The atom of an element has 9 protons, 9 electrons and 10 neutrons.
What is the mass number of the element?

Explanation

To find the mass number of the element, we can follow these steps:

**Step 1: Understand the definition of mass number.**
The mass number of an atom is the total number of protons and neutrons in its nucleus.

**Step 2: Identify the number of protons and neutrons.**
From the question, we know:
- Number of protons = 9
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