Which of the following elements has a mass number of 40 and atomic number of 20?
Calcium
The atomic number equals the number of protons in an atom, and the mass number equals the total number of protons and neutrons. Here atomic number = 20, so the element has 20 protons. Looking at the periodic table, the element with 20 protons is calcium (Ca).
Its most common isotope has mass number 40, meaning 20 protons + 20 neutrons, which matches the data given. Potassium has atomic number 19, sodium 11 and neon 10, so none of them can have atomic number 20.
Hence, the answer is Calcium.
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