Problem 21 - Chemistry: The Central Science

Question

(a) Define atomic number and mass number. (b) Which of these can vary without changing the identity of the element?

Step-by-Step Solution

Answer

(a) The atomic number (\(Z\)) is the number of protons in the nucleus of an atom and uniquely identifies an element. The mass number (\(A\)) is the total number of protons and neutrons in the nucleus of an atom and is equal to the atomic number (\(Z\)) plus the number of neutrons (\(N\)); \(A = Z + N\). (b) The mass number can vary without changing the identity of the element, as it depends on both the number of protons and neutrons. When the number of neutrons varies but the protons remain the same, we have isotopes of the same element.

Step 1: Atomic number definition

The atomic number refers to the number of protons in the nucleus of an atom. It is represented by the symbol \(Z\). The atomic number uniquely identifies an element, as no two elements have the same number of protons.

Step 2: Mass number definition

The mass number, represented by the symbol \(A\), is the total number of protons and neutrons in the nucleus of an atom. It is an integer value that is equal to the atomic number \((Z)\) plus the number of neutrons \((N)\). Thus, \(A = Z + N\).

Step 3: Determining which can vary without changing the identity of the element

As mentioned earlier, the atomic number uniquely identifies an element. If the atomic number changes, it means that the number of protons in the nucleus has changed, and thereby, it is a different element. On the other hand, the mass number can vary without changing the identity of the element because it depends on both the number of protons and the number of neutrons. When the number of neutrons varies but the number of protons remains the same, we have isotopes of the same element. So, the mass number can vary without changing the identity of the element.