Problem 25 - 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 an atom's nucleus, determining its position in the periodic table and its identity. The mass number (A) is the total number of protons and neutrons in the nucleus. (b) The mass number can vary without changing the element's identity, as different isotopes have varying numbers of neutrons but the same number of protons (atomic number).

Step 1: Define Atomic Number

The atomic number, often represented by the symbol \(Z\), is the number of protons in the nucleus of an atom. The atomic number is a unique property of an element and is used to define its position in the periodic table. Each element has a specific number of protons, and therefore, a specific atomic number. For example, hydrogen has an atomic number of 1, which means it has one proton in its nucleus.

Step 2: Define Mass Number

The mass number, often represented by the symbol \(A\), is the total number of protons and neutrons in the nucleus of an atom. The mass number does not include electrons, as their mass is negligible compared to the mass of protons and neutrons. The mass number is not unique for a specific element, as the number of neutrons can vary for atoms of the same element (known as isotopes).

Step 3: Determine Which Can Vary Without Changing the Element's Identity

As we defined in the previous steps, the atomic number is directly related to the number of protons, which determines the element's identity. In contrast, the mass number is related to the total number of protons and neutrons. Since the number of neutrons can vary and create different isotopes of the same element, the mass number can change without impacting the element's identity. However, changing the atomic number would result in a different element due to a change in the number of protons. So, the mass number can vary without changing the identity of the element.