Atomic Structure
6 marks · NormalA sample contains molecules of ammonia, .
State the meanings of the terms 'atom' and 'molecule'.
Each ammonia molecule contains one nitrogen atom (N) and three hydrogen atoms (H). The nitrogen atom in this molecule has a mass number of 15 and an atomic number of 7. Each hydrogen atom has a mass number of 1 and an atomic number of 1.
i. Calculate the total number of protons in one molecule of ammonia, using your answer to part (a) and the information above.
ii. Calculate the total number of neutrons in one ammonia molecule, using the mass numbers given above.
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) An atom is the smallest part of an element that can still be recognised as that element. A molecule is two or more atoms chemically bonded together. · must show Definition of atom matches or closely paraphrases chemistry textbook definition, Definition of molecule as two or more atoms chemically bonded
(b) 10 · must show 1 nitrogen atom (atomic number 7) gives 7 protons, 3 hydrogen atoms (atomic number 1) gives 3 protons, Total protons = 7 + 3 = 10
(c) 8 · must show Nitrogen: mass number 15 - atomic number 7 = 8 neutrons, Hydrogen: mass 1 - atomic number 1 = 0 neutrons, for each hydrogen, Total neutrons = 8 + 0 + 0 + 0 = 8
Common mistakes: Confusing protons with neutrons; Using mass number for protons
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Marking points
B1 (a) · 1 mark · Correct definition of atom (the smallest part of an element that can still be recognised as that element or similar).
B2 (a) · 1 mark · Correct definition of molecule (two or more atoms chemically bonded together).
M1 (b) · 1 mark · Identifies number of protons using atomic numbers: 1 N atom × 7 + 3 H atoms × 1. (follow-through)
A1 (b) · 1 mark · Correct answer: Total protons in one molecule = 10. (follow-through)
M2 (c) · 1 mark · Calculates neutrons by subtracting atomic number from mass number for nitrogen atom. (follow-through)
A2 (c) · 1 mark · Correct answer: Total neutrons in one molecule = 8. (follow-through)
Worked solution
a. An atom is the smallest part of an element that can still be recognised as that element. A molecule is made up of two or more atoms chemically bonded together.
b. i. Number of protons per molecule:
Nitrogen atom: atomic number = 7, so 7 protons.
Each hydrogen atom: atomic number = 1, so 1 proton per atom.
There are 3 hydrogens: 3 × 1 = 3 protons.
So, there are 10 protons in one ammonia molecule.
c. ii. Number of neutrons per molecule:
For nitrogen: mass number = 15, atomic number = 7
So, 8 neutrons from nitrogen.
Each hydrogen: mass number = 1, atomic number = 1, so 1 - 1 = 0 neutrons per hydrogen atom (total 0).
Total neutrons in the molecule = 8 + 0 + 0 + 0 = 8.