Two atoms are different isotopes of the same element if they have the same blank but different blank.
Our job in this question is to fill in the blanks correctly. So we’re starting out with two atoms, which are different isotopes of the same atomic element. To figure out what goes in these blanks, it’s helpful to recall how atomic elements are written out. Imagine that we invent a new atomic element and we name it “somethingium”. And we abbreviate it So. If we had a particular atom of somethingium, this new element, here’s how we write it out.
First, we would write the abbreviation for somethingium, So. Then, at the bottom left-hand corner, we would write one number. We can call that N one. And the top-left corner, we would write another number. We can call that N two. This number on bottom, what we’ve called N one, is the number of protons contained in this element, somethingium. This number is known as the atomic number of somethingium. That’s the number of protons it has. And that’s what characterizes the element. Then, in the top-left corner, there is another number. This number is equal to the number of protons in somethingium plus the number of neutrons in the nucleus.
In other words, this number will almost always be higher than N one. Because protons and neutrons are by far the most massive elements in any atom, that is, by comparison electrons have very low mass, the name given to N two, the sum of the protons and neutrons in an atom, is the mass number. One important thing to know about these two numbers, atomic number and mass number, is that, for a given element, atomic number is always the same. That will never change because that’s tied to what an element’s identity is.
But mass number, on the other hand, can change. And the identity of the element can still stay the same. It can still be somethingium. We do that by adding or subtracting neutrons from this atom. That wouldn’t change the identity of the atom. But it would change its mass number. When two atoms are made of the same element, say somethingium, but they have different mass numbers, they’re called isotopes of one another. That prefix, ISO, means same. The two atoms are made of the same element. But because they don’t have the same number of neutrons in them, their mass number, the sum of neutrons plus protons, isn’t the same.
All this takes us back to the blanks in our statement that we want to fill in. Two atoms that are different isotopes of the same element must have the same atomic number. That’s what it means to be of the same element. But because these two atoms aren’t identical but are isotopes, that means they have a different mass number from one another. So our completed sentence reads like this. Two atoms are different isotopes of the same element if they have the same atomic number but different mass numbers. That completes the blanks in this sentence.