What element has an atomic number of 20 and a mass number of 40?

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Answer

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Hint: $Ca$ is a group 2 element and its atomic number is 20 and mass number is 40 . Atomic number is the total number of protons present in an atom and mass number is the sum of the total number of neutrons and protons present in the atom.

Complete step by step answer:

The atomic number (Z) is the total charge on the nucleus while mass number (A) is the weight of the nucleus . Calcium has mass number 40 and atomic number 20.We know that the atomic number is the total number of protons present in an atom.The number of protons is the same as that of the electrons to keep the atom electrically neutral .And the mass number is the sum of the total number of neutrons and protons present in the atom.Therefore, in calcium atom,number of protons =20number of neutrons =mass number - number of protonsthat is , $40 - 20 = 20$ So the number of protons and neutrons becomes the same.Therefore option A is correct .and, number of electrons= number of protons= number of neutrons = 20.Therefore , option B is also correct .The number of nucleons which is referred to as the mass number of an atom is 40 in case of calcium atom which is double the number of neutrons .Therefore option D is correct

So, the correct answer is Option C .

Note:

 It should be noted that the number of electrons is always equal to the number of protons to keep the element electrically neutral . The number of neutrons may be or may not be equal to the number of electrons and protons .