In this post on general chemistry, I will review a few different topics in one post. At the end of this post, we will still be just halfway through our second chapter on general chemistry, which is a bit longer than the first.
For quite a few posts, I’ve been discussing Dalton’s atomic theory, which is undoubtedly one of the most important concepts you’ll learn in your earliest weeks of learning general chemistry. We started off with just the notion that an atom of an element has a distinct mass, and then we moved on to what types of atomic particles exist within our scope of study, and we even discussed what isotopes are. In most cases, regardless of the origin of the element, the percentages of the different and distinct isotopes of an element have, for the most part, remained the same over time. So, what Dalton actually calculated were average atomic masses, or more aptly, relative masses. Most of the work you will do in general chemistry involves the use of naturally occurring mixtures of elements, so it is important to understand the concept of average atomic masses (Ebbing and Gammon 2009).
Like I mentioned, the masses of atoms were not actually measured in Dalton’s time. Rather, they were average masses that were approximated, relative to one another. To understand how this might have worked, consider an example where someone burns hydrogen gas in oxygen. The result of the experiment is the creation of a water, which is composed of both hydrogen and oxygen. Using this experiment, you find that when you react 1.0000 gram of hydrogen with 7.9367 grams of oxygen, water is formed. The part that you need to know in order to find the atomic mass of each element, which is the relative numbers of each type of atom in water, was actually the part that Dalton struggled with. Dalton actually assumed incorrectly that the formula for water is HO. Now, however, we know that water contains two hydrogen atoms and one oxygen atom. So, logically, you can come to the conclusion that the atomic mass of oxygen is 15.873 times that of the mass of oxygen, using the calculation 2*7.9367 = 15.873 (Ebbing and Gammon 2009).
Today, we used a carbon-12 mass scale, as opposed to the hydrogen-based atomic scale that was developed by Dalton. And, rather than use relative atomic masses obtained through manual experimentation, like Dalton did, a mass spectrometer is credited for the measurements of atomic mass that are used today. The mass spectrometer compares the mass of an atom to the mass of another atom that is chosen as a standard; in this case, carbon-12. Arbitrarily, this isotope of carbon has been assigned a mass of 12 atomic mass units. An atomic mass unit (amu) is, then, a unit of measurement of mass that is equal to exactly one-twelfth of the mass of the carbon-12 atom. And, an atomic mass is thus defined as the average atomic mass of an element in atomic mass units (Ebbing and Gammon 2009).
Earlier mass spectrometers, just like modern ones, all measure the mass-to-charge ratios of positively-charged atoms; the difference is that older models may use techniques of doing so that are different from the ones that more modern machines use. Mass spectrometers display a vast amount of data. When used correctly, a mass spectrum is produced, which shows the relative numbers of atoms for various masses. For example, consider a same of neon. A sample of neon has three isotopes: Ne-20, Ne-21, and Ne-22. These isotopes are all identifiable using the information provided by a mass spectrometer The information provided by the spectrum thus allows you to calculate the atomic mass of neon: the masses of all three isotopes mention above, and the relative number, also known as the fractional abundance, of each isotope. Specifically, the fractional abundance of an isotope is described as the portion of the sum total of atoms that is made up of a specific isotope. For the example of neon that we used, the fractional abundance are 0.9051 for neon-20, 0.0027 for neon-21, and 0.0922 for neon-22. Using this information, you can multiple the fractional abundance by each isotopic mass and then adding the numbers together to get the atomic mass of the element. For neon, the atomic mass that you would be is 20.179 amu (Ebbing and Gammon 2009).
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