The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these elements is 40. What is the name given to such a pair of elements?

Explanation

To solve the question, let's break it down step by step:

### Step 1: Understand the Definitions
- **Atomic Number**: The number of protons in the nucleus of an atom, which determines the element's identity.
- **Mass Number**: The total number of protons and neutrons in the nucleus of an atom.

### Step 2: Identify the Atomic Numbers
- The atomic number of calcium (Ca) is 20.
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