Which of the following nuclides has the lowest number of neutrons? 12N, 21F, 11B, 12C, 11Be.
To answer this question, we need to make sure that we’re confident with nuclide notation. Let’s look at sodium as an example. In this nuclide notation, we have the chemical element symbol in the middle, in this case Na. And then, we have two numbers. The top number is our mass number, with the bottom being our atomic number.
Remember that the atomic number is unique to each element. So sodium is the only element with an atomic number of 11. The atomic number is also equal to the number of protons in a sodium atom. Because protons have a positive charge and an element is inherently neutral, we have to balance this out with an equal number of electrons. So for sodium, an atom contains 11 protons and also contains 11 electrons.
Now, let’s think about the mass number at the top. The mass number is equal to the atomic number plus the number of neutrons in each atom of this element. So from this, we can see that, to calculate the number of neutrons in an element, we do the mass number minus the atomic number. So let’s do that for each of the elements in the question.
I’ve used the periodic table to add in the atomic numbers for each of the elements. So now, we have the mass number and the atomic number for each option. So to work out the number of neutrons for each option, we do the mass number. Take away the atomic number. For nitrogen, this is 12 minus seven which equals five neutrons. For fluorine, 21 minus nine which gives 12 neutrons. For boron, it’s 11 minus five equals six neutrons. For carbon, we have 12 minus six equals six neutrons. And finally for beryllium, we have 11 minus four equals seven neutrons.
The question asks for the nuclide with the lowest number of neutrons. And from our list, we can see that the answer is nitrogen with five neutrons. So this is the correct answer.