Question Video: Converting from the Mass Number and the Atomic Number to the Number of Neutrons | Nagwa Question Video: Converting from the Mass Number and the Atomic Number to the Number of Neutrons | Nagwa

# Question Video: Converting from the Mass Number and the Atomic Number to the Number of Neutrons Chemistry • First Year of Secondary School

An atom of calcium has a mass number of 42 and an atomic number of 20. How many neutrons are there in its nucleus?

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### Video Transcript

An atom of calcium has a mass number of 42 and an atomic number of 20. How many neutrons are there in its nucleus?

An atom consists of a nucleus surrounded by an electron cloud. In the nucleus, we can find protons and neutrons. And electrons occupy the electron cloud. We won’t need it for this question, but it’s important to remember that atoms are neutral. So they have the same number of positive protons and negative electrons. The next thing we need to ask ourselves is, what do we mean by an atom of calcium?

Calcium is the name of an element. And we can find information about calcium on the periodic table of elements. The first thing you might notice is that the number in the box for calcium is the same as the atomic number of the atom of calcium. The atomic number is simply the number of protons in an atom or ion of a particular element. So the first thing we realize is that we have 20 protons in our atom of calcium. And while it doesn’t make a difference in this question, we also have 20 electrons.

The other piece of information in the question is that this atom has a mass number of 42. The mass number is the number of protons and neutrons in the atom or ion. So this tells us that if we take 20 and add the number of neutrons, we’ll get 42. All this sets us up nicely to answer the question. How many neutrons are there in the nucleus of the atom of calcium? The number of neutrons must be 42 minus the number of protons, which is 20. Therefore, the number of neutrons in an atom of calcium with a mass number of 42 is 22.

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