number of protons given as 9.
B3 (b) · 1 mark · Number of electrons given as 9 (for a neutral atom).
B4 (b) · 1 mark · Number of neutrons calculated as 10 (mass number – atomic number).
Worked solution
a) Mass number is the total number of protons and neutrons in the nucleus of an atom.
b) Protons = atomic number = 9. Electrons = 9 (as atom is neutral). Neutrons = mass number – atomic number = 19 – 9 = 10.