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Worked example: using the mass number equation

The sum of the number of neutrons and protons in an atom is called its mass number. If we know the mass number of an atom, we can determine the number of neutrons in its nucleus. Created by Khan Academy.

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  • piceratops ultimate style avatar for user ShivamIsSmart
    How many known elements are there? Are there any left unknown?
    (5 votes)
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    • boggle green style avatar for user TDJ
      There are currently 118 known elements. 94 of these are naturally occurring, and the others are man-made. Generally, the larger atoms are, the more unstable they become. For example, the element 118, Oganesson, lasts for just 0.9 milliseconds, before it decays! Any elements beyond the periodic table would most likely be too unstable to be used for anything.
      (14 votes)
  • aqualine tree style avatar for user Julie
    What would the mass number be used for?
    (1 vote)
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    • aqualine seed style avatar for user Kennedy
      An atom's mass number (symbol: "A") is used to determine the total number of protons and neutrons in the nucleus of an atom. I.e.,

      Protons + neutrons = mass number

      With an atom's mass number, you can figure out the number of neutrons in the nucleus. The mass number is useful when studying isotopes (atoms of the same element but with different numbers of neutrons). The mass number allows scientists to identify and compare different isotopes of an element based on their nuclear composition. For example, we can compare Carbon isotopes: carbon-12 (C-12), carbon-13 (C-13), and carbon-14 (C-14). These isotopes have mass numbers of 12, 13, and 14, respectively.

      Studying isotopes and their nuclear composition is valuable because we can gain insights into various aspects of an element's behavior. For instance, different isotopes of an element might have different stability, radioactivity, or chemical properties. This type of information is vital in many fields, not only in chemistry, but also in physics, geology, archaeology, and environmental science. That's because you can gather a lot of data from techniques like mass spectrometry (which measures the relative abundance of isotopes in a sample, to determine their ratios). We can learn a lot about the origin, age, and processes involved in various natural systems.

      This all starts with isotopes. And it's the atom's mass number that allows us to tell what isotope we're dealing with. So, that's what it's for.
      (6 votes)
  • male robot hal style avatar for user KEVIN
    Why did we not incorporate the decimal value in the calculation? Thanks, and these lessons are great!
    (3 votes)
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  • stelly orange style avatar for user Volcannon1752
    I’ve been trying to figure out how to write superscript and subscript(which if you don’t know, superscript is small text that goes slightly above the main character, and subscript is the same but goes slightly under)in the same line, but I just can’t do it. Can anyone help me? The best I can do is ₃⁶ C or something like that.
    (2 votes)
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  • blobby green style avatar for user 28808aal0
    is atomic mass and mass number the same ?
    (0 votes)
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  • primosaur tree style avatar for user Alicebel
    is the mass number always greater than the atomic number/protons?
    (1 vote)
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  • blobby green style avatar for user giftfelix621
    In the process of an elements having different mass number you mentioned isotopes. What is Isotopes
    (1 vote)
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  • blobby green style avatar for user Her24
    Does atomic or proton number refer to the mass of protons? it seemed like we were substructing apples from oranges,help anyone?
    (0 votes)
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Video transcript

- [Instructor] Hi, everyone. In this video we're going to practice using the mass number equation. This equation represents the fact that the mass number of an atom is equal to its number of protons plus its number of neutrons. Let's use the mass number equation to answer the following question. How many neutrons are in an isotope of sodium with the following notation? To answer this question, we'll first need to rearrange our mass number equation to solve for the number of neutrons. Pause the video and take a moment to try this for yourself. The rearranged equation is as follows. The number of neutrons equals the mass number minus the number of protons. Based on this notation, we know that this sodium isotope has a mass number of 23 and an atomic number of 11. If we subtract the atomic number 11 from the mass number 23, we're left with 12, which is the number of neutrons in this sodium isotope. Okay, more practice, this time with an isotope of platinum. How many neutrons does this isotope have based on the following notation, Pt-195? Again, we'll need to use the rearranged mass number equation to solve for the number of neutrons. This particular notation provides the chemical symbol of platinum and its mass number. We can look up platinum on the periodic table to find its atomic number, which is 78, meaning that every platinum atom has 78 protons. We can subtract the atomic number 78 from the mass number 195 to get 117, so this isotope of platinum has 117 neutrons.