Question

The mass number of a nucleus is

• A. always less than its atomic number
• B. always more than its atomic number
• C. sometimes equal to its atomic number
• D. sometimes less than and sometimes more than its atomic number

Answer 1

The Correct Option is C

The question is about understanding what the mass number of a nucleus signifies and how it relates to the atomic number.

The mass number of a nucleus is defined as the sum of the number of protons and neutrons in the nucleus. It is represented as A = Z + N, where:

• A is the mass number.
• Z is the atomic number (the number of protons).
• N is the number of neutrons.

To understand the options:

1. Always less than its atomic number: This cannot be true because the mass number includes the atomic number and always includes additional neutrons, making it impossible to be less than the atomic number.
2. Always more than its atomic number: While the mass number is often more than the atomic number due to the presence of neutrons, there are cases where the mass number can be equal to the atomic number if the number of neutrons is zero (e.g., for hydrogen-1 with just one proton and no neutron).
3. Sometimes equal to its atomic number: This option is correct. As mentioned, for certain isotopes like hydrogen-1, where no neutrons are present, the mass number equals the atomic number.
4. Sometimes less than and sometimes more than its atomic number: This is incorrect because the mass number can never be less than the atomic number as it includes the number of protons.

Based on the above reasoning, the correct answer is that the mass number is sometimes equal to its atomic number.