# Question Video: Using the Mass Number and Chemical Symbol to Determine Which Atom Has the Largest Number of Neutrons Chemistry • 7th Grade

Which of the following atoms has the largest number of neutrons?

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### Video Transcript

Which of the following atoms has the largest number of neutrons? (A) 19 F, (B) 28 Si, (C) 39 Ca, (D) 39 K, or (E) 23 Na.

To answer this question, we need to determine how many neutrons each of the given atoms has. We have been given information about each atom in nuclide notation. Nuclide notation, or AZE notation, is written in this general format. In nuclide notation, 𝐸 represents the chemical symbol of the element as it appears on the periodic table, 𝐴 represents the mass number, and 𝑍 represents the atomic number. The mass number is the total number of protons and neutrons in an atom or ion, while the atomic number only represents the number of protons in the atom or ion.

We can see that nuclide notation does not directly express the number of neutrons in an atom. But as mass number is the sum of the number of protons and neutrons, the number of neutrons in an atom can be calculated by subtracting the number of protons or the atomic number from the mass number.

Looking at the nuclide notations provided, we can see that we have been given the mass number for each atom, but the atomic number has not been provided. The atomic number of an atom is often omitted from nuclide notation. This is because each element has its own unique atomic number. The atomic number of each element can be found on the periodic table and is most commonly written above the chemical symbol in each cell.

By using a periodic table, we can determine that the atomic number of fluorine is nine, silicon is 14, calcium is 20, potassium is 19, and sodium is 11. Now we can subtract the atomic number from the mass number for each atom to determine how many neutrons each atom has. One atom of fluorine 19 has 10 neutrons. One atom of silicon 28 has 14 neutrons. One atom of calcium has 19 neutrons. One atom of potassium has 20 neutrons. And one atom of sodium has 12 neutrons.

We can now see that of the atoms provided, the atom with the largest number of neutrons is potassium 39. So the correct answer is answer choice (D) 39 K.