Video: Understanding the Meaning of the Atomic Number

Which of the following is another name for “atomic number”? [A] Nucleon number [B] Mass number [C] Electron number [D] Proton number [E] Neutron number.

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Video Transcript

Which of the following is another name for atomic number? a) Nucleon number. b) Mass number. c) Electron number. d) Proton number. e) Neutron number.

Now this name, atomic number, may ring a bell because whenever we write down the symbol for an atomic isotope, we include this number. Let’s say that we had an atom of a made-up element we’ll call element A. When we write down in symbolic form a particular isotope of this element, it’s normal to write the atomic number at the lower left of the symbol for it and the mass number at the upper left of the symbol for this element. We can recall that these two terms indicate something about this isotope. The atomic number tells us the number of protons in the nucleus. And the mass number tells us the number of protons plus the number of neutrons.

By the way, because protons and neutrons are both elements of the nucleus, sometimes they’re called nucleons. Knowing all this, let’s look at our answer options once more. Since a nucleon is either a proton or a neutron, that means nucleon number is the same thing as mass number, the number of protons and the number of neutrons in the nucleus. We’ve seen that mass number and atomic number refer to different things. This means that option a, as well as option b, doesn’t describe atomic number.

Option c talks about electron number. Those are parts of an atom that aren’t in the nuclear core. The atomic number doesn’t refer to electrons, so we’ll cross that choice off as well. Then, we get to option d proton number. We’ve seen that atomic number is an indication of this number. And then, looking ahead at option e neutron number, well, that’s included in the mass number, but it’s not referred to by atomic number. Option e then is off our list. We find that indeed it’s option d proton number, which is another name for atomic number.

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