Ontario Grade 9 Science Class Practice Exam

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What does the atomic number represent?

The mass of an atom in atomic mass units.
The number of protons in an atom's nucleus.
The total number of electrons in an orbit around the sun.
A measurement of electric current.

The correct answer is: The number of protons in an atom's nucleus.

The atomic number represents the number of protons in an atom's nucleus. Option A is incorrect because the atomic mass is a measurement that takes into account the mass of both protons and neutrons in an atom. Option C is incorrect because the total number of electrons in an orbit around the sun would be too large of a number to accurately represent the atomic number. Option D is incorrect because atomic number is not a measurement of electric current.