The periodic table
4 marks · NormalThe symbol for tin is Sn. An atom of tin has atomic number 50 and mass number 120.
State the number of electrons in a neutral atom of tin.
Calculate the number of neutrons in this tin atom.
Explain, using numbers, why the atomic number and the number of protons in tin are always the same.
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(a) 50
(b) 70
(c) The atomic number is defined as the number of protons in the nucleus of the atom. For tin, the atomic number is 50, so it has 50 protons. · must show States that atomic number equals protons, Uses the value for tin as an example (50)
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Marking points
B1 (a) · 1 mark · States correct number of electrons in a neutral atom of tin (50).
M1 (b) · 1 mark · Correctly calculates neutrons: 120 - 50 = 70.
B2 (c) · 1 mark · Explains that atomic number is the number of protons.
A1 (c) · 1 mark · Uses value for tin (50) or clear statement about atomic number/protons.
Worked solution
(a) Number of electrons is equal to atomic number, so tin has 50 electrons.
(b) Number of neutrons = mass number - atomic number = 120 - 50 = 70
(c) The atomic number is always the number of protons in the nucleus by definition. For tin, atomic number = 50, so it has 50 protons.