The mass number of argon is 40. Which one of the following statements is correct?
- A.The number of protons in argon is 22.
- B.The number of neutrons in argon is 18.
- C.The number of electrons in argon is 18.
- D.The sum of numbers of protons and electrons in argon is 40.
Correct answer
Explanation
The correct answer is C, the number of electrons in argon is 18. Argon has atomic number 18, and a neutral atom has as many electrons as protons, so it has 18 electrons.
The atomic number (Z) is the number of protons and fixes which element an atom is; for argon Z = 18. The mass number (A) is the number of protons plus neutrons, so the neutrons in argon-40 are 40 − 18 = 22. Electrons weigh very little and are not counted in the mass number. Argon is a noble gas of group 18 with the configuration 2, 8, 8, and it is the third most abundant gas in air, about 0.93 per cent, used to fill electric bulbs.
A is wrong because 22 is the number of neutrons, not protons; argon has 18 protons.
B is wrong because argon-40 has 22 neutrons, not 18.
D is wrong because protons plus electrons come to 18 + 18 = 36; it is protons plus neutrons that make 40.
Exam tip: protons = Z, neutrons = A − Z, electrons = Z in a neutral atom.
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