In my last post, I went over briefly what the atomic number for an element represents. I redefined what an element is based on our updated knowledge of atomic structure and the atomic number. Now, I will go over what the mass number is.
It was mentioned in a previous post what the neutron was. The neutron, too, was discovered during alpha particle-scattering experiments. Though it’s not entirely essential to commit to memory, the neutron was in part discovered by British physicist James Chadwick, who in 1932 performed an experiment wherein beryllium metal was irradiated with alpha rays, a strongly penetrating radiation that is garnered from the metal. Chadwick’s experimental results showed that the penetrating radiation obtained from the beryllium metal consisted of neutrons, or neutral particles. Thus, it was found that a neutron is a nuclear particle with a mass that is almost the exact same as the proton, but with no electric charge whatsoever. In this same experiment, it was found that neutrons are knocked out when alpha particles strike beryllium nuclei (Ebbing and Gammon 2009).
If you've ever looked at a periodic table of elements, you will see that sodium, abbreviated Na, has an atomic number of 11. Given what you’ve just learned, you should be able to define explicitly what this means. Because the atomic number provides the number of protons in the nucleus in an atom, the number 11 for sodium means that 11 protons exist in the nucleus of any and all sodium atoms individually. Because protons have a positive charge, the charge on a sodium atom’s nucleus is +11e, where e is the electron charge. This is usually just written as +11, which makes things a lot easier (Ebbing and Gammon 2009). Look at the periodic table of elements and do this with any element; you do not have to memorize the atomic numbers of the elements. Just remember what the atomic number is and stands for.
Now that we’ve briefly gone over the atomic number, let’s move on to the mass number, and at the end of it all, we will have a short review of everything that we’ve learned so far about protons, electronics, neutrons, and more.
What is the mass number? The mass number is another characteristic of an element’s nucleus, just as the atomic number is. The mass number, however, is somewhat of an extension of the atomic number, and it actually describes the sum total of individual protons and neutrons in a nucleus. Let’s go back to the sodium atom. We already established the fact that an atom of naturally-occurring sodium contains 11 protons within its nucleus. What is its mass number? The mass number of a sodium atom, per any periodic table of elements in any textbook and internet sources, is 23. This means that there are a total of 23 protons and neutrons in the nucleus of a sodium atom. And because we already know ho many protons exist in the nucleus of a sodium atom, we can determine using the mass number how many neutrons exist in the nucleus of a sodium atom. This is actually given by 23 minus 11, which equals 12. 12 neutrons are in the nucleus of a naturally-occurring sodium atom (Ebbing and Gammon 2009).
Thus, a nuclide is defined as an atom that is characterized by a distinct and certain atomic number as well as mass number. Shorthand notation for a nuclide symbol includes the mass number as a superscript and the atomic number as a subscript, both simultaneously directly preceding the abbreviation for the element in question (Ebbing and Gammon 2009).
As we went over before, atoms are normally electrically neutral, meaning that the number of protons in its nucleus is equal to the number of electrons that surround the nucleus of the atom. So, although the atomic number describes the number of protons in the nucleus of an atom, the number of electrons is typically the same, at least for any electrically neutral atom (Ebbing and Gammon 2009).
So, while the nuclei of all atoms of a given element all have the same atomic number, the mass number may differ for different nuclei. This occurrence can be described by defining the term isotope. An isotope of an atom has a nucleus with the same atomic number of the atom, but the mass numbers differ. In other words, both atoms may have the same number of protons in their nuclei, but the number of neutrons differs between isotopes. While sodium only has one isotope that occurs naturally, which was already described (sodium-23), other elements, such as oxygen, have a mixture of different isotopes, including oxygen-16, oxygen-17, and oxygen-18 (Ebbing and Gammon 2009).
Now that we have gone over protons, electrons, neutrons, the atomic number, the mass number, isotopes, and more, let’s do a short review of a few important concepts. Below is a table, inspired by Ebbing and Gammon, which contains some information that is common knowledge about some of the particles we’ve learned about. And also included is a diagram that you can use to help you understand isotopes a little better. Featured are a few different, relatively stable isotopes of the hydrogen atom. The nuclide symbol for each different isotope is included. Use the information in this post and the previous post on chemistry to help you understand the diagram. If you can understand it fully, then you've studied well!
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