Ontario Grade 9 Science Class Practice Exam

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Master Grade 9 Science with our comprehensive practice exam. Test your knowledge, identify weak areas, and boost your confidence for academic success.

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What is indicated by an atom's mass number?

The total number of electrons and neutrons
The total number of electrons and protons
The number of protons in its nucleus
The total number of protons and neutrons in its nucleus

The correct answer is: The total number of protons and neutrons in its nucleus

An atom's mass number, also known as atomic mass, is the sum of the number of protons and neutrons in its nucleus. This means that both A and B are incorrect, as they only account for part of the mass number. C is also incorrect because it only refers to the number of protons, but neglects to include the mass of the neutrons. Therefore, D is the correct option as it includes both protons and neutrons, making it the total mass of the atom.