Question 1

What determines the identity of an element?

Accepted Answer

The atomic number Z determines the identity because it equals the number of protons in the nucleus. Changing only the neutron number changes the mass number and produces an isotope of the same element.

Question 2

How is the neutron number calculated for a nuclide?

Accepted Answer

Use A = Z + N, or N = A - Z. For carbon-14, Z is 6 and A is 14, so the nucleus contains 14 - 6 = 8 neutrons.

Question 3

What makes two nuclides isotopes?

Accepted Answer

Isotopes have the same atomic number Z but different mass numbers A. Their proton counts are equal, while their neutron counts differ, as in hydrogen-1, hydrogen-2 and hydrogen-3.

Question 4

How does nuclear radius vary with mass number?

Accepted Answer

Nuclear radius follows R = R0 A^(1/3), where R0 is approximately 1.2 × 10^-15 m. The relation uses mass number A rather than measured atomic mass, so radius increases much more slowly than mass.

Question 5

Is the mass number the same as atomic mass?

Accepted Answer

No. Mass number A is an exact integer equal to protons plus neutrons. Atomic mass is the mass of an atom, expressed in unified atomic mass unit u, and may be a non-integer value.

Isotopes & Nuclear Size — Inside the Nucleus

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