Question Video: Naming an Isotope from its Nuclide Symbol

Which isotope is represented by this nuclide symbol?

03:47

Video Transcript

Which isotope is represented by this nuclide symbol? The symbol consists of the letters Na with 24 in the top left and 11 in the bottom left.

Nuclide notation is used to summarize various features of elements, isotopes, and their atoms or ions. The first feature is a capital letter or a capital letter followed by a lowercase letter, which is the element symbol. The first step of digesting nuclide notation is to look up the element symbol on the periodic table of elements.

In this case, the letters are Na. A quick scan of the periodic table will reveal that Na corresponds to an element in group one and period three. When we read that cell on the periodic table, we find that Na is the symbol for the element sodium. So, we know our nuclide symbol has something to do with sodium.

Now, sodium is an element. All atoms or ions of sodium consist of a nucleus surrounded by an electron cloud. It’s the number of protons in the nucleus of an atom or ion that determines the element. For sodium, that number is 11 which we can tell from the atomic number on the periodic table. The atomic number is simply the number of protons in an atom or ion of that particular element. And for sodium, that means 11 protons.

In nuclide notation, we put the atomic number in the bottom left-hand corner. So, so far, we understand the meaning of two of the terms in the nuclide symbol. It’s just the 24 in the top left that’s mysterious. To understand that, it’s worth looking at what an isotope is. In long form, an isotope is a type of atom or ion of an element with a specific number of neutrons per nucleus. When we look up the element on the periodic table, that only tells us the number of protons in the nuclei, not the number of neutrons. For that, we need to know the isotope.

To understand that, we need to remember what this number in the top left-hand corner of the nuclide symbol is for. In nuclide notation, the number in the top left is known as the mass number. The atomic number is the number of protons in an atom or ion of that particular isotope, while the mass number is the number of protons plus the number of neutrons in an atom or ion of that particular isotope. So, in this particular isotope, all the atoms or ions have nuclei containing 11 protons and 13 neutrons. That’s 24 minus 11.

However, that doesn’t quite get us to the end of the question. We need to know how to name this particular isotope. When we name isotopes, we take the element name, which tells us the number of protons. And then we take the mass number, which we can use to calculate the number of neutrons. We know the element we’re dealing with is sodium. And we can read the mass number 24 from the nuclide symbol. So, we worked out that an isotope with this particular symbol means an isotope where each nucleus contains 11 protons and 13 neutrons, and we call it sodium-24.

If you ever need to do this quickly, this is what you can do. Look up the symbol on the periodic table and find the name for the element. And then take the mass number in the top left and put it after the element name with a hyphen. Using either method, you can be sure that the isotope represented by this particular nuclide symbol is sodium-24.

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